Python simple calculator

calculator.py

def add(x, y):
    return x + y


def sub(x, y):
    return x - y


def mul(x, y):
    return x * y


def div(x, y):
    return x / y


ops = {'+': add, '-': sub, '*': mul, '/': div}


def tokens_to_numbers(tokens):
    for i in range(len(tokens)):
        if is_number(tokens[i]):
            tokens[i] = float(tokens[i])

    return tokens


def parse(operation):
    operation = operation.replace(' ', '')

    tokens = []

    for i in range(len(operation)):
        if operation[i].isdigit() and i > 0 and (tokens[-1].isdigit()
                                             or tokens[-1][-1] == '.'):
            tokens[-1] += operation[i]
        elif operation[i] == '.' and i > 0:
            tokens[-1] += operation[i]
        else:
            tokens.append(operation[i])

    tokens = tokens_to_numbers(tokens)

    return tokens


def find_index(tokens, operator):
    if operator in tokens:
        return tokens.index(operator)
    else:
        return 0


def lower(x, y):
    if x == 0:
        return y

    if y == 0:
        return x
        
    if x > y:
        return y
    else:
        return x


def find_next(tokens):
    precedence = lower(find_index(tokens, '*'), find_index(tokens, '/'))
    
    if precedence == 0:
        precedence = lower(find_index(tokens, '+'), find_index(tokens, '-'))

    return precedence
    

def calculate(tokens):
    next_operation = find_next(tokens)

    while(len(tokens) > 1):
        next_operation = find_next(tokens)
        result = ops[tokens[next_operation]](tokens[next_operation - 1], tokens[next_operation + 1])

        tokens[next_operation] = result
        tokens.pop(next_operation + 1)
        tokens.pop(next_operation - 1)

    return tokens[0]


def is_number(value):
  try:
    float(value)
    return True
  except:
    return False


def calculator(operation):
    tokens = parse(operation)

    return calculate(tokens)


def main():
    operation = input('Enter the operation to solve: ')

    print(calculator(operation))


if __name__ == '__main__':
    main()