How to store and use mathematical operators as python variable

Question

I am new to python and was trying to write something like this below (code A) so it does eactly like code B. I want to make use of user input of mathematical operators as do_what variable. How can we write this code (A) in python so it would work like code B?

code A

num1 = input("Enter a number: ")
num2 = input("Enter another number: ")
do_what = input("Enter a calculation symbol for calculation you want to perform: ")
result = float(num1) do_what float(num2)
print("result is: " + str(result))

code B

num1 = input("Enter a number: ")
num2 = input("Enter another number: ")
result = int(num1) + int(num2)
print("result is: " + str(result))

Not sure what infix means, but it could be variable- +, -, X etc — MAPK, Feb 06 '19 at 17:35

score 10 · Accepted Answer · answered Feb 06 '19 at 17:38

You can use the operator module for common operators and make a lookup dictionary to map symbols to functions. If you want operators not in that module, you can simply define custom functions and add look them up the same way:

import operator

operatorlookup = {
    '+': operator.add,
    '-': operator.sub,
    '*': operator.mul,
    '/': operator.truediv
}

num1 = input("Enter a number: ")
num2 = input("Enter another number: ")
do_what = input("Enter calculation symbols for calculation you want to perform: ")

op = operatorlookup.get(do_what)

if op is not None:
    result = op(float(num1), float(num2))
else:
    result = "Unknown operator"
print("result is: " + str(result))

Sheldore · Answer 2 · 2019-02-06T17:50:59.243

6

You might also be interested in knowing about the inbuilt function eval. This reduces the if and else loops to a single statement for this particular example of yours

num1 = input("Enter a number: ")
num2 = input("Enter another number: ")
do_what = input("Enter calculation symbols for calculation you want to perform: ")

result = eval(num1 + do_what + num2)
print("result is: %s" %result)

# Enter a number: 3
# Enter another number: 18
# Enter calculation symbols for calculation you want to perform: *
# result is: 54

EDIT

To make eval slightly safe in this particular case, you can perhaps use something like this

if do_what in ['+', '-', '*', '**', '/']:
    result = eval(num1 + do_what + num2)
    print("result is: %s" %result)
else:
    print ("Enter an arithmetic operator")

edited Feb 06 '19 at 17:50

answered Feb 06 '19 at 17:40

Sheldore

37,862
7
57
71

I have seen a bunch of times where `eval` is not recommended but it seems to be getting the job done. Are there downsides that one needs to worry about? – hqkhan Feb 06 '19 at 17:45
@hqkhan: Have a look at [this](https://stackoverflow.com/a/9384005/4932316) answer. As per one comment, it seems that as long as your `input` is not directly within `eval()`, you are fine – Sheldore Feb 06 '19 at 17:47
@hqkhan: Also, [this](https://stackoverflow.com/questions/35804961/python-eval-is-it-still-dangerous-if-i-disable-builtins-and-attribute-access) is worth reading – Sheldore Feb 06 '19 at 17:47
1

@hqkhan: Check my edit. That makes it slightly safer – Sheldore Feb 06 '19 at 17:51

score 2 · Answer 3 · answered Feb 06 '19 at 17:35

2

num1 = input("Enter a number: ")
num2 = input("Enter another number: ")
do_what = input("Enter calculation symbols for calculation you want to perform: ")
if do_what=='+':
    result = float(num1) + float(num2)
elif do_what=='-':
    result = float(num1) - float(num2)
elif do_what=='*':
    result = float(num1) * float(num2)
elif do_what=='/':
    result = float(num1) / float(num2)
print("result is: " + str(result))

answered Feb 06 '19 at 17:35

Igor Dragushhak

567
1
3
14

Thanks! this works, but would be nice to just replace the symbol with `do_what` – MAPK Feb 06 '19 at 17:38

score 0 · Answer 4 · answered Feb 06 '19 at 17:42

If security is not your concern you could use eval to achieve this

result = eval(str(num1) + do_what + str(num2))

The problem is that eval literally evaluates everything you write as python code, so don't do this if you mean for this to be used by others

How to store and use mathematical operators as python variable

4 Answers4