125 – Enforce positional arguments

You can use the forward slash / in a function signature to force arguments to be passed in as positional only. In other words, use / to disallow arguments from being passed as keyword arguments.

This useful when the names of the arguments are inconsequential. For example, in a mathematics context, p and q are very common names for polynomials, but the names themselves have no meaning. So, if you’re defining a function that adds two polynomials (using the idiom to operate on two lists of numbers), you might want p and q to only be passed in positionally:

from itertools import starmap, zip_longest
from operator import add

def add_polynomials(p, q, /):
    """Function to add two polynomials as lists of coefficients."""
    return list(starmap(add, zip_longest(p, q, fillvalue=0)))

add_polynomials(poly1, poly2)  # Ok.
add_polynomials(p=poly1, q=poly2)  # TypeError!

Another example use case is from the Cyclopts package, a package to create CLIs. In Cyclopts, you use the forward slash / to declare that a parameter is an argument and not an option.