Could you expand on this? For example, this works just fine:

    def silly_append(item, orig=[]):
       return orig + [item]

Edit: Oh, I think you probably mean in cases where you're mutating the input list.

I assume you refer to the `is None` idiom. That happens often enough, but I count it as exactly one use case, and I think it's usually poorly considered anyway. Again, you probably don't actually want the default value to be an empty list, because it doesn't make a lot of sense to mutate something that the caller isn't actually required to provide (unless the caller never provides it and you're just abusing the default-argument behaviour for some kind of cache).

Using, for example, `()` as a default argument, and cleaning up your logic to not do those mutations, is commonly simpler and more expressive. A lot of the community has the idea that a tuple should represent heterogeneous fixed-length data and a list should be homogeneous; but I consider (im)mutability to be a much more interesting property of types.