In Python the eval function takes two additional parameters beyond the expression parameter, namely locals and globals which populate the variables the expression has access to, so a simple solution is just to pass your kwargs to eval's locals and everything works as expected.

import re

def calc(function, **kwargs):
    if re.fullmatch(r'[0-9a-z+\-*/()]+', function) != None:
        return eval(function, locals=kwargs)
    return None

print(calc('x+y', x=10, y=20)) # Outputs 30

Obligatory warning on never using eval nor relying on regex as a security measure, but this works perfectly for a small concept as this.

You can read more about globals and locals in python here: What's the difference between globals(), locals(), and vars()?