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def mutable(x, l = []):
    l.append(x)
    print(f"My values {l=}")
>>> mutable("Example 1")
My values l=['Example 1']
>>> mutable("Example 2")
My values l=['Example 1', 'Example 2']

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Python Anti-Patterns: Mutable Arguments

Never make a function argument a mutable default, like a list or dictionary. Function arguments are evaluated once on module load. That means you get the same object every time you call the function, like the example above shows.

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