lint: Catch use of [] or {} as default parameter values in Python functions

Author: practicalswift

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}