Merge bitcoin/bitcoin#22229: test: consolidate to f-strings (part 1)

68faa87881f5334b2528db4adc72ec19d94316a3 test: use f-strings in mining_*.py tests (fanquake)
c2a5d560df2824df5731100c2584e8ad7a3d7bc2 test: use f-strings in interface_*.py tests (fanquake)
86d958262dff43002820d58ccb8958e2dbfb9d5b test: use f-strings in feature_proxy.py (fanquake)
31bdb33dcb8345df1bb94b28e811252a918d7dcb test: use f-strings in feature_segwit.py (fanquake)
b166d54c3cbb0c028210cee977b3dcde5ac5474f test: use f-strings in feature_versionbits_warning.py (fanquake)
cf6d66bf941d946600047d712c7cd15d7605322e test: use f-strings in feature_settings.py (fanquake)
6651d77f22862716f5bd7d0b31cfbd3937ab7b1d test: use f-strings in feature_pruning.py (fanquake)
961f5813ba65b6a601081912c4ece96c2679794d test: use f-strings in feature_notifications.py (fanquake)
1a546e6f6ca95772f0d7dbc2792477becbb8ea63 test: use f-strings in feature_minchainwork.py (fanquake)
6679eceacc915a8ea7cd7063f103ffc5eb9da884 test: use f-strings in feature_logging.py (fanquake)
fb633933ab570e945d2a366f37eeff39f516c613 test: use f-strings in feature_loadblock.py (fanquake)
e9ca8b254d4b9567831c0e113ce1c0a2b4795a95 test: use f-strings in feature_help.py (fanquake)
ff7e3309995a8960ac371741b2b00c6da40f7490 test: use f-strings in feature_filelock.py (fanquake)
d5a6adc5e478fa5c6e562377eea873dc38e66578 test: use f-strings in feature_fee_estimation.py (fanquake)
a2de33cbdc79202bccddb4beadfde88266ac979f test: use f-strings in feature_dersig.py (fanquake)
a2502cc63fd308be8af840962da9c53339433fa6 test: use f-strings in feature_dbcrash.py (fanquake)
3e2f84e7a96cb4b97b609ac853f78edd0ed43f82 test: use f-strings in feature_csv_activation.py (fanquake)
e2f1fd8ee92fa421b6d293169044d6ddd5a9b8df test: use f-strings in feature_config_args.py (fanquake)
36d33d32b1b498b61f56d552f6e2c1d064f978c3 test: use f-strings in feature_cltv.py (fanquake)
dca173cc044270b30782b1e3355e9dcb8c534295 test: use f-strings in feature_blocksdir.py (fanquake)
5453e8706278918ac51a725e81599cfa18c8cdbc test: use f-strings in feature_backwards_compatibility.py (fanquake)
6f3d5ad67ac8e7b50abae1a2949898d858e38106 test: use f-strings in feature_asmap.py (fanquake)

Pull request description:

  Rather than using 3 different ways to build/format strings (sometimes all in the same test, i.e [`feature_config_args.py`](https://github.com/bitcoin/bitcoin/blob/master/test/functional/feature_config_args.py)), consolidate to using [f-strings (3.6+)](https://docs.python.org/3/reference/lexical_analysis.html#f-strings), which are generally more concise / readable, as well as more performant than existing methods.

  This deals with the `feature_*.py`, `interface_*.py` and `mining_*.py` tests.

  See also: [PEP 498](https://www.python.org/dev/peps/pep-0498/)

ACKs for top commit:
  mjdietzx:
    reACK 68faa87881f5334b2528db4adc72ec19d94316a3
  Zero-1729:
    crACK 68faa87881f5334b2528db4adc72ec19d94316a3

Tree-SHA512: d4e1a42e07d96d2c552387a46da1534223c4ce408703d7568ad2ef580797dd68d9695b8d19666b567af37f44de6e430e8be5db5d5404ba8fcecf9f5b026a6efb

26 files changed, 171 insertions, 175 deletions

diff --git a/test/functional/feature_asmap.py b/test/functional/feature_asmap.py
index 5fcecb4882..704dd6126b 100755
--- a/test/functional/feature_asmap.py
+++ b/test/functional/feature_asmap.py
@@ -31,8 +31,8 @@ ASMAP = '../../src/test/data/asmap.raw' # path to unit test skeleton asmap
 VERSION = 'fec61fa21a9f46f3b17bdcd660d7f4cd90b966aad3aec593c99b35f0aca15853'
 
 def expected_messages(filename):
-    return ['Opened asmap file "{}" (59 bytes) from disk'.format(filename),
-            'Using asmap version {} for IP bucketing'.format(VERSION)]
+    return [f'Opened asmap file "{filename}" (59 bytes) from disk',
+            f'Using asmap version {VERSION} for IP bucketing']
 
 class AsmapTest(BitcoinTestFramework):
     def set_test_params(self):
@@ -50,7 +50,7 @@ class AsmapTest(BitcoinTestFramework):
         filename = os.path.join(self.datadir, 'my-map-file.map')
         shutil.copyfile(self.asmap_raw, filename)
         with self.node.assert_debug_log(expected_messages(filename)):
-            self.start_node(0, ['-asmap={}'.format(filename)])
+            self.start_node(0, [f'-asmap={filename}'])
         os.remove(filename)
 
     def test_asmap_with_relative_path(self):
@@ -60,13 +60,13 @@ class AsmapTest(BitcoinTestFramework):
         filename = os.path.join(self.datadir, name)
         shutil.copyfile(self.asmap_raw, filename)
         with self.node.assert_debug_log(expected_messages(filename)):
-            self.start_node(0, ['-asmap={}'.format(name)])
+            self.start_node(0, [f'-asmap={name}'])
         os.remove(filename)
 
     def test_default_asmap(self):
         shutil.copyfile(self.asmap_raw, self.default_asmap)
         for arg in ['-asmap', '-asmap=']:
-            self.log.info('Test bitcoind {} (using default map file)'.format(arg))
+            self.log.info(f'Test bitcoind {arg} (using default map file)')
             self.stop_node(0)
             with self.node.assert_debug_log(expected_messages(self.default_asmap)):
                 self.start_node(0, [arg])
@@ -75,7 +75,7 @@ class AsmapTest(BitcoinTestFramework):
     def test_default_asmap_with_missing_file(self):
         self.log.info('Test bitcoind -asmap with missing default map file')
         self.stop_node(0)
-        msg = "Error: Could not find asmap file \"{}\"".format(self.default_asmap)
+        msg = f"Error: Could not find asmap file \"{self.default_asmap}\""
         self.node.assert_start_raises_init_error(extra_args=['-asmap'], expected_msg=msg)
 
     def test_empty_asmap(self):
@@ -83,7 +83,7 @@ class AsmapTest(BitcoinTestFramework):
         self.stop_node(0)
         with open(self.default_asmap, "w", encoding="utf-8") as f:
             f.write("")
-        msg = "Error: Could not parse asmap file \"{}\"".format(self.default_asmap)
+        msg = f"Error: Could not parse asmap file \"{self.default_asmap}\""
         self.node.assert_start_raises_init_error(extra_args=['-asmap'], expected_msg=msg)
         os.remove(self.default_asmap)
 
diff --git a/test/functional/feature_backwards_compatibility.py b/test/functional/feature_backwards_compatibility.py
index e0ba835f99..978080703e 100755
--- a/test/functional/feature_backwards_compatibility.py
+++ b/test/functional/feature_backwards_compatibility.py
@@ -366,7 +366,7 @@ class BackwardsCompatibilityTest(BitcoinTestFramework):
             assert_equal(load_res['warning'], '')
             wallet = node_master.get_wallet_rpc("u1_v16")
             info = wallet.getaddressinfo(v16_addr)
-            descriptor = "wpkh([" + info["hdmasterfingerprint"] + hdkeypath[1:] + "]" + v16_pubkey + ")"
+            descriptor = f"wpkh([{info['hdmasterfingerprint']}{hdkeypath[1:]}]{v16_pubkey})"
             assert_equal(info["desc"], descsum_create(descriptor))
 
             # Now copy that same wallet back to 0.16 to make sure no automatic upgrade breaks it
@@ -389,7 +389,7 @@ class BackwardsCompatibilityTest(BitcoinTestFramework):
             node_master.loadwallet("u1_v17")
             wallet = node_master.get_wallet_rpc("u1_v17")
             info = wallet.getaddressinfo(address)
-            descriptor = "wpkh([" + info["hdmasterfingerprint"] + hdkeypath[1:] + "]" + pubkey + ")"
+            descriptor = f"wpkh([{info['hdmasterfingerprint']}{hdkeypath[1:]}]{pubkey})"
             assert_equal(info["desc"], descsum_create(descriptor))
 
             # Now copy that same wallet back to 0.17 to make sure no automatic upgrade breaks it
diff --git a/test/functional/feature_blocksdir.py b/test/functional/feature_blocksdir.py
index 7bfad52c24..d3276e64ee 100755
--- a/test/functional/feature_blocksdir.py
+++ b/test/functional/feature_blocksdir.py
@@ -24,10 +24,10 @@ class BlocksdirTest(BitcoinTestFramework):
         initialize_datadir(self.options.tmpdir, 0, self.chain)
         self.log.info("Starting with nonexistent blocksdir ...")
         blocksdir_path = os.path.join(self.options.tmpdir, 'blocksdir')
-        self.nodes[0].assert_start_raises_init_error(["-blocksdir=" + blocksdir_path], 'Error: Specified blocks directory "{}" does not exist.'.format(blocksdir_path))
+        self.nodes[0].assert_start_raises_init_error([f"-blocksdir={blocksdir_path}"], f'Error: Specified blocks directory "{blocksdir_path}" does not exist.')
         os.mkdir(blocksdir_path)
         self.log.info("Starting with existing blocksdir ...")
-        self.start_node(0, ["-blocksdir=" + blocksdir_path])
+        self.start_node(0, [f"-blocksdir={blocksdir_path}"])
         self.log.info("mining blocks..")
         self.nodes[0].generatetoaddress(10, self.nodes[0].get_deterministic_priv_key().address)
         assert os.path.isfile(os.path.join(blocksdir_path, self.chain, "blocks", "blk00000.dat"))
diff --git a/test/functional/feature_cltv.py b/test/functional/feature_cltv.py
index 7c14f5d5a6..dc5bffe33e 100755
--- a/test/functional/feature_cltv.py
+++ b/test/functional/feature_cltv.py
@@ -135,7 +135,7 @@ class BIP65Test(BitcoinTestFramework):
         block.nVersion = 3
         block.solve()
 
-        with self.nodes[0].assert_debug_log(expected_msgs=['{}, bad-version(0x00000003)'.format(block.hash)]):
+        with self.nodes[0].assert_debug_log(expected_msgs=[f'{block.hash}, bad-version(0x00000003)']):
             peer.send_and_ping(msg_block(block))
             assert_equal(int(self.nodes[0].getbestblockhash(), 16), tip)
             peer.sync_with_ping()
@@ -173,8 +173,7 @@ class BIP65Test(BitcoinTestFramework):
             block.hashMerkleRoot = block.calc_merkle_root()
             block.solve()
 
-            with self.nodes[0].assert_debug_log(expected_msgs=['CheckInputScripts on {} failed with {}'.format(
-                                                block.vtx[-1].hash, expected_cltv_reject_reason)]):
+            with self.nodes[0].assert_debug_log(expected_msgs=[f'CheckInputScripts on {block.vtx[-1].hash} failed with {expected_cltv_reject_reason}']):
                 peer.send_and_ping(msg_block(block))
                 assert_equal(int(self.nodes[0].getbestblockhash(), 16), tip)
                 peer.sync_with_ping()
diff --git a/test/functional/feature_config_args.py b/test/functional/feature_config_args.py
index 24c8a8987a..d5f0ae480b 100755
--- a/test/functional/feature_config_args.py
+++ b/test/functional/feature_config_args.py
@@ -24,7 +24,7 @@ class ConfArgsTest(BitcoinTestFramework):
 
         inc_conf_file_path = os.path.join(self.nodes[0].datadir, 'include.conf')
         with open(os.path.join(self.nodes[0].datadir, 'bitcoin.conf'), 'a', encoding='utf-8') as conf:
-            conf.write('includeconf={}\n'.format(inc_conf_file_path))
+            conf.write(f'includeconf={inc_conf_file_path}\n')
 
         self.nodes[0].assert_start_raises_init_error(
             expected_msg='Error: Error parsing command line arguments: Invalid parameter -dash_cli=1',
@@ -43,13 +43,13 @@ class ConfArgsTest(BitcoinTestFramework):
         if self.is_wallet_compiled():
             with open(inc_conf_file_path, 'w', encoding='utf8') as conf:
                 conf.write("wallet=foo\n")
-            self.nodes[0].assert_start_raises_init_error(expected_msg='Error: Config setting for -wallet only applied on %s network when in [%s] section.' % (self.chain, self.chain))
+            self.nodes[0].assert_start_raises_init_error(expected_msg=f'Error: Config setting for -wallet only applied on {self.chain} network when in [{self.chain}] section.')
 
         main_conf_file_path = os.path.join(self.options.tmpdir, 'node0', 'bitcoin_main.conf')
-        util.write_config(main_conf_file_path, n=0, chain='', extra_config='includeconf={}\n'.format(inc_conf_file_path))
+        util.write_config(main_conf_file_path, n=0, chain='', extra_config=f'includeconf={inc_conf_file_path}\n')
         with open(inc_conf_file_path, 'w', encoding='utf-8') as conf:
             conf.write('acceptnonstdtxn=1\n')
-        self.nodes[0].assert_start_raises_init_error(extra_args=["-conf={}".format(main_conf_file_path)], expected_msg='Error: acceptnonstdtxn is not currently supported for main chain')
+        self.nodes[0].assert_start_raises_init_error(extra_args=[f"-conf={main_conf_file_path}"], expected_msg='Error: acceptnonstdtxn is not currently supported for main chain')
 
         with open(inc_conf_file_path, 'w', encoding='utf-8') as conf:
             conf.write('nono\n')
@@ -69,14 +69,14 @@ class ConfArgsTest(BitcoinTestFramework):
 
         inc_conf_file2_path = os.path.join(self.nodes[0].datadir, 'include2.conf')
         with open(os.path.join(self.nodes[0].datadir, 'bitcoin.conf'), 'a', encoding='utf-8') as conf:
-            conf.write('includeconf={}\n'.format(inc_conf_file2_path))
+            conf.write(f'includeconf={inc_conf_file2_path}\n')
 
         with open(inc_conf_file_path, 'w', encoding='utf-8') as conf:
             conf.write('testnot.datadir=1\n')
         with open(inc_conf_file2_path, 'w', encoding='utf-8') as conf:
             conf.write('[testnet]\n')
         self.restart_node(0)
-        self.nodes[0].stop_node(expected_stderr='Warning: ' + inc_conf_file_path + ':1 Section [testnot] is not recognized.' + os.linesep + inc_conf_file2_path + ':1 Section [testnet] is not recognized.')
+        self.nodes[0].stop_node(expected_stderr=f'Warning: {inc_conf_file_path}:1 Section [testnot] is not recognized.{os.linesep}{inc_conf_file2_path}:1 Section [testnet] is not recognized.')
 
         with open(inc_conf_file_path, 'w', encoding='utf-8') as conf:
             conf.write('')  # clear
@@ -105,8 +105,8 @@ class ConfArgsTest(BitcoinTestFramework):
                     'Command-line arg: rpcpassword=****',
                     'Command-line arg: rpcuser=****',
                     'Command-line arg: torpassword=****',
-                    'Config file arg: %s="1"' % self.chain,
-                    'Config file arg: [%s] server="1"' % self.chain,
+                    f'Config file arg: {self.chain}="1"',
+                    f'Config file arg: [{self.chain}] server="1"',
                 ],
                 unexpected_msgs=[
                     'alice:f7efda5c189b999524f151318c0c86$d5b51b3beffbc0',
@@ -235,7 +235,7 @@ class ConfArgsTest(BitcoinTestFramework):
 
         # Check that using -datadir argument on non-existent directory fails
         self.nodes[0].datadir = new_data_dir
-        self.nodes[0].assert_start_raises_init_error(['-datadir=' + new_data_dir], 'Error: Specified data directory "' + new_data_dir + '" does not exist.')
+        self.nodes[0].assert_start_raises_init_error([f'-datadir={new_data_dir}'], f'Error: Specified data directory "{new_data_dir}" does not exist.')
 
         # Check that using non-existent datadir in conf file fails
         conf_file = os.path.join(default_data_dir, "bitcoin.conf")
@@ -243,21 +243,21 @@ class ConfArgsTest(BitcoinTestFramework):
         # datadir needs to be set before [chain] section
         conf_file_contents = open(conf_file, encoding='utf8').read()
         with open(conf_file, 'w', encoding='utf8') as f:
-            f.write("datadir=" + new_data_dir + "\n")
+            f.write(f"datadir={new_data_dir}\n")
             f.write(conf_file_contents)
 
-        self.nodes[0].assert_start_raises_init_error(['-conf=' + conf_file], 'Error: Error reading configuration file: specified data directory "' + new_data_dir + '" does not exist.')
+        self.nodes[0].assert_start_raises_init_error([f'-conf={conf_file}'], f'Error: Error reading configuration file: specified data directory "{new_data_dir}" does not exist.')
 
         # Create the directory and ensure the config file now works
         os.mkdir(new_data_dir)
-        self.start_node(0, ['-conf='+conf_file])
+        self.start_node(0, [f'-conf={conf_file}'])
         self.stop_node(0)
         assert os.path.exists(os.path.join(new_data_dir, self.chain, 'blocks'))
 
         # Ensure command line argument overrides datadir in conf
         os.mkdir(new_data_dir_2)
         self.nodes[0].datadir = new_data_dir_2
-        self.start_node(0, ['-datadir='+new_data_dir_2, '-conf='+conf_file])
+        self.start_node(0, [f'-datadir={new_data_dir_2}', f'-conf={conf_file}'])
         assert os.path.exists(os.path.join(new_data_dir_2, self.chain, 'blocks'))
 
 
diff --git a/test/functional/feature_csv_activation.py b/test/functional/feature_csv_activation.py
index 1ac1a0563f..bc0050d47a 100755
--- a/test/functional/feature_csv_activation.py
+++ b/test/functional/feature_csv_activation.py
@@ -247,7 +247,7 @@ class BIP68_112_113Test(BitcoinTestFramework):
         self.send_blocks(test_blocks)
 
         assert_equal(self.tipheight, CSV_ACTIVATION_HEIGHT - 2)
-        self.log.info("Height = {}, CSV not yet active (will activate for block {}, not {})".format(self.tipheight, CSV_ACTIVATION_HEIGHT, CSV_ACTIVATION_HEIGHT - 1))
+        self.log.info(f"Height = {self.tipheight}, CSV not yet active (will activate for block {CSV_ACTIVATION_HEIGHT}, not {CSV_ACTIVATION_HEIGHT - 1})")
         assert not softfork_active(self.nodes[0], 'csv')
 
         # Test both version 1 and version 2 transactions for all tests
diff --git a/test/functional/feature_dbcrash.py b/test/functional/feature_dbcrash.py
index 6d8e5430f8..5bbcd20016 100755
--- a/test/functional/feature_dbcrash.py
+++ b/test/functional/feature_dbcrash.py
@@ -102,7 +102,7 @@ class ChainstateWriteCrashTest(BitcoinTestFramework):
         # perhaps we generated a test case that blew up our cache?
         # TODO: If this happens a lot, we should try to restart without -dbcrashratio
         # and make sure that recovery happens.
-        raise AssertionError("Unable to successfully restart node %d in allotted time", node_index)
+        raise AssertionError(f"Unable to successfully restart node {node_index} in allotted time")
 
     def submit_block_catch_error(self, node_index, block):
         """Try submitting a block to the given node.
@@ -114,10 +114,10 @@ class ChainstateWriteCrashTest(BitcoinTestFramework):
             self.nodes[node_index].submitblock(block)
             return True
         except (http.client.CannotSendRequest, http.client.RemoteDisconnected) as e:
-            self.log.debug("node %d submitblock raised exception: %s", node_index, e)
+            self.log.debug(f"node {node_index} submitblock raised exception: {e}")
             return False
         except OSError as e:
-            self.log.debug("node %d submitblock raised OSError exception: errno=%s", node_index, e.errno)
+            self.log.debug(f"node {node_index} submitblock raised OSError exception: errno={e.errno}")
             if e.errno in [errno.EPIPE, errno.ECONNREFUSED, errno.ECONNRESET]:
                 # The node has likely crashed
                 return False
@@ -142,15 +142,15 @@ class ChainstateWriteCrashTest(BitcoinTestFramework):
         # Deliver each block to each other node
         for i in range(3):
             nodei_utxo_hash = None
-            self.log.debug("Syncing blocks to node %d", i)
+            self.log.debug(f"Syncing blocks to node {i}")
             for (block_hash, block) in blocks:
                 # Get the block from node3, and submit to node_i
-                self.log.debug("submitting block %s", block_hash)
+                self.log.debug(f"submitting block {block_hash}")
                 if not self.submit_block_catch_error(i, block):
                     # TODO: more carefully check that the crash is due to -dbcrashratio
                     # (change the exit code perhaps, and check that here?)
                     self.wait_for_node_exit(i, timeout=30)
-                    self.log.debug("Restarting node %d after block hash %s", i, block_hash)
+                    self.log.debug(f"Restarting node {i} after block hash {block_hash}")
                     nodei_utxo_hash = self.restart_node(i, block_hash)
                     assert nodei_utxo_hash is not None
                     self.restart_counts[i] += 1
@@ -167,7 +167,7 @@ class ChainstateWriteCrashTest(BitcoinTestFramework):
             # - we only update the utxo cache after a node restart, since flushing
             # the cache is a no-op at that point
             if nodei_utxo_hash is not None:
-                self.log.debug("Checking txoutsetinfo matches for node %d", i)
+                self.log.debug(f"Checking txoutsetinfo matches for node {i}")
                 assert_equal(nodei_utxo_hash, node3_utxo_hash)
 
     def verify_utxo_hash(self):
@@ -218,14 +218,14 @@ class ChainstateWriteCrashTest(BitcoinTestFramework):
         # Start by creating a lot of utxos on node3
         initial_height = self.nodes[3].getblockcount()
         utxo_list = create_confirmed_utxos(self.nodes[3].getnetworkinfo()['relayfee'], self.nodes[3], 5000)
-        self.log.info("Prepped %d utxo entries", len(utxo_list))
+        self.log.info(f"Prepped {len(utxo_list)} utxo entries")
 
         # Sync these blocks with the other nodes
         block_hashes_to_sync = []
         for height in range(initial_height + 1, self.nodes[3].getblockcount() + 1):
             block_hashes_to_sync.append(self.nodes[3].getblockhash(height))
 
-        self.log.debug("Syncing %d blocks with other nodes", len(block_hashes_to_sync))
+        self.log.debug(f"Syncing {len(block_hashes_to_sync)} blocks with other nodes")
         # Syncing the blocks could cause nodes to crash, so the test begins here.
         self.sync_node3blocks(block_hashes_to_sync)
 
@@ -235,18 +235,18 @@ class ChainstateWriteCrashTest(BitcoinTestFramework):
         # each time through the loop, generate a bunch of transactions,
         # and then either mine a single new block on the tip, or some-sized reorg.
         for i in range(40):
-            self.log.info("Iteration %d, generating 2500 transactions %s", i, self.restart_counts)
+            self.log.info(f"Iteration {i}, generating 2500 transactions {self.restart_counts}")
             # Generate a bunch of small-ish transactions
             self.generate_small_transactions(self.nodes[3], 2500, utxo_list)
             # Pick a random block between current tip, and starting tip
             current_height = self.nodes[3].getblockcount()
             random_height = random.randint(starting_tip_height, current_height)
-            self.log.debug("At height %d, considering height %d", current_height, random_height)
+            self.log.debug(f"At height {current_height}, considering height {random_height}")
             if random_height > starting_tip_height:
                 # Randomly reorg from this point with some probability (1/4 for
                 # tip, 1/5 for tip-1, ...)
                 if random.random() < 1.0 / (current_height + 4 - random_height):
-                    self.log.debug("Invalidating block at height %d", random_height)
+                    self.log.debug(f"Invalidating block at height {random_height}")
                     self.nodes[3].invalidateblock(self.nodes[3].getblockhash(random_height))
 
             # Now generate new blocks until we pass the old tip height
@@ -258,10 +258,10 @@ class ChainstateWriteCrashTest(BitcoinTestFramework):
                     # new address to avoid mining a block that has just been invalidated
                     address=self.nodes[3].getnewaddress(),
                 ))
-            self.log.debug("Syncing %d new blocks...", len(block_hashes))
+            self.log.debug(f"Syncing {len(block_hashes)} new blocks...")
             self.sync_node3blocks(block_hashes)
             utxo_list = self.nodes[3].listunspent()
-            self.log.debug("Node3 utxo count: %d", len(utxo_list))
+            self.log.debug(f"Node3 utxo count: {len(utxo_list)}")
 
         # Check that the utxo hashes agree with node3
         # Useful side effect: each utxo cache gets flushed here, so that we
@@ -269,7 +269,7 @@ class ChainstateWriteCrashTest(BitcoinTestFramework):
         self.verify_utxo_hash()
 
         # Check the test coverage
-        self.log.info("Restarted nodes: %s; crashes on restart: %d", self.restart_counts, self.crashed_on_restart)
+        self.log.info(f"Restarted nodes: {self.restart_counts}; crashes on restart: {self.crashed_on_restart}")
 
         # If no nodes were restarted, we didn't test anything.
         assert self.restart_counts != [0, 0, 0]
@@ -280,7 +280,7 @@ class ChainstateWriteCrashTest(BitcoinTestFramework):
         # Warn if any of the nodes escaped restart.
         for i in range(3):
             if self.restart_counts[i] == 0:
-                self.log.warning("Node %d never crashed during utxo flush!", i)
+                self.log.warning(f"Node {i} never crashed during utxo flush!")
 
 
 if __name__ == "__main__":
diff --git a/test/functional/feature_dersig.py b/test/functional/feature_dersig.py
index 5dd6cb6cb2..fd46385cd4 100755
--- a/test/functional/feature_dersig.py
+++ b/test/functional/feature_dersig.py
@@ -104,7 +104,7 @@ class BIP66Test(BitcoinTestFramework):
         block.rehash()
         block.solve()
 
-        with self.nodes[0].assert_debug_log(expected_msgs=['{}, bad-version(0x00000002)'.format(block.hash)]):
+        with self.nodes[0].assert_debug_log(expected_msgs=[f'{block.hash}, bad-version(0x00000002)']):
             peer.send_and_ping(msg_block(block))
             assert_equal(int(self.nodes[0].getbestblockhash(), 16), tip)
             peer.sync_with_ping()
@@ -134,7 +134,7 @@ class BIP66Test(BitcoinTestFramework):
         block.rehash()
         block.solve()
 
-        with self.nodes[0].assert_debug_log(expected_msgs=['CheckInputScripts on {} failed with non-mandatory-script-verify-flag (Non-canonical DER signature)'.format(block.vtx[-1].hash)]):
+        with self.nodes[0].assert_debug_log(expected_msgs=[f'CheckInputScripts on {block.vtx[-1].hash} failed with non-mandatory-script-verify-flag (Non-canonical DER signature)']):
             peer.send_and_ping(msg_block(block))
             assert_equal(int(self.nodes[0].getbestblockhash(), 16), tip)
             peer.sync_with_ping()
diff --git a/test/functional/feature_fee_estimation.py b/test/functional/feature_fee_estimation.py
index 5322b02414..a8f3e12e07 100755
--- a/test/functional/feature_fee_estimation.py
+++ b/test/functional/feature_fee_estimation.py
@@ -72,7 +72,7 @@ def small_txpuzzle_randfee(from_node, conflist, unconflist, amount, min_fee, fee
             total_in += t["amount"]
             tx.vin.append(CTxIn(COutPoint(int(t["txid"], 16), t["vout"]), b""))
         if total_in <= amount + fee:
-            raise RuntimeError("Insufficient funds: need %d, have %d" % (amount + fee, total_in))
+            raise RuntimeError(f"Insufficient funds: need {amount + fee}, have {total_in}")
     tx.vout.append(CTxOut(int((total_in - amount - fee) * COIN), P2SH_1))
     tx.vout.append(CTxOut(int(amount * COIN), P2SH_2))
     # These transactions don't need to be signed, but we still have to insert
@@ -124,8 +124,7 @@ def check_raw_estimates(node, fees_seen):
             assert_greater_than(feerate, 0)
 
             if feerate + delta < min(fees_seen) or feerate - delta > max(fees_seen):
-                raise AssertionError("Estimated fee (%f) out of range (%f,%f)"
-                                     % (feerate, min(fees_seen), max(fees_seen)))
+                raise AssertionError(f"Estimated fee ({feerate}) out of range ({min(fees_seen)},{max(fees_seen)})")
 
 def check_smart_estimates(node, fees_seen):
     """Call estimatesmartfee and verify that the estimates meet certain invariants."""
@@ -138,11 +137,9 @@ def check_smart_estimates(node, fees_seen):
         assert_greater_than(feerate, 0)
 
         if feerate + delta < min(fees_seen) or feerate - delta > max(fees_seen):
-            raise AssertionError("Estimated fee (%f) out of range (%f,%f)"
-                                 % (feerate, min(fees_seen), max(fees_seen)))
+            raise AssertionError(f"Estimated fee ({feerate}) out of range ({min(fees_seen)},{max(fees_seen)})")
         if feerate - delta > last_feerate:
-            raise AssertionError("Estimated fee (%f) larger than last fee (%f) for lower number of confirms"
-                                 % (feerate, last_feerate))
+            raise AssertionError(f"Estimated fee ({feerate}) larger than last fee ({last_feerate}) for lower number of confirms")
         last_feerate = feerate
 
         if i == 0:
diff --git a/test/functional/feature_filelock.py b/test/functional/feature_filelock.py
index 2798d11b0a..e09107802b 100755
--- a/test/functional/feature_filelock.py
+++ b/test/functional/feature_filelock.py
@@ -22,11 +22,11 @@ class FilelockTest(BitcoinTestFramework):
 
     def run_test(self):
         datadir = os.path.join(self.nodes[0].datadir, self.chain)
-        self.log.info("Using datadir {}".format(datadir))
+        self.log.info(f"Using datadir {datadir}")
 
         self.log.info("Check that we can't start a second bitcoind instance using the same datadir")
-        expected_msg = "Error: Cannot obtain a lock on data directory {0}. {1} is probably already running.".format(datadir, self.config['environment']['PACKAGE_NAME'])
-        self.nodes[1].assert_start_raises_init_error(extra_args=['-datadir={}'.format(self.nodes[0].datadir), '-noserver'], expected_msg=expected_msg)
+        expected_msg = f"Error: Cannot obtain a lock on data directory {datadir}. {self.config['environment']['PACKAGE_NAME']} is probably already running."
+        self.nodes[1].assert_start_raises_init_error(extra_args=[f'-datadir={self.nodes[0].datadir}', '-noserver'], expected_msg=expected_msg)
 
         if self.is_wallet_compiled():
             def check_wallet_filelock(descriptors):
@@ -38,7 +38,7 @@ class FilelockTest(BitcoinTestFramework):
                     expected_msg = "Error: SQLiteDatabase: Unable to obtain an exclusive lock on the database, is it being used by another bitcoind?"
                 else:
                     expected_msg = "Error: Error initializing wallet database environment"
-                self.nodes[1].assert_start_raises_init_error(extra_args=['-walletdir={}'.format(wallet_dir), '-wallet=' + wallet_name, '-noserver'], expected_msg=expected_msg, match=ErrorMatch.PARTIAL_REGEX)
+                self.nodes[1].assert_start_raises_init_error(extra_args=[f'-walletdir={wallet_dir}', f'-wallet={wallet_name}', '-noserver'], expected_msg=expected_msg, match=ErrorMatch.PARTIAL_REGEX)
 
             if self.is_bdb_compiled():
                 check_wallet_filelock(False)
diff --git a/test/functional/feature_help.py b/test/functional/feature_help.py
index babe9bfc80..837e95c128 100755
--- a/test/functional/feature_help.py
+++ b/test/functional/feature_help.py
@@ -40,14 +40,14 @@ class HelpTest(BitcoinTestFramework):
         # Node should exit immediately and output help to stdout.
         output, _ = self.get_node_output(ret_code_expected=0)
         assert b'Options' in output
-        self.log.info("Help text received: {} (...)".format(output[0:60]))
+        self.log.info(f"Help text received: {output[0:60]} (...)")
 
         self.log.info("Start bitcoin with -version for version information")
         self.nodes[0].start(extra_args=['-version'])
         # Node should exit immediately and output version to stdout.
         output, _ = self.get_node_output(ret_code_expected=0)
         assert b'version' in output
-        self.log.info("Version text received: {} (...)".format(output[0:60]))
+        self.log.info(f"Version text received: {output[0:60]} (...)")
 
         # Test that arguments not in the help results in an error
         self.log.info("Start bitcoind with -fakearg to make sure it does not start")
@@ -55,7 +55,7 @@ class HelpTest(BitcoinTestFramework):
         # Node should exit immediately and output an error to stderr
         _, output = self.get_node_output(ret_code_expected=1)
         assert b'Error parsing command line arguments' in output
-        self.log.info("Error message received: {} (...)".format(output[0:60]))
+        self.log.info(f"Error message received: {output[0:60]} (...)")
 
 
 if __name__ == '__main__':
diff --git a/test/functional/feature_loadblock.py b/test/functional/feature_loadblock.py
index 14f64d63a2..e5db6de085 100755
--- a/test/functional/feature_loadblock.py
+++ b/test/functional/feature_loadblock.py
@@ -45,17 +45,17 @@ class LoadblockTest(BitcoinTestFramework):
 
         self.log.info("Create linearization config file")
         with open(cfg_file, "a", encoding="utf-8") as cfg:
-            cfg.write("datadir={}\n".format(data_dir))
-            cfg.write("rpcuser={}\n".format(node_url.username))
-            cfg.write("rpcpassword={}\n".format(node_url.password))
-            cfg.write("port={}\n".format(node_url.port))
-            cfg.write("host={}\n".format(node_url.hostname))
-            cfg.write("output_file={}\n".format(bootstrap_file))
-            cfg.write("max_height=100\n")
-            cfg.write("netmagic=fabfb5da\n")
-            cfg.write("input={}\n".format(blocks_dir))
-            cfg.write("genesis={}\n".format(genesis_block))
-            cfg.write("hashlist={}\n".format(hash_list.name))
+            cfg.write(f"datadir={data_dir}\n")
+            cfg.write(f"rpcuser={node_url.username}\n")
+            cfg.write(f"rpcpassword={node_url.password}\n")
+            cfg.write(f"port={node_url.port}\n")
+            cfg.write(f"host={node_url.hostname}\n")
+            cfg.write(f"output_file={bootstrap_file}\n")
+            cfg.write(f"max_height=100\n")
+            cfg.write(f"netmagic=fabfb5da\n")
+            cfg.write(f"input={blocks_dir}\n")
+            cfg.write(f"genesis={genesis_block}\n")
+            cfg.write(f"hashlist={hash_list.name}\n")
 
         base_dir = self.config["environment"]["SRCDIR"]
         linearize_dir = os.path.join(base_dir, "contrib", "linearize")
@@ -72,7 +72,7 @@ class LoadblockTest(BitcoinTestFramework):
                        check=True)
 
         self.log.info("Restart second, unsynced node with bootstrap file")
-        self.restart_node(1, extra_args=["-loadblock=" + bootstrap_file])
+        self.restart_node(1, extra_args=[f"-loadblock={bootstrap_file}"])
         assert_equal(self.nodes[1].getblockcount(), 100)  # start_node is blocking on all block files being imported
 
         assert_equal(self.nodes[1].getblockchaininfo()['blocks'], 100)
diff --git a/test/functional/feature_logging.py b/test/functional/feature_logging.py
index afcbcf099a..722219518a 100755
--- a/test/functional/feature_logging.py
+++ b/test/functional/feature_logging.py
@@ -29,7 +29,7 @@ class LoggingTest(BitcoinTestFramework):
 
         # test alternative log file name outside datadir
         tempname = os.path.join(self.options.tmpdir, "foo.log")
-        self.restart_node(0, ["-debuglogfile=%s" % tempname])
+        self.restart_node(0, [f"-debuglogfile={tempname}"])
         assert os.path.isfile(tempname)
 
         # check that invalid log (relative) will cause error
@@ -37,26 +37,26 @@ class LoggingTest(BitcoinTestFramework):
         invalidname = os.path.join("foo", "foo.log")
         self.stop_node(0)
         exp_stderr = r"Error: Could not open debug log file \S+$"
-        self.nodes[0].assert_start_raises_init_error(["-debuglogfile=%s" % (invalidname)], exp_stderr, match=ErrorMatch.FULL_REGEX)
+        self.nodes[0].assert_start_raises_init_error([f"-debuglogfile={invalidname}"], exp_stderr, match=ErrorMatch.FULL_REGEX)
         assert not os.path.isfile(os.path.join(invdir, "foo.log"))
 
         # check that invalid log (relative) works after path exists
         self.stop_node(0)
         os.mkdir(invdir)
-        self.start_node(0, ["-debuglogfile=%s" % (invalidname)])
+        self.start_node(0, [f"-debuglogfile={invalidname}"])
         assert os.path.isfile(os.path.join(invdir, "foo.log"))
 
         # check that invalid log (absolute) will cause error
         self.stop_node(0)
         invdir = os.path.join(self.options.tmpdir, "foo")
         invalidname = os.path.join(invdir, "foo.log")
-        self.nodes[0].assert_start_raises_init_error(["-debuglogfile=%s" % invalidname], exp_stderr, match=ErrorMatch.FULL_REGEX)
+        self.nodes[0].assert_start_raises_init_error([f"-debuglogfile={invalidname}"], exp_stderr, match=ErrorMatch.FULL_REGEX)
         assert not os.path.isfile(os.path.join(invdir, "foo.log"))
 
         # check that invalid log (absolute) works after path exists
         self.stop_node(0)
         os.mkdir(invdir)
-        self.start_node(0, ["-debuglogfile=%s" % (invalidname)])
+        self.start_node(0, [f"-debuglogfile={invalidname}"])
         assert os.path.isfile(os.path.join(invdir, "foo.log"))
 
         # check that -nodebuglogfile disables logging
@@ -67,7 +67,7 @@ class LoggingTest(BitcoinTestFramework):
         assert not os.path.isfile(default_log_path)
 
         # just sanity check no crash here
-        self.restart_node(0, ["-debuglogfile=%s" % os.devnull])
+        self.restart_node(0, [f"-debuglogfile={os.devnull}"])
 
 
 if __name__ == '__main__':
diff --git a/test/functional/feature_minchainwork.py b/test/functional/feature_minchainwork.py
index 803feb509a..3950690cf0 100755
--- a/test/functional/feature_minchainwork.py
+++ b/test/functional/feature_minchainwork.py
@@ -45,16 +45,16 @@ class MinimumChainWorkTest(BitcoinTestFramework):
         # Start building a chain on node0.  node2 shouldn't be able to sync until node1's
         # minchainwork is exceeded
         starting_chain_work = REGTEST_WORK_PER_BLOCK # Genesis block's work
-        self.log.info("Testing relay across node %d (minChainWork = %d)", 1, self.node_min_work[1])
+        self.log.info(f"Testing relay across node 1 (minChainWork = {self.node_min_work[1]})")
 
         starting_blockcount = self.nodes[2].getblockcount()
 
         num_blocks_to_generate = int((self.node_min_work[1] - starting_chain_work) / REGTEST_WORK_PER_BLOCK)
-        self.log.info("Generating %d blocks on node0", num_blocks_to_generate)
+        self.log.info(f"Generating {num_blocks_to_generate} blocks on node0")
         hashes = self.nodes[0].generatetoaddress(num_blocks_to_generate,
                                                  self.nodes[0].get_deterministic_priv_key().address)
 
-        self.log.info("Node0 current chain work: %s", self.nodes[0].getblockheader(hashes[-1])['chainwork'])
+        self.log.info(f"Node0 current chain work: {self.nodes[0].getblockheader(hashes[-1])['chainwork']}")
 
         # Sleep a few seconds and verify that node2 didn't get any new blocks
         # or headers.  We sleep, rather than sync_blocks(node0, node1) because
@@ -63,7 +63,7 @@ class MinimumChainWorkTest(BitcoinTestFramework):
         time.sleep(3)
 
         self.log.info("Verifying node 2 has no more blocks than before")
-        self.log.info("Blockcounts: %s", [n.getblockcount() for n in self.nodes])
+        self.log.info(f"Blockcounts: {[n.getblockcount() for n in self.nodes]}")
         # Node2 shouldn't have any new headers yet, because node1 should not
         # have relayed anything.
         assert_equal(len(self.nodes[2].getchaintips()), 1)
@@ -84,7 +84,7 @@ class MinimumChainWorkTest(BitcoinTestFramework):
         # continue the test.
 
         self.sync_all()
-        self.log.info("Blockcounts: %s", [n.getblockcount() for n in self.nodes])
+        self.log.info(f"Blockcounts: {[n.getblockcount() for n in self.nodes]}")
 
 if __name__ == '__main__':
     MinimumChainWorkTest().main()
diff --git a/test/functional/feature_notifications.py b/test/functional/feature_notifications.py
index 6fc8773ee3..3f8fe9ccd5 100755
--- a/test/functional/feature_notifications.py
+++ b/test/functional/feature_notifications.py
@@ -20,7 +20,7 @@ FILE_CHARS_DISALLOWED = '/\\?%*:|"<>' if os.name == 'nt' else '/'
 UNCONFIRMED_HASH_STRING = 'unconfirmed'
 
 def notify_outputname(walletname, txid):
-    return txid if os.name == 'nt' else '{}_{}'.format(walletname, txid)
+    return txid if os.name == 'nt' else f'{walletname}_{txid}'
 
 
 class NotificationsTest(BitcoinTestFramework):
@@ -39,11 +39,11 @@ class NotificationsTest(BitcoinTestFramework):
 
         # -alertnotify and -blocknotify on node0, walletnotify on node1
         self.extra_args = [[
-            "-alertnotify=echo > {}".format(os.path.join(self.alertnotify_dir, '%s')),
-            "-blocknotify=echo > {}".format(os.path.join(self.blocknotify_dir, '%s')),
+            f"-alertnotify=echo > {os.path.join(self.alertnotify_dir, '%s')}",
+            f"-blocknotify=echo > {os.path.join(self.blocknotify_dir, '%s')}",
         ], [
             "-rescan",
-            "-walletnotify=echo %h_%b > {}".format(os.path.join(self.walletnotify_dir, notify_outputname('%w', '%s'))),
+            f"-walletnotify=echo %h_%b > {os.path.join(self.walletnotify_dir, notify_outputname('%w', '%s'))}",
         ]]
         self.wallet_names = [self.default_wallet_name, self.wallet]
         super().setup_network()
@@ -54,12 +54,12 @@ class NotificationsTest(BitcoinTestFramework):
             seed = "cTdGmKFWpbvpKQ7ejrdzqYT2hhjyb3GPHnLAK7wdi5Em67YLwSm9"
             xpriv = "tprv8ZgxMBicQKsPfHCsTwkiM1KT56RXbGGTqvc2hgqzycpwbHqqpcajQeMRZoBD35kW4RtyCemu6j34Ku5DEspmgjKdt2qe4SvRch5Kk8B8A2v"
             desc_imports = [{
-                "desc": descsum_create("wpkh(" + xpriv + "/0/*)"),
+                "desc": descsum_create(f"wpkh({xpriv}/0/*)"),
                 "timestamp": 0,
                 "active": True,
                 "keypool": True,
             },{
-                "desc": descsum_create("wpkh(" + xpriv + "/1/*)"),
+                "desc": descsum_create(f"wpkh({xpriv}/1/*)"),
                 "timestamp": 0,
                 "active": True,
                 "keypool": True,
diff --git a/test/functional/feature_proxy.py b/test/functional/feature_proxy.py
index 162814815e..2fb5e328f5 100755
--- a/test/functional/feature_proxy.py
+++ b/test/functional/feature_proxy.py
@@ -97,14 +97,14 @@ class ProxyTest(BitcoinTestFramework):
         # Note: proxies are not used to connect to local nodes. This is because the proxy to
         # use is based on CService.GetNetwork(), which returns NET_UNROUTABLE for localhost.
         args = [
-            ['-listen', '-proxy=%s:%i' % (self.conf1.addr),'-proxyrandomize=1'],
-            ['-listen', '-proxy=%s:%i' % (self.conf1.addr),'-onion=%s:%i' % (self.conf2.addr),
-                '-i2psam=%s:%i' % (self.i2p_sam), '-i2pacceptincoming=0', '-proxyrandomize=0'],
-            ['-listen', '-proxy=%s:%i' % (self.conf2.addr),'-proxyrandomize=1'],
+            ['-listen', f'-proxy={self.conf1.addr[0]}:{self.conf1.addr[1]}','-proxyrandomize=1'],
+            ['-listen', f'-proxy={self.conf1.addr[0]}:{self.conf1.addr[1]}',f'-onion={self.conf2.addr[0]}:{self.conf2.addr[1]}',
+                f'-i2psam={self.i2p_sam[0]}:{self.i2p_sam[1]}', '-i2pacceptincoming=0', '-proxyrandomize=0'],
+            ['-listen', f'-proxy={self.conf2.addr[0]}:{self.conf2.addr[1]}','-proxyrandomize=1'],
             []
         ]
         if self.have_ipv6:
-            args[3] = ['-listen', '-proxy=[%s]:%i' % (self.conf3.addr),'-proxyrandomize=0', '-noonion']
+            args[3] = ['-listen', f'-proxy=[{self.conf3.addr[0]}]:{self.conf3.addr[1]}','-proxyrandomize=0', '-noonion']
         self.add_nodes(self.num_nodes, extra_args=args)
         self.start_nodes()
 
@@ -116,7 +116,7 @@ class ProxyTest(BitcoinTestFramework):
     def node_test(self, node, proxies, auth, test_onion=True):
         rv = []
         addr = "15.61.23.23:1234"
-        self.log.debug("Test: outgoing IPv4 connection through node for address {}".format(addr))
+        self.log.debug(f"Test: outgoing IPv4 connection through node for address {addr}")
         node.addnode(addr, "onetry")
         cmd = proxies[0].queue.get()
         assert isinstance(cmd, Socks5Command)
@@ -132,7 +132,7 @@ class ProxyTest(BitcoinTestFramework):
 
         if self.have_ipv6:
             addr = "[1233:3432:2434:2343:3234:2345:6546:4534]:5443"
-            self.log.debug("Test: outgoing IPv6 connection through node for address {}".format(addr))
+            self.log.debug(f"Test: outgoing IPv6 connection through node for address {addr}")
             node.addnode(addr, "onetry")
             cmd = proxies[1].queue.get()
             assert isinstance(cmd, Socks5Command)
@@ -148,7 +148,7 @@ class ProxyTest(BitcoinTestFramework):
 
         if test_onion:
             addr = "pg6mmjiyjmcrsslvykfwnntlaru7p5svn6y2ymmju6nubxndf4pscryd.onion:8333"
-            self.log.debug("Test: outgoing onion connection through node for address {}".format(addr))
+            self.log.debug(f"Test: outgoing onion connection through node for address {addr}")
             node.addnode(addr, "onetry")
             cmd = proxies[2].queue.get()
             assert isinstance(cmd, Socks5Command)
@@ -162,7 +162,7 @@ class ProxyTest(BitcoinTestFramework):
             self.network_test(node, addr, network=NET_ONION)
 
         addr = "node.noumenon:8333"
-        self.log.debug("Test: outgoing DNS name connection through node for address {}".format(addr))
+        self.log.debug(f"Test: outgoing DNS name connection through node for address {addr}")
         node.addnode(addr, "onetry")
         cmd = proxies[3].queue.get()
         assert isinstance(cmd, Socks5Command)
@@ -218,12 +218,12 @@ class ProxyTest(BitcoinTestFramework):
         n1 = networks_dict(self.nodes[1].getnetworkinfo())
         assert_equal(NETWORKS, n1.keys())
         for net in ['ipv4', 'ipv6']:
-            assert_equal(n1[net]['proxy'], '%s:%i' % (self.conf1.addr))
+            assert_equal(n1[net]['proxy'], f'{self.conf1.addr[0]}:{self.conf1.addr[1]}')
             assert_equal(n1[net]['proxy_randomize_credentials'], False)
-        assert_equal(n1['onion']['proxy'], '%s:%i' % (self.conf2.addr))
+        assert_equal(n1['onion']['proxy'], f'{self.conf2.addr[0]}:{self.conf2.addr[1]}')
         assert_equal(n1['onion']['proxy_randomize_credentials'], False)
         assert_equal(n1['onion']['reachable'], True)
-        assert_equal(n1['i2p']['proxy'], '%s:%i' % (self.i2p_sam))
+        assert_equal(n1['i2p']['proxy'], f'{self.i2p_sam[0]}:{self.i2p_sam[1]}')
         assert_equal(n1['i2p']['proxy_randomize_credentials'], False)
         assert_equal(n1['i2p']['reachable'], True)
 
@@ -234,7 +234,7 @@ class ProxyTest(BitcoinTestFramework):
                 expected_proxy = ''
                 expected_randomize = False
             else:
-                expected_proxy = '%s:%i' % (self.conf2.addr)
+                expected_proxy = f'{self.conf2.addr[0]}:{self.conf2.addr[1]}'
                 expected_randomize = True
             assert_equal(n2[net]['proxy'], expected_proxy)
             assert_equal(n2[net]['proxy_randomize_credentials'], expected_randomize)
@@ -248,7 +248,7 @@ class ProxyTest(BitcoinTestFramework):
                 if net == NET_I2P:
                     expected_proxy = ''
                 else:
-                    expected_proxy = '[%s]:%i' % (self.conf3.addr)
+                    expected_proxy = f'[{self.conf3.addr[0]}]:{self.conf3.addr[1]}'
                 assert_equal(n3[net]['proxy'], expected_proxy)
                 assert_equal(n3[net]['proxy_randomize_credentials'], False)
             assert_equal(n3['onion']['reachable'], False)
diff --git a/test/functional/feature_pruning.py b/test/functional/feature_pruning.py
index e9aae4733b..2f0868e733 100755
--- a/test/functional/feature_pruning.py
+++ b/test/functional/feature_pruning.py
@@ -148,7 +148,7 @@ class PruneTest(BitcoinTestFramework):
     def test_height_min(self):
         assert os.path.isfile(os.path.join(self.prunedir, "blk00000.dat")), "blk00000.dat is missing, pruning too early"
         self.log.info("Success")
-        self.log.info("Though we're already using more than 550MiB, current usage: %d" % calc_usage(self.prunedir))
+        self.log.info(f"Though we're already using more than 550MiB, current usage: {calc_usage(self.prunedir)}")
         self.log.info("Mining 25 more blocks should cause the first block file to be pruned")
         # Pruning doesn't run until we're allocating another chunk, 20 full blocks past the height cutoff will ensure this
         mine_large_blocks(self.nodes[0], 25)
@@ -158,7 +158,7 @@ class PruneTest(BitcoinTestFramework):
 
         self.log.info("Success")
         usage = calc_usage(self.prunedir)
-        self.log.info("Usage should be below target: %d" % usage)
+        self.log.info(f"Usage should be below target: {usage}")
         assert_greater_than(550, usage)
 
     def create_chain_with_staleblocks(self):
@@ -181,18 +181,18 @@ class PruneTest(BitcoinTestFramework):
             self.connect_nodes(0, 2)
             self.sync_blocks(self.nodes[0:3])
 
-        self.log.info("Usage can be over target because of high stale rate: %d" % calc_usage(self.prunedir))
+        self.log.info(f"Usage can be over target because of high stale rate: {calc_usage(self.prunedir)}")
 
     def reorg_test(self):
         # Node 1 will mine a 300 block chain starting 287 blocks back from Node 0 and Node 2's tip
         # This will cause Node 2 to do a reorg requiring 288 blocks of undo data to the reorg_test chain
 
         height = self.nodes[1].getblockcount()
-        self.log.info("Current block height: %d" % height)
+        self.log.info(f"Current block height: {height}")
 
         self.forkheight = height - 287
         self.forkhash = self.nodes[1].getblockhash(self.forkheight)
-        self.log.info("Invalidating block %s at height %d" % (self.forkhash, self.forkheight))
+        self.log.info(f"Invalidating block {self.forkhash} at height {self.forkheight}")
         self.nodes[1].invalidateblock(self.forkhash)
 
         # We've now switched to our previously mined-24 block fork on node 1, but that's not what we want
@@ -204,7 +204,7 @@ class PruneTest(BitcoinTestFramework):
             curhash = self.nodes[1].getblockhash(self.forkheight - 1)
 
         assert self.nodes[1].getblockcount() == self.forkheight - 1
-        self.log.info("New best height: %d" % self.nodes[1].getblockcount())
+        self.log.info(f"New best height: {self.nodes[1].getblockcount()}")
 
         # Disconnect node1 and generate the new chain
         self.disconnect_nodes(0, 1)
@@ -218,8 +218,8 @@ class PruneTest(BitcoinTestFramework):
         self.connect_nodes(1, 2)
         self.sync_blocks(self.nodes[0:3], timeout=120)
 
-        self.log.info("Verify height on node 2: %d" % self.nodes[2].getblockcount())
-        self.log.info("Usage possibly still high because of stale blocks in block files: %d" % calc_usage(self.prunedir))
+        self.log.info(f"Verify height on node 2: {self.nodes[2].getblockcount()}")
+        self.log.info(f"Usage possibly still high because of stale blocks in block files: {calc_usage(self.prunedir)}")
 
         self.log.info("Mine 220 more large blocks so we have requisite history")
 
@@ -227,7 +227,7 @@ class PruneTest(BitcoinTestFramework):
         self.sync_blocks(self.nodes[0:3], timeout=120)
 
         usage = calc_usage(self.prunedir)
-        self.log.info("Usage should be below target: %d" % usage)
+        self.log.info(f"Usage should be below target: {usage}")
         assert_greater_than(550, usage)
 
     def reorg_back(self):
@@ -235,7 +235,7 @@ class PruneTest(BitcoinTestFramework):
         assert_raises_rpc_error(-1, "Block not available (pruned data)", self.nodes[2].getblock, self.forkhash)
         with self.nodes[2].assert_debug_log(expected_msgs=['block verification stopping at height', '(pruning, no data)']):
             self.nodes[2].verifychain(checklevel=4, nblocks=0)
-        self.log.info("Will need to redownload block %d" % self.forkheight)
+        self.log.info(f"Will need to redownload block {self.forkheight}")
 
         # Verify that we have enough history to reorg back to the fork point
         # Although this is more than 288 blocks, because this chain was written more recently
@@ -259,7 +259,7 @@ class PruneTest(BitcoinTestFramework):
         # At this point node 2 is within 288 blocks of the fork point so it will preserve its ability to reorg
         if self.nodes[2].getblockcount() < self.mainchainheight:
             blocks_to_mine = first_reorg_height + 1 - self.mainchainheight
-            self.log.info("Rewind node 0 to prev main chain to mine longer chain to trigger redownload. Blocks needed: %d" % blocks_to_mine)
+            self.log.info(f"Rewind node 0 to prev main chain to mine longer chain to trigger redownload. Blocks needed: {blocks_to_mine}")
             self.nodes[0].invalidateblock(curchainhash)
             assert_equal(self.nodes[0].getblockcount(), self.mainchainheight)
             assert_equal(self.nodes[0].getbestblockhash(), self.mainchainhash2)
@@ -296,7 +296,7 @@ class PruneTest(BitcoinTestFramework):
             assert_equal(ret, node.getblockchaininfo()['pruneheight'])
 
         def has_block(index):
-            return os.path.isfile(os.path.join(self.nodes[node_number].datadir, self.chain, "blocks", "blk{:05}.dat".format(index)))
+            return os.path.isfile(os.path.join(self.nodes[node_number].datadir, self.chain, "blocks", f"blk{index:05}.dat"))
 
         # should not prune because chain tip of node 3 (995) < PruneAfterHeight (1000)
         assert_raises_rpc_error(-1, "Blockchain is too short for pruning", node.pruneblockchain, height(500))
diff --git a/test/functional/feature_segwit.py b/test/functional/feature_segwit.py
index cbd8521499..6f1cecce58 100755
--- a/test/functional/feature_segwit.py
+++ b/test/functional/feature_segwit.py
@@ -65,7 +65,7 @@ def find_spendable_utxo(node, min_value):
         if utxo['spendable']:
             return utxo
 
-    raise AssertionError("Unspent output equal or higher than %s not found" % min_value)
+    raise AssertionError(f"Unspent output equal or higher than {min_value} not found")
 
 txs_mined = {} # txindex from txid to blockhash
 
diff --git a/test/functional/feature_settings.py b/test/functional/feature_settings.py
index 5a0236401d..26048d37f6 100755
--- a/test/functional/feature_settings.py
+++ b/test/functional/feature_settings.py
@@ -83,7 +83,7 @@ class SettingsTest(BitcoinTestFramework):
         with altsettings.open("w") as fp:
             fp.write('{"key": "value"}')
         with node.assert_debug_log(expected_msgs=['Setting file arg: key = "value"']):
-            self.start_node(0, extra_args=["-settings={}".format(altsettings)])
+            self.start_node(0, extra_args=[f"-settings={altsettings}"])
             self.stop_node(0)
 
 
diff --git a/test/functional/feature_versionbits_warning.py b/test/functional/feature_versionbits_warning.py
index 1c9e237d78..a7ec5c48e3 100755
--- a/test/functional/feature_versionbits_warning.py
+++ b/test/functional/feature_versionbits_warning.py
@@ -21,7 +21,7 @@ VB_TOP_BITS = 0x20000000
 VB_UNKNOWN_BIT = 27       # Choose a bit unassigned to any deployment
 VB_UNKNOWN_VERSION = VB_TOP_BITS | (1 << VB_UNKNOWN_BIT)
 
-WARN_UNKNOWN_RULES_ACTIVE = "Unknown new rules activated (versionbit {})".format(VB_UNKNOWN_BIT)
+WARN_UNKNOWN_RULES_ACTIVE = f"Unknown new rules activated (versionbit {VB_UNKNOWN_BIT})"
 VB_PATTERN = re.compile("Unknown new rules activated.*versionbit")
 
 class VersionBitsWarningTest(BitcoinTestFramework):
@@ -34,7 +34,7 @@ class VersionBitsWarningTest(BitcoinTestFramework):
         # Open and close to create zero-length file
         with open(self.alert_filename, 'w', encoding='utf8'):
             pass
-        self.extra_args = [["-alertnotify=echo %s >> \"" + self.alert_filename + "\""]]
+        self.extra_args = [[f"-alertnotify=echo %s >> \"{self.alert_filename}\""]]
         self.setup_nodes()
 
     def send_blocks_with_version(self, peer, numblocks, version):
diff --git a/test/functional/interface_bitcoin_cli.py b/test/functional/interface_bitcoin_cli.py
index dfa448a1a8..4b5363ec49 100755
--- a/test/functional/interface_bitcoin_cli.py
+++ b/test/functional/interface_bitcoin_cli.py
@@ -87,12 +87,12 @@ class TestBitcoinCli(BitcoinTestFramework):
         user, password = get_auth_cookie(self.nodes[0].datadir, self.chain)
 
         self.log.info("Test -stdinrpcpass option")
-        assert_equal(BLOCKS, self.nodes[0].cli('-rpcuser={}'.format(user), '-stdinrpcpass', input=password).getblockcount())
-        assert_raises_process_error(1, 'Incorrect rpcuser or rpcpassword', self.nodes[0].cli('-rpcuser={}'.format(user), '-stdinrpcpass', input='foo').echo)
+        assert_equal(BLOCKS, self.nodes[0].cli(f'-rpcuser={user}', '-stdinrpcpass', input=password).getblockcount())
+        assert_raises_process_error(1, 'Incorrect rpcuser or rpcpassword', self.nodes[0].cli(f'-rpcuser={user}', '-stdinrpcpass', input='foo').echo)
 
         self.log.info("Test -stdin and -stdinrpcpass")
-        assert_equal(['foo', 'bar'], self.nodes[0].cli('-rpcuser={}'.format(user), '-stdin', '-stdinrpcpass', input=password + '\nfoo\nbar').echo())
-        assert_raises_process_error(1, 'Incorrect rpcuser or rpcpassword', self.nodes[0].cli('-rpcuser={}'.format(user), '-stdin', '-stdinrpcpass', input='foo').echo)
+        assert_equal(['foo', 'bar'], self.nodes[0].cli(f'-rpcuser={user}', '-stdin', '-stdinrpcpass', input=f'{password}\nfoo\nbar').echo())
+        assert_raises_process_error(1, 'Incorrect rpcuser or rpcpassword', self.nodes[0].cli(f'-rpcuser={user}', '-stdin', '-stdinrpcpass', input='foo').echo)
 
         self.log.info("Test connecting to a non-existing server")
         assert_raises_process_error(1, "Could not connect to the server", self.nodes[0].cli('-rpcport=1').echo)
@@ -150,8 +150,8 @@ class TestBitcoinCli(BitcoinTestFramework):
             w1 = self.nodes[0].get_wallet_rpc(wallets[0])
             w2 = self.nodes[0].get_wallet_rpc(wallets[1])
             w3 = self.nodes[0].get_wallet_rpc(wallets[2])
-            rpcwallet2 = '-rpcwallet={}'.format(wallets[1])
-            rpcwallet3 = '-rpcwallet={}'.format(wallets[2])
+            rpcwallet2 = f'-rpcwallet={wallets[1]}'
+            rpcwallet3 = f'-rpcwallet={wallets[2]}'
             w1.walletpassphrase(password, self.rpc_timeout)
             w2.encryptwallet(password)
             w1.sendtoaddress(w2.getnewaddress(), amounts[1])
@@ -162,7 +162,7 @@ class TestBitcoinCli(BitcoinTestFramework):
 
             self.log.info("Test -getinfo with multiple wallets and -rpcwallet returns specified wallet balance")
             for i in range(len(wallets)):
-                cli_get_info_string = self.nodes[0].cli('-getinfo', '-rpcwallet={}'.format(wallets[i])).send_cli()
+                cli_get_info_string = self.nodes[0].cli('-getinfo', f'-rpcwallet={wallets[i]}').send_cli()
                 cli_get_info = cli_get_info_string_to_dict(cli_get_info_string)
                 assert 'Balances' not in cli_get_info_string
                 assert_equal(cli_get_info["Wallet"], wallets[i])
@@ -296,7 +296,7 @@ class TestBitcoinCli(BitcoinTestFramework):
         self.log.info("Test -version with node stopped")
         self.stop_node(0)
         cli_response = self.nodes[0].cli('-version').send_cli()
-        assert "{} RPC client version".format(self.config['environment']['PACKAGE_NAME']) in cli_response
+        assert f"{self.config['environment']['PACKAGE_NAME']} RPC client version" in cli_response
 
         self.log.info("Test -rpcwait option successfully waits for RPC connection")
         self.nodes[0].start()  # start node without RPC connection
diff --git a/test/functional/interface_http.py b/test/functional/interface_http.py
index d007490f80..075224c011 100755
--- a/test/functional/interface_http.py
+++ b/test/functional/interface_http.py
@@ -24,8 +24,8 @@ class HTTPBasicsTest (BitcoinTestFramework):
         # lowlevel check for http persistent connection #
         #################################################
         url = urllib.parse.urlparse(self.nodes[0].url)
-        authpair = url.username + ':' + url.password
-        headers = {"Authorization": "Basic " + str_to_b64str(authpair)}
+        authpair = f'{url.username}:{url.password}'
+        headers = {"Authorization": f"Basic {str_to_b64str(authpair)}"}
 
         conn = http.client.HTTPConnection(url.hostname, url.port)
         conn.connect()
@@ -42,7 +42,7 @@ class HTTPBasicsTest (BitcoinTestFramework):
         conn.close()
 
         #same should be if we add keep-alive because this should be the std. behaviour
-        headers = {"Authorization": "Basic " + str_to_b64str(authpair), "Connection": "keep-alive"}
+        headers = {"Authorization": f"Basic {str_to_b64str(authpair)}", "Connection": "keep-alive"}
 
         conn = http.client.HTTPConnection(url.hostname, url.port)
         conn.connect()
@@ -59,7 +59,7 @@ class HTTPBasicsTest (BitcoinTestFramework):
         conn.close()
 
         #now do the same with "Connection: close"
-        headers = {"Authorization": "Basic " + str_to_b64str(authpair), "Connection":"close"}
+        headers = {"Authorization": f"Basic {str_to_b64str(authpair)}", "Connection":"close"}
 
         conn = http.client.HTTPConnection(url.hostname, url.port)
         conn.connect()
@@ -70,8 +70,8 @@ class HTTPBasicsTest (BitcoinTestFramework):
 
         #node1 (2nd node) is running with disabled keep-alive option
         urlNode1 = urllib.parse.urlparse(self.nodes[1].url)
-        authpair = urlNode1.username + ':' + urlNode1.password
-        headers = {"Authorization": "Basic " + str_to_b64str(authpair)}
+        authpair = f'{urlNode1.username}:{urlNode1.password}'
+        headers = {"Authorization": f"Basic {str_to_b64str(authpair)}"}
 
         conn = http.client.HTTPConnection(urlNode1.hostname, urlNode1.port)
         conn.connect()
@@ -81,8 +81,8 @@ class HTTPBasicsTest (BitcoinTestFramework):
 
         #node2 (third node) is running with standard keep-alive parameters which means keep-alive is on
         urlNode2 = urllib.parse.urlparse(self.nodes[2].url)
-        authpair = urlNode2.username + ':' + urlNode2.password
-        headers = {"Authorization": "Basic " + str_to_b64str(authpair)}
+        authpair = f'{urlNode2.username}:{urlNode2.password}'
+        headers = {"Authorization": f"Basic {str_to_b64str(authpair)}"}
 
         conn = http.client.HTTPConnection(urlNode2.hostname, urlNode2.port)
         conn.connect()
@@ -94,13 +94,13 @@ class HTTPBasicsTest (BitcoinTestFramework):
         # Check excessive request size
         conn = http.client.HTTPConnection(urlNode2.hostname, urlNode2.port)
         conn.connect()
-        conn.request('GET', '/' + ('x'*1000), '', headers)
+        conn.request('GET', f'/{"x"*1000}', '', headers)
         out1 = conn.getresponse()
         assert_equal(out1.status, http.client.NOT_FOUND)
 
         conn = http.client.HTTPConnection(urlNode2.hostname, urlNode2.port)
         conn.connect()
-        conn.request('GET', '/' + ('x'*10000), '', headers)
+        conn.request('GET', f'/{"x"*10000}', '', headers)
         out1 = conn.getresponse()
         assert_equal(out1.status, http.client.BAD_REQUEST)
 
diff --git a/test/functional/interface_rest.py b/test/functional/interface_rest.py
index d644a420fc..47c95f5673 100755
--- a/test/functional/interface_rest.py
+++ b/test/functional/interface_rest.py
@@ -57,7 +57,7 @@ class RESTTest (BitcoinTestFramework):
             rest_uri += '.hex'
 
         conn = http.client.HTTPConnection(self.url.hostname, self.url.port)
-        self.log.debug('%s %s %s', http_method, rest_uri, body)
+        self.log.debug(f'{http_method} {rest_uri} {body}')
         if http_method == 'GET':
             conn.request('GET', rest_uri)
         elif http_method == 'POST':
@@ -92,11 +92,11 @@ class RESTTest (BitcoinTestFramework):
 
         self.log.info("Test the /tx URI")
 
-        json_obj = self.test_rest_request("/tx/{}".format(txid))
+        json_obj = self.test_rest_request(f"/tx/{txid}")
         assert_equal(json_obj['txid'], txid)
 
         # Check hex format response
-        hex_response = self.test_rest_request("/tx/{}".format(txid), req_type=ReqType.HEX, ret_type=RetType.OBJ)
+        hex_response = self.test_rest_request(f"/tx/{txid}", req_type=ReqType.HEX, ret_type=RetType.OBJ)
         assert_greater_than_or_equal(int(hex_response.getheader('content-length')),
                                      json_obj['size']*2)
 
@@ -114,7 +114,7 @@ class RESTTest (BitcoinTestFramework):
         assert_equal(self.nodes[1].getbalance(), Decimal("0.1"))
 
         # Check chainTip response
-        json_obj = self.test_rest_request("/getutxos/{}-{}".format(*spending))
+        json_obj = self.test_rest_request(f"/getutxos/{spending[0]}-{spending[1]}")
         assert_equal(json_obj['chaintipHash'], bb_hash)
 
         # Make sure there is one utxo
@@ -123,7 +123,7 @@ class RESTTest (BitcoinTestFramework):
 
         self.log.info("Query a spent TXO using the /getutxos URI")
 
-        json_obj = self.test_rest_request("/getutxos/{}-{}".format(*spent))
+        json_obj = self.test_rest_request(f"/getutxos/{spent[0]}-{spent[1]}")
 
         # Check chainTip response
         assert_equal(json_obj['chaintipHash'], bb_hash)
@@ -136,7 +136,7 @@ class RESTTest (BitcoinTestFramework):
 
         self.log.info("Query two TXOs using the /getutxos URI")
 
-        json_obj = self.test_rest_request("/getutxos/{}-{}/{}-{}".format(*(spending + spent)))
+        json_obj = self.test_rest_request(f"/getutxos/{spending[0]}-{spending[1]}/{spent[0]}-{spent[1]}")
 
         assert_equal(len(json_obj['utxos']), 1)
         assert_equal(json_obj['bitmap'], "10")
@@ -163,32 +163,32 @@ class RESTTest (BitcoinTestFramework):
 
         # do a tx and don't sync
         txid = self.nodes[0].sendtoaddress(self.nodes[1].getnewaddress(), 0.1)
-        json_obj = self.test_rest_request("/tx/{}".format(txid))
+        json_obj = self.test_rest_request(f"/tx/{txid}")
         # get the spent output to later check for utxo (should be spent by then)
         spent = (json_obj['vin'][0]['txid'], json_obj['vin'][0]['vout'])
         # get n of 0.1 outpoint
         n, = filter_output_indices_by_value(json_obj['vout'], Decimal('0.1'))
         spending = (txid, n)
 
-        json_obj = self.test_rest_request("/getutxos/{}-{}".format(*spending))
+        json_obj = self.test_rest_request(f"/getutxos/{spending[0]}-{spending[1]}")
         assert_equal(len(json_obj['utxos']), 0)
 
-        json_obj = self.test_rest_request("/getutxos/checkmempool/{}-{}".format(*spending))
+        json_obj = self.test_rest_request(f"/getutxos/checkmempool/{spending[0]}-{spending[1]}")
         assert_equal(len(json_obj['utxos']), 1)
 
-        json_obj = self.test_rest_request("/getutxos/{}-{}".format(*spent))
+        json_obj = self.test_rest_request(f"/getutxos/{spent[0]}-{spent[1]}")
         assert_equal(len(json_obj['utxos']), 1)
 
-        json_obj = self.test_rest_request("/getutxos/checkmempool/{}-{}".format(*spent))
+        json_obj = self.test_rest_request(f"/getutxos/checkmempool/{spent[0]}-{spent[1]}")
         assert_equal(len(json_obj['utxos']), 0)
 
         self.nodes[0].generate(1)
         self.sync_all()
 
-        json_obj = self.test_rest_request("/getutxos/{}-{}".format(*spending))
+        json_obj = self.test_rest_request(f"/getutxos/{spending[0]}-{spending[1]}")
         assert_equal(len(json_obj['utxos']), 1)
 
-        json_obj = self.test_rest_request("/getutxos/checkmempool/{}-{}".format(*spending))
+        json_obj = self.test_rest_request(f"/getutxos/checkmempool/{spending[0]}-{spending[1]}")
         assert_equal(len(json_obj['utxos']), 1)
 
         # Do some invalid requests
@@ -197,11 +197,11 @@ class RESTTest (BitcoinTestFramework):
         self.test_rest_request("/getutxos/checkmempool", http_method='POST', req_type=ReqType.JSON, status=400, ret_type=RetType.OBJ)
 
         # Test limits
-        long_uri = '/'.join(["{}-{}".format(txid, n_) for n_ in range(20)])
-        self.test_rest_request("/getutxos/checkmempool/{}".format(long_uri), http_method='POST', status=400, ret_type=RetType.OBJ)
+        long_uri = '/'.join([f"{txid}-{n_}" for n_ in range(20)])
+        self.test_rest_request(f"/getutxos/checkmempool/{long_uri}", http_method='POST', status=400, ret_type=RetType.OBJ)
 
-        long_uri = '/'.join(['{}-{}'.format(txid, n_) for n_ in range(15)])
-        self.test_rest_request("/getutxos/checkmempool/{}".format(long_uri), http_method='POST', status=200)
+        long_uri = '/'.join([f'{txid}-{n_}' for n_ in range(15)])
+        self.test_rest_request(f"/getutxos/checkmempool/{long_uri}", http_method='POST', status=200)
 
         self.nodes[0].generate(1)  # generate block to not affect upcoming tests
         self.sync_all()
@@ -215,42 +215,42 @@ class RESTTest (BitcoinTestFramework):
 
         # Check result if block is not in the active chain
         self.nodes[0].invalidateblock(bb_hash)
-        assert_equal(self.test_rest_request('/headers/1/{}'.format(bb_hash)), [])
-        self.test_rest_request('/block/{}'.format(bb_hash))
+        assert_equal(self.test_rest_request(f'/headers/1/{bb_hash}'), [])
+        self.test_rest_request(f'/block/{bb_hash}')
         self.nodes[0].reconsiderblock(bb_hash)
 
         # Check binary format
-        response = self.test_rest_request("/block/{}".format(bb_hash), req_type=ReqType.BIN, ret_type=RetType.OBJ)
+        response = self.test_rest_request(f"/block/{bb_hash}", req_type=ReqType.BIN, ret_type=RetType.OBJ)
         assert_greater_than(int(response.getheader('content-length')), BLOCK_HEADER_SIZE)
         response_bytes = response.read()
 
         # Compare with block header
-        response_header = self.test_rest_request("/headers/1/{}".format(bb_hash), req_type=ReqType.BIN, ret_type=RetType.OBJ)
+        response_header = self.test_rest_request(f"/headers/1/{bb_hash}", req_type=ReqType.BIN, ret_type=RetType.OBJ)
         assert_equal(int(response_header.getheader('content-length')), BLOCK_HEADER_SIZE)
         response_header_bytes = response_header.read()
         assert_equal(response_bytes[:BLOCK_HEADER_SIZE], response_header_bytes)
 
         # Check block hex format
-        response_hex = self.test_rest_request("/block/{}".format(bb_hash), req_type=ReqType.HEX, ret_type=RetType.OBJ)
+        response_hex = self.test_rest_request(f"/block/{bb_hash}", req_type=ReqType.HEX, ret_type=RetType.OBJ)
         assert_greater_than(int(response_hex.getheader('content-length')), BLOCK_HEADER_SIZE*2)
         response_hex_bytes = response_hex.read().strip(b'\n')
         assert_equal(response_bytes.hex().encode(), response_hex_bytes)
 
         # Compare with hex block header
-        response_header_hex = self.test_rest_request("/headers/1/{}".format(bb_hash), req_type=ReqType.HEX, ret_type=RetType.OBJ)
+        response_header_hex = self.test_rest_request(f"/headers/1/{bb_hash}", req_type=ReqType.HEX, ret_type=RetType.OBJ)
         assert_greater_than(int(response_header_hex.getheader('content-length')), BLOCK_HEADER_SIZE*2)
         response_header_hex_bytes = response_header_hex.read(BLOCK_HEADER_SIZE*2)
         assert_equal(response_bytes[:BLOCK_HEADER_SIZE].hex().encode(), response_header_hex_bytes)
 
         # Check json format
-        block_json_obj = self.test_rest_request("/block/{}".format(bb_hash))
+        block_json_obj = self.test_rest_request(f"/block/{bb_hash}")
         assert_equal(block_json_obj['hash'], bb_hash)
-        assert_equal(self.test_rest_request("/blockhashbyheight/{}".format(block_json_obj['height']))['blockhash'], bb_hash)
+        assert_equal(self.test_rest_request(f"/blockhashbyheight/{block_json_obj['height']}")['blockhash'], bb_hash)
 
         # Check hex/bin format
-        resp_hex = self.test_rest_request("/blockhashbyheight/{}".format(block_json_obj['height']), req_type=ReqType.HEX, ret_type=RetType.OBJ)
+        resp_hex = self.test_rest_request(f"/blockhashbyheight/{block_json_obj['height']}", req_type=ReqType.HEX, ret_type=RetType.OBJ)
         assert_equal(resp_hex.read().decode('utf-8').rstrip(), bb_hash)
-        resp_bytes = self.test_rest_request("/blockhashbyheight/{}".format(block_json_obj['height']), req_type=ReqType.BIN, ret_type=RetType.BYTES)
+        resp_bytes = self.test_rest_request(f"/blockhashbyheight/{block_json_obj['height']}", req_type=ReqType.BIN, ret_type=RetType.BYTES)
         blockhash = resp_bytes[::-1].hex()
         assert_equal(blockhash, bb_hash)
 
@@ -264,7 +264,7 @@ class RESTTest (BitcoinTestFramework):
         self.test_rest_request("/blockhashbyheight/", ret_type=RetType.OBJ, status=400)
 
         # Compare with json block header
-        json_obj = self.test_rest_request("/headers/1/{}".format(bb_hash))
+        json_obj = self.test_rest_request(f"/headers/1/{bb_hash}")
         assert_equal(len(json_obj), 1)  # ensure that there is one header in the json response
         assert_equal(json_obj[0]['hash'], bb_hash)  # request/response hash should be the same
 
@@ -276,7 +276,7 @@ class RESTTest (BitcoinTestFramework):
         # See if we can get 5 headers in one response
         self.nodes[1].generate(5)
         self.sync_all()
-        json_obj = self.test_rest_request("/headers/5/{}".format(bb_hash))
+        json_obj = self.test_rest_request(f"/headers/5/{bb_hash}")
         assert_equal(len(json_obj), 5)  # now we should have 5 header objects
 
         self.log.info("Test tx inclusion in the /mempool and /block URIs")
@@ -306,13 +306,13 @@ class RESTTest (BitcoinTestFramework):
         self.sync_all()
 
         # Check if the 3 tx show up in the new block
-        json_obj = self.test_rest_request("/block/{}".format(newblockhash[0]))
+        json_obj = self.test_rest_request(f"/block/{newblockhash[0]}")
         non_coinbase_txs = {tx['txid'] for tx in json_obj['tx']
                             if 'coinbase' not in tx['vin'][0]}
         assert_equal(non_coinbase_txs, set(txs))
 
         # Check the same but without tx details
-        json_obj = self.test_rest_request("/block/notxdetails/{}".format(newblockhash[0]))
+        json_obj = self.test_rest_request(f"/block/notxdetails/{newblockhash[0]}")
         for tx in txs:
             assert tx in json_obj['tx']
 
diff --git a/test/functional/interface_rpc.py b/test/functional/interface_rpc.py
index 4d5666f414..89a7d29b24 100755
--- a/test/functional/interface_rpc.py
+++ b/test/functional/interface_rpc.py
@@ -16,7 +16,7 @@ def expect_http_status(expected_http_status, expected_rpc_code,
                        fcn, *args):
     try:
         fcn(*args)
-        raise AssertionError("Expected RPC error %d, got none" % expected_rpc_code)
+        raise AssertionError(f"Expected RPC error {expected_rpc_code}, got none")
     except JSONRPCException as exc:
         assert_equal(exc.error["code"], expected_rpc_code)
         assert_equal(exc.http_status, expected_http_status)
diff --git a/test/functional/interface_zmq.py b/test/functional/interface_zmq.py
index 15f352d68c..61b96a8250 100755
--- a/test/functional/interface_zmq.py
+++ b/test/functional/interface_zmq.py
@@ -132,7 +132,7 @@ class ZMQTest (BitcoinTestFramework):
             socket = self.ctx.socket(zmq.SUB)
             subscribers.append(ZMQSubscriber(socket, topic.encode()))
 
-        self.restart_node(0, ["-zmqpub%s=%s" % (topic, address) for topic, address in services] +
+        self.restart_node(0, [f"-zmqpub{topic}={address}" for topic, address in services] +
                              self.extra_args[0])
 
         for i, sub in enumerate(subscribers):
@@ -184,7 +184,7 @@ class ZMQTest (BitcoinTestFramework):
         rawtx = subs[3]
 
         num_blocks = 5
-        self.log.info("Generate %(n)d blocks (and %(n)d coinbase txes)" % {"n": num_blocks})
+        self.log.info(f"Generate {num_blocks} blocks (and {num_blocks} coinbase txes)")
         genhashes = self.nodes[0].generatetoaddress(num_blocks, ADDRESS_BCRT1_UNSPENDABLE)
 
         self.sync_all()
@@ -504,7 +504,7 @@ class ZMQTest (BitcoinTestFramework):
             if mempool_sequence is not None:
                 zmq_mem_seq = mempool_sequence
                 if zmq_mem_seq > get_raw_seq:
-                    raise Exception("We somehow jumped mempool sequence numbers! zmq_mem_seq: {} > get_raw_seq: {}".format(zmq_mem_seq, get_raw_seq))
+                    raise Exception(f"We somehow jumped mempool sequence numbers! zmq_mem_seq: {zmq_mem_seq} > get_raw_seq: {get_raw_seq}")
 
         # 4) Moving forward, we apply the delta to our local view
         #    remaining txs(5) + 1 rbf(A+R) + 1 block connect + 1 final tx
@@ -520,7 +520,7 @@ class ZMQTest (BitcoinTestFramework):
                         assert mempool_sequence > expected_sequence
                         r_gap += mempool_sequence - expected_sequence
                     else:
-                        raise Exception("WARNING: txhash has unexpected mempool sequence value: {} vs expected {}".format(mempool_sequence, expected_sequence))
+                        raise Exception(f"WARNING: txhash has unexpected mempool sequence value: {mempool_sequence} vs expected {expected_sequence}")
             if label == "A":
                 assert hash_str not in mempool_view
                 mempool_view.add(hash_str)
diff --git a/test/functional/mining_basic.py b/test/functional/mining_basic.py
index 01fc02f27e..23711646a2 100755
--- a/test/functional/mining_basic.py
+++ b/test/functional/mining_basic.py
@@ -65,10 +65,10 @@ class MiningTest(BitcoinTestFramework):
         assert_equal(mining_info['currentblockweight'], 4000)
 
         self.log.info('test blockversion')
-        self.restart_node(0, extra_args=['-mocktime={}'.format(t), '-blockversion=1337'])
+        self.restart_node(0, extra_args=[f'-mocktime={t}', '-blockversion=1337'])
         self.connect_nodes(0, 1)
         assert_equal(1337, self.nodes[0].getblocktemplate(NORMAL_GBT_REQUEST_PARAMS)['version'])
-        self.restart_node(0, extra_args=['-mocktime={}'.format(t)])
+        self.restart_node(0, extra_args=[f'-mocktime={t}'])
         self.connect_nodes(0, 1)
         assert_equal(VERSIONBITS_TOP_BITS + (1 << VERSIONBITS_DEPLOYMENT_TESTDUMMY_BIT), self.nodes[0].getblocktemplate(NORMAL_GBT_REQUEST_PARAMS)['version'])
         self.restart_node(0)