How Can I Avoid Unexpected Behavior When Using Default Parameters in Python Functions?

Avoiding Default Parameter Pitfalls in Python

In Python, it can be tempting to use an empty list as a default parameter. However, this can lead to unexpected behavior due to the early binding of default parameters.

Consider a function like this:

def my_func(working_list=[]):
    working_list.append("a")
    print(working_list)

Initially, the default parameter works as expected. However, subsequent calls continue to update the same list, resulting in a printed list that grows with each invocation.

To avoid this, explicitly test for the default parameter's existence in the function body and assign a new empty list if necessary:

def my_func(working_list=None):
    if working_list is None:
        working_list = []
    working_list.append("a")
    print(working_list)

Alternatively, you can use a one-liner:

working_list = [] if working_list is None else working_list

Aside: PEP 8 recommends using is None for comparisons with singletons like None. Avoid using == None, as it can lead to erroneous results.

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