How to pass an operator to a python function?

Question

I'd like to pass a math operator, along with the numeric values to compare, to a function. Here is my broken code:

def get_truth(inp,relate,cut):    
    if inp print(relate) cut:
        return True
    else:
        return False

and call it with

get_truth(1.0,'>',0.0)

which should return True.

grc · Accepted Answer · 2013-09-03T12:06:44.883

103

Have a look at the operator module:

import operator
get_truth(1.0, operator.gt, 0.0)

...

def get_truth(inp, relate, cut):    
    return relate(inp, cut)
    # you don't actually need an if statement here

edited Sep 03 '13 at 12:06

answered Sep 03 '13 at 12:01

grc

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In the end, I just got rid of my function and used operator. 'if operator.gt(1.0,0.0):' – philshem Sep 03 '13 at 12:43
Couldn't you just use `if 1.0 > 0.0:`? – grc Sep 03 '13 at 13:38
3

I want to be able to change the operator dynamically (so I may have to go back to the function solution). – philshem Sep 03 '13 at 15:28
1

for `in` operator see `operator.contains(a,b)`. Note that this checks presence of b in a not the other way round. – Ilia Gilmijarow Sep 25 '18 at 14:28

score 58 · Answer 2 · edited Jun 15 '21 at 05:12

58

Make a mapping of strings and operator functions. Also, you don't need if/else condition:

import operator


def get_truth(inp, relate, cut):
    ops = {'>': operator.gt,
           '<': operator.lt,
           '>=': operator.ge,
           '<=': operator.le,
           '==': operator.eq}
    return ops[relate](inp, cut)


print(get_truth(1.0, '>', 0.0)) # prints True
print(get_truth(1.0, '<', 0.0)) # prints False
print(get_truth(1.0, '>=', 0.0)) # prints True
print(get_truth(1.0, '<=', 0.0)) # prints False
print(get_truth(1.0, '==', 0.0)) # prints False

FYI, eval() is evil: Why is using 'eval' a bad practice?

edited Jun 15 '21 at 05:12

Michael Dorner

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answered Sep 03 '13 at 12:02

alecxe

462,703
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1,088
1,195

3

+1. Also, this answer demonstrates that you can directly return the value of `ops[relate]`, instead of explicitly testing it and returning a literal `True` or `False`. – chepner Sep 03 '13 at 12:05
1

I recommend to replace `=` with `==` – leoll2 Mar 12 '21 at 15:39
Beautiful! But please use `==` for comparison! – Michael Dorner Jun 14 '21 at 06:58

Viktor Kerkez · Answer 3 · 2013-09-03T12:07:32.033

13

Use the operator module. It contains all the standard operators that you can use in python. Then use the operator as a functions:

import operator

def get_truth(inp, op, cut):
    return op(inp, cut):

get_truth(1.0, operator.gt, 0.0)

If you really want to use strings as operators, then create a dictionary mapping from string to operator function as @alecxe suggested.

edited Sep 03 '13 at 12:07

answered Sep 03 '13 at 12:01

Viktor Kerkez

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score 1 · Answer 4 · edited May 10 '20 at 09:04

1

Use the operator module instead:

import operator
def get_truth(inp, relate, cut):
    rel_ops = {
        '>': operator.gt,
        '<': operator.lt,
        '>=': operator.ge,
        '<=': operator.le,
        '==': operator.eq,
        '!=': operator.ne
    }
    return rel_ops[relate](inp, cut)

edited May 10 '20 at 09:04

E_net4

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answered Sep 03 '13 at 12:01

Paul Evans

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@PaulEvans: because mapping `<` to `operator.lt` is trivial. – Martijn Pieters Sep 03 '13 at 12:10
Thanks @PaulEvans. I will go with the operator method for reasons desribed here: http://stackoverflow.com/questions/1832940/is-using-eval-in-python-a-bad-practice – philshem Sep 03 '13 at 12:13

score -4 · Answer 5 · answered Sep 03 '13 at 12:07

-4

>>> def get_truth(inp,relate,cut):
...     if eval("%s%s%s" % (inp,relate,cut)):
...         return True
...     else:
...         return False
...
>>> get_truth(1.0,'>',0.0)
True
>>>

answered Sep 03 '13 at 12:07

amadain

2,724
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11

No, please don't use `eval()` when better solutions are available. – Martijn Pieters Sep 03 '13 at 12:11
what is your problem with eval??? You are downvoting everybody who has given a working solution that uses eval – amadain Sep 03 '13 at 12:13
8

`eval()` poses a security risk, and is way overused as a 'solution' whenever people do not know a better option exists. – Martijn Pieters Sep 03 '13 at 12:15
**[Do not ever use `eval` (or `exec`) on data that could possibly come from outside the program in any form. It is a critical security risk. You allow the author of the data to run arbitrary code on your computer](https://stackoverflow.com/questions/1832940/why-is-using-eval-a-bad-practice). It [cannot easily be sandboxed, and proper sandboxing is harder than using a proper tool for the job.](https://stackoverflow.com/questions/3068139)** – Karl Knechtel Mar 29 '23 at 01:09

How to pass an operator to a python function?

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