Evaluate a math expression with order of operations

MathExpression.py

""" Evaluates a math expression with the correct order of operations. """


def editExpression(inputExpr):
    """ Edit the math expression by removing spaces and enclosing it in parentheses. """
    expression = "("
    for ch in inputExpr:
        if ch != ' ':
            expression += ch
    expression += ')'
    return expression


def precedence(operator):
    """ Return the given operators precedent to ensure the correct order of operations. """
    if operator == '(':
        return 0
    elif operator == ')':
        return 1
    elif operator == '+' or operator == '-':
        return 2
    elif (operator == '*' or operator == 'x' or operator == '×') or \
            (operator == '/' or operator == '÷') or operator == '%':
        return 3
    elif operator == '^':
        return 4
    else:  # not an operator
        return -1


def getNumber(expression, index):
    """ Retrive a number from the expression string (and move the index accordingly). """
    is_neg = False
    if expression[index] == '-':
        is_neg = True
        index += 1
    if expression[index] == '+':
        index += 1
    num = 0
    divBy = 0
    while index < len(expression):
        currChar = expression[index]
        if currChar == '.':
            # The "divBy *= 10" line must be written after the if block
            divBy = 0.1  # divBy will be "1" by the end of this iteration
        elif ord('0') <= ord(currChar) and ord(currChar) <= ord('9'):
            num = (num * 10) + (ord(currChar) - ord('0'))
        else:  # not part of the number
            break
        divBy *= 10
        index += 1
    if divBy != 0:
        num /= divBy
    if is_neg:
        num *= -1
    return num, index


def operate(operator, num2, num1):
    """ Run the operator (operate) on the two given numbers; return the result. """
    if operator == '+':
        return num1 + num2
    elif operator == '-':
        return num1 - num2
    elif operator == '*' or operator == 'x' or operator == '×':
        return num1 * num2
    elif operator == '/' or operator == '÷':
        return num1 / num2
    elif operator == '%':
        return num1 % num2
    elif operator == '^':
        return pow(num1, num2)
    else:  # not an operator
        return 0


def evaluateExpression(expression):
    """ Evaluates a given math expression and returns the result """
    operators = []
    numbers = []
    index = 0
    was_oper = True
    while index < len(expression):
        if was_oper and expression[index] != '(':
            # read a number (could be negative and/or floating point)
            num, index = getNumber(expression, index)
            numbers.append(num)
            was_oper = False
            continue
        prec = precedence(expression[index])
        if expression[index] == '(':
            prec = 10  # ignoring the below while loop
        while len(operators) > 0 and prec <= precedence(operators[-1]):
            numbers.append(operate(operators.pop(-1),
                           numbers.pop(-1), numbers.pop(-1)))
        if expression[index] == ')':
            operators.pop(-1)  # '('
            was_oper = False
        else:  # regular operator or '('
            operators.append(expression[index])
            was_oper = True
        index += 1
    return numbers[0]


# Main code: handles the user interactions (input and output)
print("Welcome the my math expression calculator!")
print("If you want to stop evaluating expression enter \"q\".")
expression = editExpression(input("Enter a math expression: "))
while expression != "(q)":  # ask for user expressions until quit
    print("Your expression evaluates to:", evaluateExpression(expression))
    expression = editExpression(input("Enter a new math expression: "))