For what it's worth, mutable default parameters do have their use. def foo(x, cache = {}): value = cache.get(x) if value is None:

For what it's worth, mutable default parameters do have their use. def foo(x, cache = {}): value = cache.get(x) if value is None: ... cache[x] = value return value This simple example, thankfully, is obviated by the @cache decorator as of py3.9, but I still occasionally find uses for the feature.