Web Reference: Python’s default arguments are evaluated once when the function is defined, not each time the function is called (like it is in say, Ruby). This means that if you use a mutable default argument and mutate it, you will and have mutated that object for all future calls to the function as well. Default variables are evaluated at function definition (which happens the first time it is called), and NOT each time the function is called. Consequently, if you mutate a mutable default argument any subsequent function call can only use the mutated object. Jan 25, 2026 · Learn why mutable default arguments in Python cause unexpected behavior and how to fix this common bug with the None pattern.
YouTube Excerpt: From "The Hitchhiker's Guide to Python" https://docs.python-guide.org/writing/gotchas/
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