Parsing a addition/subtraction/multiplication/division sign from a string

Question

I went through a lesson of creating a simple calculator using Python, and I'm trying to simplify it. The issue is as follows: I know that it is possible to parse a string (number) into a float using "float()", but I'm trying to parse a addition/subtraction/multiplication/division sign from a string into a float or integer or any other format that would perform the action. Here's a sample of what I'm trying to do:

while True:
    user_input = input("""
        quit - exit program
        add - addition
        sub - subtraction
        mul - multiplication
        div - division
        Please choose function:""")
    actions = ("+-*/")
    user_input_1 = float(input("first number:"))
    user_input_2 = float(input("second number:"))
    operation = user_input_1 float(action) user_input_2
    if user_input == "add":
        action = actions[0]
        answer = operation
        print (answer)

If the user_input is "add" user_input_1 is "5" user_input_2 is "7" then the print(answer) should result in 12

This is just the first part of the code, and I'm getting a syntax error. The issue is most likely the inability to parse the addition sign using "float()". Is there another way of parsing the signs?

Answer 1

Even though you should be posting the full traceback, the SyntaxError comes from this line:

operation = user_input_1 float(action) user_input_2

It is neither a valid Python expression nor a statement.

A solution that doesn't involve eval: You can use the operator module and a dictionary from "sign" to the operator itself, as seen in this very basic (and error prone) example:

import operator

operations_dict = {'+': operator.add,
                   '-': operator.sub} # extend to other operators you see fit

a = float(input('first num'))
b = float(input('second_num'))
sign = input('operator')
print(operations_dict[sign](a, b))
>> first num
   1
>> second num
   2
>> operator
   +
>> 3.0

Answer 2

You can use eval in the form eval('1 + 2'), which will give you desired result. You can read more on eval here https://docs.python.org/2/library/functions.html#eval

but please keep in mind the following statement

The user can expose hidden values in the program, or call a dangerous function (dangerous_function("/etc/passwd")). Of course in most cases (desktop programs) the user can't do any more than they could do by writing their own python script, but in some applications (web apps, kiosk computers), this could be a risk.

which is from http://lybniz2.sourceforge.net/safeeval.html

Answer 3

In your case this should work :

def operation(action): 
    return user_input_1 + action + user_input_2

if user_input == "add":
    action = actions[0]
    answer = operation(action)
    print (eval(answer))

but that's not the best way to do the operations. You can simply add or divide like this or you can construct a dictionary for the operations:

def operation(action): 
    if(action == "add"):
        return user_input_1 + user_input_2
    elif action == "sub":
        return user_input_1 - user_input_2
    elif action == "div":
        return user_input_1 / user_input_2
    # and so on....

answer = operation(user_input)
print (answer)

Answer 4

I do not recommend using eval() however; setting some rules won't hurt using eval(). This is really basic calculator and its easy to understand.

allowed_characters = "0123456789+-/*= "
print("""
Basic calculator application.
Operators:
    +   addition
    -   subtraction
    *   multiplication
    /   division
Type the desired operation and press ENTER. 
(Example 23 and 46 to multiple
Type 23 * 46 then press ENTER.)
""")
while True:
    data = input("Please enter your calculation: ")
    if data == "q":
        print("Exiting...")
        break
    for s in data:
        if s not in allowed_characters:
            print("Invalid character!")
            quit()
    calculate = eval(data)
    print(calculate)