Extracting from **kwargs

I am writing a pretty basic calculator for functions (for example f(x)=x+5), and the first step is writing a quick function to calculate the result if we already know all the variables. Source code is:

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

But I need to add a script that changes the function to add in the values instead of the variables. For example, if I call my calc() as calc('x+5*y', x=2, y=7), it should change the function to simply '2+5*7', so that I could put it into eval().

Does anybody have any ideas on how this would be possible, or what are some alternative methods for this?

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()?