Detecting kwarg override in Python

Question

I am looking for a way to detect whether a keyword arg was passed in explicitly, without using **kwargs.

Here is an example:

def foo(first, second=None):
    pass

If, in this function, I find that second contains None, is there a way to know whether that None was from the default, or was passed in explicitly? Basically, I have an optional argument that could conceivably be any value or type. So I either need some sort of "unique" default such that the user would never intentionally pass in that default, or I need a way to detect whether the argument was actually passed in explicitly.

I expect I could discover it by inspecting the stack, but I feel like that's overkill.

I also know I could do it this way:

def foo(first, **kwargs):
    if 'second' in kwargs:
        # Overridden!
        pass

But I would prefer not to accept **kwargs, as it makes my function signature less useful and can hide errors.

Answer 1

You can always create a unique object and use it as the default:

_default = object()

def foo(first, second=_default):
    if second is not _default:
        # Overridden!
        pass