Enforcing keyword arguments in Python 3

November 27, 2018

Today I learned that in Python 3, one can enforce the use of keyword arguments in functions!

Consider a function for doing safe division. Several things can go wrong when doing division directly, and therefore we might want to use a function which could handle possible errors. One possible error is dividing by zero. This is an undefined operation, and it will raise an error. Another is that in Python, division always returns a floating point number, and Python does not support arbitrarily large floating point numbers. Thus one might get an overflow error. (See here.) We might want to introduce two flags for handling these types of errors in our division function, ignore_overflow and ignore_zero_division. By default, these should be False.

def safe_division(number, divisor, ignore_overflow=False, ignore_zero_division=False):
    try:
        return number / divisor
    except OverflowError:
        if ignore_overflow:
            return 0
        else:
            raise
    except ZeroDivisionError:
        if ignore_zero_division:
            return float('inf')
        else:
            raise

Calling this straightforwardly like safe_division(1, 0, True, False) is not very readable. We would have to check the actual function body to see what this function call would actually do. We can call it like

safe_division(1, 0, ignore_overflow=False, ignore_zero_division=True)

or even

safe_division(1, 0, ignore_zero_division=True, ignore_overflow=False)

(notice how we swapped the order here). However, specifying the keywords is only optional behavior for those calling the function.

A solution for this is to enforce using keyword arguments. This is possible to do in Python 2 by specifying a keyword argument dictionary, typically called **kwargs.

However, in Python 3, there is a simple way to enforce it! By adding a * in the function arguments, we force all succeeding arguments to be named. E.g. by having the first argument be *, we force all arguments to be named.

def safe_division(*, number, divisor, ignore_overflow, ignore_zero_division):
    try:
        return number / divisor
    except OverflowError:
        if ignore_overflow:
            return 0
        else:
            raise
    except ZeroDivisionError:
        if ignore_zero_division:
            return float('inf')
        else:
            raise

If we now call safe_division(1, 0, True, False), or even safe_division(1, 0) we would get an error:

>>> safe_division(1, 2, True, False)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: safe_division() takes 0 positional arguments but 4 were given

We now have to use it like this:

>>> safe_division(number=10**1000, divisor=3**-100,
ignore_overflow=True, ignore_zero_division=False)
0

This makes writing defensive Python easier!