[tests] Recommend f-strings for formatting, update feature_block to use them

2 files changed, 6 insertions, 6 deletions

diff --git a/test/functional/README.md b/test/functional/README.md
index 82b30fed51..2764acbf18 100644
--- a/test/functional/README.md
+++ b/test/functional/README.md
@@ -34,7 +34,7 @@ don't have test cases for.
 - When subclassing the BitcoinTestFramework, place overrides for the
   `set_test_params()`, `add_options()` and `setup_xxxx()` methods at the top of
   the subclass, then locally-defined helper methods, then the `run_test()` method.
-- Use `'{}'.format(x)` for string formatting, not `'%s' % x`.
+- Use `f'{x}'` for string formatting in preference to `'{}'.format(x)` or `'%s' % x`.
 
 #### Naming guidelines
 
diff --git a/test/functional/feature_block.py b/test/functional/feature_block.py
index 3cf7f3890c..158efb52c9 100755
--- a/test/functional/feature_block.py
+++ b/test/functional/feature_block.py
@@ -119,7 +119,7 @@ class FullBlockTest(BitcoinTestFramework):
         # Allow the block to mature
         blocks = []
         for i in range(NUM_BUFFER_BLOCKS_TO_GENERATE):
-            blocks.append(self.next_block("maturitybuffer.{}".format(i)))
+            blocks.append(self.next_block(f"maturitybuffer.{i}"))
             self.save_spendable_output()
         self.send_blocks(blocks)
 
@@ -151,8 +151,8 @@ class FullBlockTest(BitcoinTestFramework):
             if template.valid_in_block:
                 continue
 
-            self.log.info("Reject block with invalid tx: %s", TxTemplate.__name__)
-            blockname = "for_invalid.%s" % TxTemplate.__name__
+            self.log.info(f"Reject block with invalid tx: {TxTemplate.__name__}")
+            blockname = f"for_invalid.{TxTemplate.__name__}"
             badblock = self.next_block(blockname)
             badtx = template.get_tx()
             if TxTemplate != invalid_txs.InputMissing:
@@ -1355,12 +1355,12 @@ class FullBlockTest(BitcoinTestFramework):
 
     # save the current tip so it can be spent by a later block
     def save_spendable_output(self):
-        self.log.debug("saving spendable output %s" % self.tip.vtx[0])
+        self.log.debug(f"saving spendable output {self.tip.vtx[0]}")
         self.spendable_outputs.append(self.tip)
 
     # get an output that we previously marked as spendable
     def get_spendable_output(self):
-        self.log.debug("getting spendable output %s" % self.spendable_outputs[0].vtx[0])
+        self.log.debug(f"getting spendable output {self.spendable_outputs[0].vtx[0]}")
         return self.spendable_outputs.pop(0).vtx[0]
 
     # move the tip back to a previous block