Question

What is the most elegant and pythonic way to turn a test such as:

def is_even(n):
  return n % 2 == 0

Into a logically equivalent (but different syntactically) test such as:

if n in even_numbers:
  # assert is_even(n)
  # ...

I can think of a few ways to create the object even_numbers, but I’m interested to hear if there are nice ways I might have overlooked.

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_{Note: I’m aware this probably sound like an XY problem, so my use case is concerning the choices kwarg to argparse.ArgumentParser.add_argument, and whether it’s going to be nicer to use that or alternatively to munge the callable type kwarg.}

Answer 1

I would turn even_numbers into a container type:

class EvenNumbers(object):

    # Add other container methods as needed

    def __contains__(self, value):
        return value % 2 == 0

even_numbers = EvenNumbers()

Then,

>>> 3 in even_numbers
False
>>> 4 in even_numbers
True