Example: Parsing arithmetic operations with the re library.

#!/usr/bin/env python3

import re
import string

def remove_whitespace(in_string: str):
    return in_string.translate(str.maketrans(dict.fromkeys(string.whitespace)))

def parse_operation(expression):
    # convert to string
    ss = f"{expression}"
    ss = remove_whitespace(ss)

    # passed value is a simple int or float
    try:
        return int(ss)
    except Exception:
        pass

    try:
        return float(ss)
    except Exception:
        pass

    # passed value is either int or float followed by a %
    match = re.fullmatch(r"([\d]+(\.[\d]+)?)%", ss)
    if match:
        try:
            return int(match.group(1))
        except Exception:
            return float(match.group(1))

    # passed value is (int/float)(operator)(int/float)
    match_list = []
    match_list.append(re.fullmatch(r"([\d]+)%?([+\-*/])([\d]+)%?", ss))
    match_list.append(re.fullmatch(r"([\d]+\.[\d]+)%?([+\-*/])([\d]+\.[\d]+)%?", ss))


    for match in match_list:
        if match:
            try:
                x = int(match.group(1))
                y = int(match.group(3))
            except Exception:
                x = float(match.group(1))
                y = float(match.group(3))

            operator = match.group(2)

            if operator == '+':
                return x + y
            elif operator == '-':
                return x - y
            elif operator == '*':
                return x * y
            elif operator == '/':
                return x / y

    return None



l1 = ['0', '10+7', '20', '30', '40+20', '6']
l2 = [0, '10+7', 20, 30, '40+20', 6]

l1_result = [parse_operation(x) for x in l1]
l2_result = [parse_operation(x) for x in l2]

print("l1", l1_result)
print("l2", l2_result)


l3 = ['10%', '20%', 5, 14, 16]
l4 = ['10%+30%', '20%-7%', '5', '14', '16']

l3_result = [parse_operation(x) for x in l3]
l4_result = [parse_operation(x) for x in l4]

print("l3", l3_result)
print("l4", l4_result)


l5 = ['10% - 90%', '15.99 + 33.0', '12 / 3', '99 / 34', '25 * 4', '5', '6', '9.99']
l5_result = [parse_operation(x) for x in l5]

print("l5", l5_result)