(Python) What easiest way to detect whether an optional argument is actually passed to function?

Question

A function in Python:

def f(a=1):
  print(a)

What easiest way to detect whether the argument «a» is actually passed to f or is missed?

There is a function «IsMissing» in VBA:

IsMissing(argname)

Does anything similar exist in Python?

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I mean something like this:

def f(a=1):
  …
  …


f()
>>> The argument «a» was not passed!
f(1)
>>> The argument «a» was passed and is equal to: 1