How Can I Avoid Unexpected Behavior with Mutable Default Parameters in Python Functions?

Early-Bound Default Parameters in Python: Avoiding Common Pitfalls

When working with default parameters in Python, it's important to be aware of potential issues caused by early binding. Early-bound default parameters, like those initialized to mutable data structures (e.g., lists), can behave unexpectedly when the function is called multiple times.

Consider the following example function:

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

While calling this function for the first time will initialize an empty list as intended, subsequent calls will update the same list. This can lead to unintended behavior as the list grows with each call.

To avoid this issue, it's recommended to provide a None default value for mutable default parameters and explicitly check for it within the function body. Here's how you can modify the example function:

def my_func(working_list=None):
    if working_list is None:
        working_list = []

    working_list.append("a")
    print(working_list)

By setting the default value to None and testing for it explicitly, each call to the function will create a new empty list, ensuring that the function operates as expected.

Additional Considerations

According to PEP 8 guidelines, using is or is not comparisons with None is preferred over == or !=. This ensures clarity and reduces the risk of potential errors.

Remember to keep these guidelines in mind when working with default parameters to avoid any unforeseen issues in your code.

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