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/

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."""