This PR adds initial support for type hints checking in python scripts.

Support for type hints was introduced in Python 3.5. Type hints make it easier to read and review code in my opinion. Also an IDE may discover a potential bug sooner. Yet, as PEP 484 says: "It should also be emphasized that Python will remain a dynamically typed language, and the authors have no desire to ever make type hints mandatory, even by convention."

Mypy is used in lint-python.sh to do the type checking. The package is standard so there is little chance that it will be abandoned. Mypy checks that type hints in source code are correct when they are not, it fails with an error.

Useful resources:

* https://docs.python.org/3.5/library/typing.html
* https://www.python.org/dev/peps/pep-0484/

7 files changed, 43 insertions, 33 deletions

diff --git a/test/functional/README.md b/test/functional/README.md
index 004e0afb1d..aff5f714f2 100644
--- a/test/functional/README.md
+++ b/test/functional/README.md
@@ -26,10 +26,12 @@ don't have test cases for.
   The Travis linter also checks this, but [possibly not in all cases](https://github.com/bitcoin/bitcoin/pull/14884#discussion_r239585126).
 - See [the python lint script](/test/lint/lint-python.sh) that checks for violations that
   could lead to bugs and issues in the test code.
+- Use [type hints](https://docs.python.org/3/library/typing.html) in your code to improve code readability
+  and to detect possible bugs earlier.
 - Avoid wildcard imports
 - Use a module-level docstring to describe what the test is testing, and how it
   is testing it.
-- When subclassing the BitcoinTestFramwork, place overrides for the
+- 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`.
@@ -45,7 +47,7 @@ don't have test cases for.
     - `rpc` for tests for individual RPC methods or features, eg `rpc_listtransactions.py`
     - `tool` for tests for tools, eg `tool_wallet.py`
     - `wallet` for tests for wallet features, eg `wallet_keypool.py`
-- use an underscore to separate words
+- Use an underscore to separate words
     - exception: for tests for specific RPCs or command line options which don't include underscores, name the test after the exact RPC or argument name, eg `rpc_decodescript.py`, not `rpc_decode_script.py`
 - Don't use the redundant word `test` in the name, eg `interface_zmq.py`, not `interface_zmq_test.py`
 
diff --git a/test/functional/data/invalid_txs.py b/test/functional/data/invalid_txs.py
index ae5721bec2..6e72db1d96 100644
--- a/test/functional/data/invalid_txs.py
+++ b/test/functional/data/invalid_txs.py
@@ -21,6 +21,7 @@ Invalid tx cases not covered here can be found by running:
 """
 import abc
 
+from typing import Optional
 from test_framework.messages import (
     COutPoint,
     CTransaction,
@@ -56,7 +57,7 @@ class BadTxTemplate:
     __metaclass__ = abc.ABCMeta
 
     # The expected error code given by bitcoind upon submission of the tx.
-    reject_reason = ""
+    reject_reason = ""  # type: Optional[str]
 
     # Only specified if it differs from mempool acceptance error.
     block_reject_reason = ""
diff --git a/test/functional/p2p_segwit.py b/test/functional/p2p_segwit.py
index 8a989097b4..8803086213 100755
--- a/test/functional/p2p_segwit.py
+++ b/test/functional/p2p_segwit.py
@@ -295,7 +295,7 @@ class SegWitTest(BitcoinTestFramework):
 
         return func_wrapper
 
-    @subtest
+    @subtest  # type: ignore
     def test_non_witness_transaction(self):
         """See if sending a regular transaction works, and create a utxo to use in later tests."""
         # Mine a block with an anyone-can-spend coinbase,
@@ -324,7 +324,7 @@ class SegWitTest(BitcoinTestFramework):
         self.utxo.append(UTXO(tx.sha256, 0, 49 * 100000000))
         self.nodes[0].generate(1)
 
-    @subtest
+    @subtest  # type: ignore
     def test_unnecessary_witness_before_segwit_activation(self):
         """Verify that blocks with witnesses are rejected before activation."""
 
@@ -355,7 +355,7 @@ class SegWitTest(BitcoinTestFramework):
         self.utxo.pop(0)
         self.utxo.append(UTXO(tx.sha256, 0, tx.vout[0].nValue))
 
-    @subtest
+    @subtest  # type: ignore
     def test_block_relay(self):
         """Test that block requests to NODE_WITNESS peer are with MSG_WITNESS_FLAG.
 
@@ -451,7 +451,7 @@ class SegWitTest(BitcoinTestFramework):
             self.old_node.announce_tx_and_wait_for_getdata(block4.vtx[0])
             assert block4.sha256 not in self.old_node.getdataset
 
-    @subtest
+    @subtest  # type: ignore
     def test_v0_outputs_arent_spendable(self):
         """Test that v0 outputs aren't spendable before segwit activation.
 
@@ -533,7 +533,7 @@ class SegWitTest(BitcoinTestFramework):
         self.utxo.pop(0)
         self.utxo.append(UTXO(txid, 2, value))
 
-    @subtest
+    @subtest  # type: ignore
     def test_getblocktemplate_before_lockin(self):
         txid = int(self.nodes[0].sendtoaddress(self.nodes[0].getnewaddress(), 1), 16)
 
@@ -559,7 +559,7 @@ class SegWitTest(BitcoinTestFramework):
         self.nodes[0].generate(1)
         self.sync_blocks()
 
-    @subtest
+    @subtest  # type: ignore
     def test_witness_tx_relay_before_segwit_activation(self):
 
         # Generate a transaction that doesn't require a witness, but send it
@@ -601,7 +601,7 @@ class SegWitTest(BitcoinTestFramework):
         self.utxo.pop(0)
         self.utxo.append(UTXO(tx_hash, 0, tx_value))
 
-    @subtest
+    @subtest  # type: ignore
     def test_standardness_v0(self):
         """Test V0 txout standardness.
 
@@ -679,7 +679,7 @@ class SegWitTest(BitcoinTestFramework):
         self.utxo.append(UTXO(tx3.sha256, 0, tx3.vout[0].nValue))
         assert_equal(len(self.nodes[1].getrawmempool()), 0)
 
-    @subtest
+    @subtest  # type: ignore
     def advance_to_segwit_active(self):
         """Mine enough blocks to activate segwit."""
         assert not softfork_active(self.nodes[0], 'segwit')
@@ -690,7 +690,7 @@ class SegWitTest(BitcoinTestFramework):
         assert softfork_active(self.nodes[0], 'segwit')
         self.segwit_active = True
 
-    @subtest
+    @subtest  # type: ignore
     def test_p2sh_witness(self):
         """Test P2SH wrapped witness programs."""
 
@@ -759,7 +759,7 @@ class SegWitTest(BitcoinTestFramework):
         self.utxo.pop(0)
         self.utxo.append(UTXO(spend_tx.sha256, 0, spend_tx.vout[0].nValue))
 
-    @subtest
+    @subtest  # type: ignore
     def test_witness_commitments(self):
         """Test witness commitments.
 
@@ -849,7 +849,7 @@ class SegWitTest(BitcoinTestFramework):
         self.utxo.pop(0)
         self.utxo.append(UTXO(tx3.sha256, 0, tx3.vout[0].nValue))
 
-    @subtest
+    @subtest  # type: ignore
     def test_block_malleability(self):
 
         # Make sure that a block that has too big a virtual size
@@ -889,7 +889,7 @@ class SegWitTest(BitcoinTestFramework):
         block.vtx[0].wit.vtxinwit[0].scriptWitness.stack = [ser_uint256(0)]
         test_witness_block(self.nodes[0], self.test_node, block, accepted=True)
 
-    @subtest
+    @subtest  # type: ignore
     def test_witness_block_size(self):
         # TODO: Test that non-witness carrying blocks can't exceed 1MB
         # Skipping this test for now; this is covered in p2p-fullblocktest.py
@@ -967,7 +967,7 @@ class SegWitTest(BitcoinTestFramework):
         self.utxo.pop(0)
         self.utxo.append(UTXO(block.vtx[-1].sha256, 0, block.vtx[-1].vout[0].nValue))
 
-    @subtest
+    @subtest  # type: ignore
     def test_submit_block(self):
         """Test that submitblock adds the nonce automatically when possible."""
         block = self.build_next_block()
@@ -1003,7 +1003,7 @@ class SegWitTest(BitcoinTestFramework):
         # Tip should not advance!
         assert self.nodes[0].getbestblockhash() != block_2.hash
 
-    @subtest
+    @subtest  # type: ignore
     def test_extra_witness_data(self):
         """Test extra witness data in a transaction."""
 
@@ -1076,7 +1076,7 @@ class SegWitTest(BitcoinTestFramework):
         self.utxo.pop(0)
         self.utxo.append(UTXO(tx2.sha256, 0, tx2.vout[0].nValue))
 
-    @subtest
+    @subtest  # type: ignore
     def test_max_witness_push_length(self):
         """Test that witness stack can only allow up to 520 byte pushes."""
 
@@ -1113,7 +1113,7 @@ class SegWitTest(BitcoinTestFramework):
         self.utxo.pop()
         self.utxo.append(UTXO(tx2.sha256, 0, tx2.vout[0].nValue))
 
-    @subtest
+    @subtest  # type: ignore
     def test_max_witness_program_length(self):
         """Test that witness outputs greater than 10kB can't be spent."""
 
@@ -1161,7 +1161,7 @@ class SegWitTest(BitcoinTestFramework):
         self.utxo.pop()
         self.utxo.append(UTXO(tx2.sha256, 0, tx2.vout[0].nValue))
 
-    @subtest
+    @subtest  # type: ignore
     def test_witness_input_length(self):
         """Test that vin length must match vtxinwit length."""
 
@@ -1243,7 +1243,7 @@ class SegWitTest(BitcoinTestFramework):
         self.utxo.pop()
         self.utxo.append(UTXO(tx2.sha256, 0, tx2.vout[0].nValue))
 
-    @subtest
+    @subtest  # type: ignore
     def test_tx_relay_after_segwit_activation(self):
         """Test transaction relay after segwit activation.
 
@@ -1336,7 +1336,7 @@ class SegWitTest(BitcoinTestFramework):
         self.utxo.pop(0)
         self.utxo.append(UTXO(tx3.sha256, 0, tx3.vout[0].nValue))
 
-    @subtest
+    @subtest  # type: ignore
     def test_segwit_versions(self):
         """Test validity of future segwit version transactions.
 
@@ -1418,7 +1418,7 @@ class SegWitTest(BitcoinTestFramework):
         # Add utxo to our list
         self.utxo.append(UTXO(tx3.sha256, 0, tx3.vout[0].nValue))
 
-    @subtest
+    @subtest  # type: ignore
     def test_premature_coinbase_witness_spend(self):
 
         block = self.build_next_block()
@@ -1453,7 +1453,7 @@ class SegWitTest(BitcoinTestFramework):
         test_witness_block(self.nodes[0], self.test_node, block2, accepted=True)
         self.sync_blocks()
 
-    @subtest
+    @subtest  # type: ignore
     def test_uncompressed_pubkey(self):
         """Test uncompressed pubkey validity in segwit transactions.
 
@@ -1558,7 +1558,7 @@ class SegWitTest(BitcoinTestFramework):
         test_witness_block(self.nodes[0], self.test_node, block, accepted=True)
         self.utxo.append(UTXO(tx5.sha256, 0, tx5.vout[0].nValue))
 
-    @subtest
+    @subtest  # type: ignore
     def test_signature_version_1(self):
 
         key = ECKey()
@@ -1740,7 +1740,7 @@ class SegWitTest(BitcoinTestFramework):
         for i in range(len(tx.vout)):
             self.utxo.append(UTXO(tx.sha256, i, tx.vout[i].nValue))
 
-    @subtest
+    @subtest  # type: ignore
     def test_non_standard_witness_blinding(self):
         """Test behavior of unnecessary witnesses in transactions does not blind the node for the transaction"""
 
@@ -1794,7 +1794,7 @@ class SegWitTest(BitcoinTestFramework):
         self.utxo.pop(0)
         self.utxo.append(UTXO(tx3.sha256, 0, tx3.vout[0].nValue))
 
-    @subtest
+    @subtest  # type: ignore
     def test_non_standard_witness(self):
         """Test detection of non-standard P2WSH witness"""
         pad = chr(1).encode('latin-1')
@@ -1894,7 +1894,7 @@ class SegWitTest(BitcoinTestFramework):
 
         self.utxo.pop(0)
 
-    @subtest
+    @subtest  # type: ignore
     def test_upgrade_after_activation(self):
         """Test the behavior of starting up a segwit-aware node after the softfork has activated."""
 
@@ -1916,7 +1916,7 @@ class SegWitTest(BitcoinTestFramework):
             assert_equal(self.nodes[0].getblock(block_hash), self.nodes[2].getblock(block_hash))
             height -= 1
 
-    @subtest
+    @subtest  # type: ignore
     def test_witness_sigops(self):
         """Test sigop counting is correct inside witnesses."""
 
diff --git a/test/functional/test_framework/script.py b/test/functional/test_framework/script.py
index 9102266456..bc1b5b26fc 100644
--- a/test/functional/test_framework/script.py
+++ b/test/functional/test_framework/script.py
@@ -9,6 +9,7 @@ This file is modified from python-bitcoinlib.
 import hashlib
 import struct
 import unittest
+from typing import List, Dict
 
 from .messages import (
     CTransaction,
@@ -21,7 +22,7 @@ from .messages import (
 )
 
 MAX_SCRIPT_ELEMENT_SIZE = 520
-OPCODE_NAMES = {}
+OPCODE_NAMES = {}  # type: Dict[CScriptOp, str]
 
 def hash160(s):
     return hashlib.new('ripemd160', sha256(s)).digest()
@@ -37,7 +38,7 @@ def bn2vch(v):
     # Serialize to bytes
     return encoded_v.to_bytes(n_bytes, 'little')
 
-_opcode_instances = []
+_opcode_instances = []  # type: List[CScriptOp]
 class CScriptOp(int):
     """A single script opcode"""
     __slots__ = ()
diff --git a/test/functional/test_framework/test_framework.py b/test/functional/test_framework/test_framework.py
index 5469f808d1..8e0da5295d 100755
--- a/test/functional/test_framework/test_framework.py
+++ b/test/functional/test_framework/test_framework.py
@@ -91,6 +91,9 @@ class BitcoinTestFramework(metaclass=BitcoinTestMetaClass):
 
     This class also contains various public and private helper methods."""
 
+    chain = None  # type: str
+    setup_clean_chain = None  # type: bool
+
     def __init__(self):
         """Sets test framework defaults. Do not override this method. Instead, override the set_test_params() method"""
         self.chain = 'regtest'
@@ -407,7 +410,7 @@ class BitcoinTestFramework(metaclass=BitcoinTestMetaClass):
 
     # Public helper methods. These can be accessed by the subclass test scripts.
 
-    def add_nodes(self, num_nodes, extra_args=None, *, rpchost=None, binary=None, binary_cli=None, versions=None):
+    def add_nodes(self, num_nodes: int, extra_args=None, *, rpchost=None, binary=None, binary_cli=None, versions=None):
         """Instantiate TestNode objects.
 
         Should only be called once after the nodes have been specified in
diff --git a/test/functional/test_runner.py b/test/functional/test_runner.py
index 0812470b0c..1517407700 100755
--- a/test/functional/test_runner.py
+++ b/test/functional/test_runner.py
@@ -42,7 +42,7 @@ except UnicodeDecodeError:
 if os.name != 'nt' or sys.getwindowsversion() >= (10, 0, 14393):
     if os.name == 'nt':
         import ctypes
-        kernel32 = ctypes.windll.kernel32
+        kernel32 = ctypes.windll.kernel32  # type: ignore
         ENABLE_VIRTUAL_TERMINAL_PROCESSING = 4
         STD_OUTPUT_HANDLE = -11
         STD_ERROR_HANDLE = -12
diff --git a/test/lint/lint-python.sh b/test/lint/lint-python.sh
index 86ac5a930f..b9aa6c799b 100755
--- a/test/lint/lint-python.sh
+++ b/test/lint/lint-python.sh
@@ -7,6 +7,7 @@
 # Check for specified flake8 warnings in python files.
 
 export LC_ALL=C
+export MYPY_CACHE_DIR="${BASE_ROOT_DIR}/test/.mypy_cache"
 
 enabled=(
     E101 # indentation contains mixed spaces and tabs
@@ -96,3 +97,5 @@ PYTHONWARNINGS="ignore" flake8 --ignore=B,C,E,F,I,N,W --select=$(IFS=","; echo "
         echo "$@"
     fi
 )
+
+mypy --ignore-missing-imports $(git ls-files "test/functional/*.py")
\ No newline at end of file