Merge #16726: tests: Avoid common Python default parameter gotcha when mutable dict/list:s are used as default parameter values

e4f4ea47ebf7774fb6f445adde7bf7ea71fa05a1 lint: Catch use of [] or {} as default parameter values in Python functions (practicalswift)
25dd86715039586d92176eee16e9c6644d2547f0 Avoid using mutable default parameter values (practicalswift)

Pull request description:

  Avoid common Python default parameter gotcha when mutable `dict`/`list`:s are used as default parameter values.

  Examples of this gotcha caught during review:
  * https://github.com/bitcoin/bitcoin/pull/16673#discussion_r317415261
  * https://github.com/bitcoin/bitcoin/pull/14565#discussion_r241942304

  Perhaps surprisingly this is how mutable list and dictionary default parameter values behave in Python:

  ```
  >>> def f(i, j=[], k={}):
  ...     j.append(i)
  ...     k[i] = True
  ...     return j, k
  ...
  >>> f(1)
  ([1], {1: True})
  >>> f(1)
  ([1, 1], {1: True})
  >>> f(2)
  ([1, 1, 2], {1: True, 2: True})
  ```

  In contrast to:

  ```
  >>> def f(i, j=None, k=None):
  ...     if j is None:
  ...         j = []
  ...     if k is None:
  ...         k = {}
  ...     j.append(i)
  ...     k[i] = True
  ...     return j, k
  ...
  >>> f(1)
  ([1], {1: True})
  >>> f(1)
  ([1], {1: True})
  >>> f(2)
  ([2], {2: True})
  ```

  The latter is typically the intended behaviour.

  This PR fixes two instances of this and adds a check guarding against this gotcha going forward :-)

ACKs for top commit:
  Sjors:
    Oh Python... ACK e4f4ea47ebf7774fb6f445adde7bf7ea71fa05a1. Testing tip: swap the two commits.

Tree-SHA512: 56e14d24fc866211a20185c9fdb274ed046c3aed2dc0e07699e58b6f9fa3b79f6d0c880fb02d72b7fe5cc5eb7c0ff6da0ead33123344e1a872209370c2e49e3f

3 files changed, 58 insertions, 2 deletions

diff --git a/test/functional/test_framework/messages.py b/test/functional/test_framework/messages.py
index 89a5a65e64..917efaa833 100755
--- a/test/functional/test_framework/messages.py
+++ b/test/functional/test_framework/messages.py
@@ -803,7 +803,9 @@ class HeaderAndShortIDs:
         return [ key0, key1 ]
 
     # Version 2 compact blocks use wtxid in shortids (rather than txid)
-    def initialize_from_block(self, block, nonce=0, prefill_list = [0], use_witness = False):
+    def initialize_from_block(self, block, nonce=0, prefill_list=None, use_witness=False):
+        if prefill_list is None:
+            prefill_list = [0]
         self.header = CBlockHeader(block)
         self.nonce = nonce
         self.prefilled_txn = [ PrefilledTransaction(i, block.vtx[i]) for i in prefill_list ]
diff --git a/test/functional/wallet_importmulti.py b/test/functional/wallet_importmulti.py
index e4a4ab1f35..23748e5dd7 100755
--- a/test/functional/wallet_importmulti.py
+++ b/test/functional/wallet_importmulti.py
@@ -44,8 +44,10 @@ class ImportMultiTest(BitcoinTestFramework):
     def setup_network(self):
         self.setup_nodes()
 
-    def test_importmulti(self, req, success, error_code=None, error_message=None, warnings=[]):
+    def test_importmulti(self, req, success, error_code=None, error_message=None, warnings=None):
         """Run importmulti and assert success"""
+        if warnings is None:
+            warnings = []
         result = self.nodes[1].importmulti([req])
         observed_warnings = []
         if 'warnings' in result[0]:
diff --git a/test/lint/lint-python-mutable-default-parameters.sh b/test/lint/lint-python-mutable-default-parameters.sh
new file mode 100755
index 0000000000..1f9f035d30
--- /dev/null
+++ b/test/lint/lint-python-mutable-default-parameters.sh
@@ -0,0 +1,52 @@
+#!/usr/bin/env bash
+#
+# Copyright (c) 2019 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+#
+# Detect when a mutable list or dict is used as a default parameter value in a Python function.
+
+export LC_ALL=C
+EXIT_CODE=0
+OUTPUT=$(git grep -E '^\s*def [a-zA-Z0-9_]+\(.*=\s*(\[|\{)' -- "*.py")
+if [[ ${OUTPUT} != "" ]]; then
+    echo "A mutable list or dict seems to be used as default parameter value:"
+    echo
+    echo "${OUTPUT}"
+    echo
+    cat << EXAMPLE
+This is how mutable list and dict default parameter values behave:
+
+>>> def f(i, j=[], k={}):
+...     j.append(i)
+...     k[i] = True
+...     return j, k
+...
+>>> f(1)
+([1], {1: True})
+>>> f(1)
+([1, 1], {1: True})
+>>> f(2)
+([1, 1, 2], {1: True, 2: True})
+
+The intended behaviour was likely:
+
+>>> def f(i, j=None, k=None):
+...     if j is None:
+...         j = []
+...     if k is None:
+...         k = {}
+...     j.append(i)
+...     k[i] = True
+...     return j, k
+...
+>>> f(1)
+([1], {1: True})
+>>> f(1)
+([1], {1: True})
+>>> f(2)
+([2], {2: True})
+EXAMPLE
+    EXIT_CODE=1
+fi
+exit ${EXIT_CODE}