Default is not set

Usually for optional params we use None as default in the function signature. If it is None we assume the user did not specify one.

def main(param=None, **kwargs):
    if param is None:
        param = get_system_default()
    # ...


main(param='foo')  # use 'foo'
main()             # dynamically set param

But what if None is a possible value too?

main(param=None)   # OOPS, will calculate anther value for param

The parameter still needs to be optional, because we can calculate a good default if it is not set, but how can we know if an optional parameter was explicitely set by the caller?

One way would be make it a required parameter and expose the get_system_default() function. We then change the signature of our main() so that Python ensures param is always set.

def main(param, **kwargs):
    # ...


main(param='foo')                      # use 'foo'
main(param=get_system_default())       # dynamically set param

With this approach we delegated that responsabilty to the user of our function, this is style and is perfectly feasable. But if main() is called a lot, we complicated things for our user.

Another trick is to use the **kwargs dict and check if a param is there or not.

def main(*args, **kwargs):
    if 'param' in kwargs:
        param = kwargs['param']
    else:
        param = get_system_default()
    # ...

This also works fine, but we kind of loose visibility on what parameters main() accepts. This is a bummer if you are using autocomplete tools of other helpers that analyse the code.

So my last trick kind of mitigates that by introducing a custom default value, the NotSet type.

class NotSet():
    pass


NOT_SET = NotSet()


def main(param=NOT_SET, **kwargs):
    if param is NOT_SET:
        param = get_system_default()
    # ...

This is a unique type that won't collide with any other possible value for param. If it is NOT_SET, then it is not set :)

I kind of like the first solution, but if you think that you will end up with lots of get_system_default() calls everywhere in your code, at some point switching to this NOT_SET approach would make sense.