Setting the default value of a function input to equal another input in Python

def example(a, b, c=None):
    if c is None:
        c = a
    ...

The default value for the keyword argument can't be a variable (if it is, it's converted to a fixed value when the function is defined.) Commonly used to pass arguments to a main function:

def main(argv=None):
    if argv is None:
        argv = sys.argv

If None could be a valid value, the solution is to either use *args/**kwargs magic as in carl's answer, or use a sentinel object. Libraries that do this include attrs and Marshmallow, and in my opinion it's much cleaner and likely faster.

missing = object()

def example(a, b, c=missing):
    if c is missing:
        c = a
    ...

The only way for c is missing to be true is for c to be exactly that dummy object you created there.

This general pattern is probably the best and most readable:

def exampleFunction(a, b, c = None):
    if c is None:
        c = a
    ...

You have to be careful that None is not a valid state for c.

If you want to support 'None' values, you can do something like this:

def example(a, b, *args, **kwargs):
    if 'c' in kwargs:
        c = kwargs['c']
    elif len(args) > 0:
        c = args[0]
    else:
        c = a