shunting-yard parser in python3

calc.py

import math
import operator
import re

functions = {
    "sqrt": math.sqrt,
    "lg": math.log10,
    "ln": math.log,
    "log": math.log,
    }

operators = {
    "**": (operator.pow, 30),
    "*": (operator.mul, 20),
    "/": (operator.truediv, 20),
    "+": (operator.add, 10),
    "-": (operator.sub, 10),
    }

def _parse(expr="", result=[], op_stack=[]):
    tokens = [m[0] for m in re.findall(r'(\d+(\.\d+)?|\+|\-|\*+|\/|\w+|\(|\)|,)', expr)]

    for token in tokens:
        if re.match(r'\d+(\.\d+)?', token):
            result.append(float(token))
        elif token in functions.keys() or token == "(":
            op_stack.append(token)
        elif token in operators.keys():
            while op_stack and op_stack[-1] in operators.keys() and (operators[op_stack[-1]][1] > operators[token][1] or (token != "**" and operators[op_stack[-1]][1] == operators[token][1])):
                result.append(op_stack.pop())
            op_stack.append(token)
        elif token == ")":
            while op_stack[-1] != "(":
                result.append(op_stack.pop())
            if op_stack[-1] == "(":
                op_stack.pop()
            else:
                raise "mismatched parentheses"
            if op_stack and op_stack[-1] in functions.keys():
                result.append(op_stack.pop())
        elif token == ",":
            while op_stack[-1] != "(":
                result.append(op_stack.pop())
        else:
            raise "wtf"

    while op_stack:
        if op_stack[-1] == "(":
            raise "mismatched parentheses"
        result.append(op_stack.pop())

    return result

def _compute(seq, op_stack=[]):
    for item in seq:
        if isinstance(item, float):
            op_stack.append(item)
        elif item == "log":
            op2 = op_stack.pop()
            op1 = op_stack.pop()
            op_stack.append(functions["log"](op1, op2))
        elif item in operators.keys():
            op2 = op_stack.pop()
            op1 = op_stack.pop()
            op_stack.append(operators[item][0](op1, op2))
        elif item in functions.keys():
            op_stack.append(functions[item](op_stack.pop()))
        else:
            raise "wtf"

    return op_stack.pop()

def calc(expr=""):
    return _compute(_parse(expr))

print(calc("2+2*2+lg(10)-log((2+2)**3**2, 16)"))
print(calc("log(10*10, 10)"))
print(calc("4**3**2"))