Converted lint-python-mutable-default-parameters.sh to python

Change permission

Change argument so that it's compatiable with python 3.6

Change comment to docstring

Remove .split, .append, .extend calls. Remove 'output' variable assignment

Author: KevinMusgrave

test/lint/lint-python-mutable-default-parameters.py

@@ -0,0 +1,72 @@
+#!/usr/bin/env python3
+#
+# 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.
+"""
+
+import subprocess
+import sys
+
+
+def main():
+    command = [
+        "git",
+        "grep",
+        "-E",
+        r"^\s*def [a-zA-Z0-9_]+\(.*=\s*(\[|\{)",
+        "--",
+        "*.py",
+    ]
+    output = subprocess.run(command, stdout=subprocess.PIPE, universal_newlines=True)
+    if len(output.stdout) > 0:
+        error_msg = (
+            "A mutable list or dict seems to be used as default parameter value:\n\n"
+            f"{output.stdout}\n"
+            f"{example()}"
+        )
+        print(error_msg)
+        sys.exit(1)
+    else:
+        sys.exit(0)
+
+
+def example():
+    return """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})"""
+
+
+if __name__ == "__main__":
+    main()