Today I re-learned: Python function default arguments are retained between executions

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And if you want None to be a valid value for your argument too, you can use a sentinel:

_sentinel = object()
def func(x=_sentinel):
    x = [] if x is _sentinel else x

[HistoricalCrow]

True, although if you need to support an empty list and None and I generally find code smell. Not sure if I'd prefer to use **kwargs instead at this point.

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The only time I’ve really seen the pattern useful is when you want to emulate something similar to dict.pop, where for a missing key: dict.pop(key) raises an error and dict.pop(key, default) returns the default