Do not use an empty list as a default parameter
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ID: python-security/no-empty-list-as-parameter
Language: Python
Severity: Warning
Category: Security
Description
Developers should not be setting a default argument to an empty list. Instead, use None and check if the value is defined. Using a default list can cause unwanted behavior as the value of the argument is only evaluated once when the function is defined, not when it is run. Because of this, each function call will reference the same underlying memory when the default value is used, which can lead to unwanted behavior.
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Non-Compliant Code Examples
def newFunction(arg1, arg2: int, arg3 = [], arg4: MyType = []): # do not use an empty list as a default parameter
arg3.append(arg2)
arg4.append(arg1)
print(arg3, arg4)
newFunction('a', 1)
newFunction('b', 2)
newFunction('c', 3)
# Will print:
# [1] ['a']
# [1, 2] ['a', 'b']
# [1, 2, 3] ['a', 'b', 'c']
Compliant Code Examples
def newFunction(arg1, arg2: int, arg3 = None, arg4 = None): # do not use an empty list as a default parameter
if arg3 is None:
arg3 = []
if arg4 is None:
arg4 = []
arg3.append(arg2)
arg4.append(arg1)
print(arg3, arg4)
newFunction('a', 1)
newFunction('b', 2)
newFunction('c', 3)
# Will print:
# [1] ['a']
# [2] ['b']
# [3] ['c']
