Pythonic way of ensuring that input argument to a function is int/str?

Question

Basically the argument that foo() expects should be passed as int, but there is a possibility that someone will pass it as str (which is also valid if str can be converted to int). This is what I came up with:

def foo(input_argument):
    func_name = 'foo'

    if type(input_argument) is not int or type(input_argument) is not str:
        print(
            '%s: "input_argument" expects int/str, not %s' % (
                func_name,
                type(input_argument)
            )
        )
        return None
    try:
        input_argument= int(input_argument)
    except:
        print(
            '%s: "input_argument" expects number in str/int format' % func_name
        )
        return None

Is there something that is built-in which could simplify this in a more pythonic way?

Edit: boolean type should be treated as invalid

Frankly I'd just simplify that to `input_argument = int(input_argument)`. That's a no-op if it's already an int, parses a string if possible, throws a ValueError if it's a string but not one that can be parsed to an int, or throws a TypeError. And you should pretty much never use bare `except:`. — jonrsharpe, Aug 30 '19 at 08:12
@jonrsharpe maybe i should've stated that True is not a valid type of input argument also — Sir DrinksCoffeeALot, Aug 30 '19 at 08:18
Well, technically it is, but looking from a "functional" way it isn't — Sir DrinksCoffeeALot, Aug 30 '19 at 08:23

score 1 · Answer 1 · answered Aug 30 '19 at 08:19

1

You could eventually use some data validation library, like Cerberus, but like jonsharpe said in a comment, the common way is to let python handle the error by simply try to convert the input into an integer.

Just do:

def foo(input_argument):
    input_argument= int(input_argument)
    # ... your method

Take a look there for more on the subject: https://stackoverflow.com/a/154156/4279120

answered Aug 30 '19 at 08:19

olinox14

6,177
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Our custom environment for running these scripts doesn't really support duck typing, it breaks if exception occurs so we have to handle these exceptions by ourselves, but I do get the point! – Sir DrinksCoffeeALot Aug 30 '19 at 08:21

daveoncode · Answer 2 · 2019-08-30T08:34:16.433

1

You can use type hints to get IDE support for types (the IDE will tell you if you are passing a wrong type)... anyway nothing will prevent to pass a wrong type at runtime, so you can check it as in the following snippet and raise a ValueError if the received object is not the expected one:

def foo(input: Union[int, str]):
    if not isinstance(input, (int, str)):
        raise ValueError(f'Invalid input, expected int or str, got: "{type(input)}"')

    # ...implementation

edited Aug 30 '19 at 08:34

answered Aug 30 '19 at 08:21

daveoncode

18,900
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The post is tagged with python 2.7; the syntax you use here is Python 3 only. Mypy and PyCharm support hints in comments, use those. – Martijn Pieters Aug 30 '19 at 08:59

Tim Stack · Answer 3 · 2019-08-30T08:26:10.983

I think you're overcomplicating things

import sys

def foo(input_argument):
    try:
        input_argument = int(input_argument)
    except Exception:
        print('Unexpected error occured: %s' % sys.exc_info()[1])

or with better error handling

def foo(input_argument):
    try:
        input_argument = int(input_argument)
    except ValueError:
        print('That is not a number, but a string')
    except TypeError:
        print('That is not a number')

Pythonic way of ensuring that input argument to a function is int/str?

3 Answers3