Do not use an empty list as a default parameter

Docs > Code Analysis > Static Analysis Rules > Do not use an empty list as a default parameter

Metadata

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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Avoid using empty list as default argument

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']