Infix Expression Evaluation

infix_expression_evaluation.py

def evaluate(e):
    expr = list(e)
    stack = []
    num=""
    
    while len(expr) > 0:
        c = expr.pop(0)
        if c in "0123456789." or (c is "-" and (len(stack) == 0 or str(stack[-1]) in "+-*/")):
            num += c
        else:
            if num:
                stack.append(num)
                num = ""
            
            if c in "+-*/":
                stack.append(c)
            elif c == ")" and len(stack) >= 3:

                num2 = stack.pop()
                operator = stack.pop()
                num1 = stack.pop()
                
                if operator is "+":
                    stack.append(float(num1) + float(num2))
                elif operator is "-":
                    stack.append(float(num1) - float(num2))
                elif operator is "/":
                    if num2 == 0:
                        return "ERROR"
                    stack.append(float(num1) / float(num2))
                elif operator is "*":
                    stack.append(float(num1) * float(num2))
            
    if len(stack) > 1:
        return "ERROR"

    result = stack.pop()
    if result.is_integer():
        return int(result)
    else:
        return result