Python Bugs in 60s - Avoid Mutable Defaults #shorts

Added: 2026-05-05

[00:00:00]Using mutable objects like lists as default arguments is a common bug trap in Python.

[00:00:05]When you define a function with a mutable default argument, Python doesn't create a new object each time. Instead, it uses the same object across all calls, leading to unexpected behavior.

[00:00:17]Here's what happens under the hood.

[00:00:19]The first time mutable default arg is called, it appends one to the list.

[00:00:24]The second call appends another one to the same list, producing one one.

[00:00:29]This demonstrates how reusing the same list can lead to incorrect results. To fix this, we can use none as the default value.

[00:00:37]Inside the function, we then check if the argument is none and create a new list if it is.

[00:00:43]This ensures that each function call operates on a fresh list, preventing the unwanted sharing of state. Let's see the corrected function in action.

[00:00:52]Now when fixed mutable default arg is called twice, each call returns a separate list with a single one.

[00:00:58]This approach prevents the list from accumulating unexpected values across function calls. Understanding this common mistake helps ensure your Python functions behave as expected.

[00:01:09]By using none and creating new objects as needed, you safeguard against unintended side effects and keep your code robust and reliable.

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