mutable argument in function definition

[deanhystad]

This is what you are doing:

default_target = []
def append_to(item, target=default_target):
    target.append(item)
    return target

When the module is first imported, a list object is created to serve as the default value for target. I explicitly do that in my example above, but functionally it is the same as what happens in your example. If you want a different list each time the function is called you need to do this:

def append_to(item, target=None):
    return [item] if target is None else target.append(item)

But you should reconsider wanting to do this at all. I'm having a hard time thinking of a use case where I want a default mutable argument that is returned as the value of the function.