How to check if a variable is an integer or a string?

Question

I have an application that has a couple of commands. When you type a certain command, you have to type in additional info about something/someone. Now that info has to be strictly an integer or a string, depending on the situation.

However, whatever you type into Python using raw_input() actually is a string, no matter what, so more specifically, how would I shortly and without try...except see if a variable is made of digits or characters?

You could do: `"0".isdigit()`. Documentation: http://docs.python.org/2/library/stdtypes.html#str.isdigit — mechanical_meat, May 10 '13 at 18:08
Checking an object's type in Python is a bad idea - it makes Python's dynamic nature less useful. *It's easier to ask for forgiveness than permission.* — Gareth Latty, May 10 '13 at 18:15
Why the aversion to `try`..`except` here? This is *exactly* what you'd use exception handling *for*. — Martijn Pieters, May 10 '13 at 18:23
isinstance (in answer below) is a good way to go in the case of just needing to know whether the content of a variable is a string or integer. This can happen when a user option can be either. — gseattle, Mar 21 '17 at 07:56

Martijn Pieters · Accepted Answer · 2014-10-10T08:54:13.317

103

In my opinion you have two options:

Just try to convert it to an int, but catch the exception:
```
try:
    value = int(value)
except ValueError:
    pass  # it was a string, not an int.
```
This is the Ask Forgiveness approach.
Explicitly test if there are only digits in the string:
```
value.isdigit()
```
str.isdigit() returns True only if all characters in the string are digits (0-9).

The unicode / Python 3 str type equivalent is unicode.isdecimal() / str.isdecimal(); only Unicode decimals can be converted to integers, as not all digits have an actual integer value (U+00B2 SUPERSCRIPT 2 is a digit, but not a decimal, for example).

This is often called the Ask Permission approach, or Look Before You Leap.

The latter will not detect all valid int() values, as whitespace and + and - are also allowed in int() values. The first form will happily accept ' +10 ' as a number, the latter won't.

If your expect that the user normally will input an integer, use the first form. It is easier (and faster) to ask for forgiveness rather than for permission in that case.

edited Oct 10 '14 at 08:54

answered May 10 '13 at 18:12

Martijn Pieters

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The are other options. Better say that those two options are just what you would recommend yourself. – Alexei Sholik May 10 '13 at 18:18
@android: The other options are overkill for most situations. Yet, quantified it. – Martijn Pieters May 10 '13 at 18:20
The problem is that I don't really understand try...except, so if you could explain it more thoroughly, that would be great. – Gloripaxis May 10 '13 at 19:59
1

@user2154354: Operations can throw exceptions; these cut through the normal flow of code, they 'fall down' through the stack; functions immediately return, until something 'catches' the exception. The `try:`..`except:` statement does the catching. `int()` can throw a `ValueError` exception, but we specifically catch it again to detect that the string was not convertable to an integer. See the [Python tutorial](http://docs.python.org/2/tutorial/errors.html) for more details. – Martijn Pieters May 10 '13 at 20:07
@Veedrac: indeed, pointed out now. – Martijn Pieters Oct 10 '14 at 08:55
`isdigit()` retunrs `False` for negative numbers – Mawg says reinstate Monica Apr 14 '16 at 14:48
@Mawg: yes, which is why I was explicit about it only returning true if all characters are digits. `-` is not a digit. – Martijn Pieters Apr 14 '16 at 17:33
True. But it only works for positive integers, not all integers :-( – Mawg says reinstate Monica Apr 14 '16 at 19:22

score 38 · Answer 2 · answered May 10 '13 at 18:11

38

if you want to check what it is:

>>>isinstance(1,str)
False
>>>isinstance('stuff',str)
True
>>>isinstance(1,int)
True
>>>isinstance('stuff',int)
False

if you want to get ints from raw_input

>>>x=raw_input('enter thing:')
enter thing: 3
>>>try: x = int(x)
   except: pass

>>>isinstance(x,int)
True

answered May 10 '13 at 18:11

TehTris

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Use basestring instead of str, to catch unicode, see http://stackoverflow.com/questions/1979004/what-is-the-difference-between-isinstanceaaa-basestring-and-isinstanceaaa – gseattle Mar 21 '17 at 07:39
2

That's perfect for me, it works for positive and negative numbers ... cool, thanks – A.Joly Aug 03 '17 at 14:36
one liner: `print(isinstance('stuff',int))` – Shōgun8 Nov 02 '22 at 04:52

score 8 · Answer 3 · answered May 10 '13 at 18:08

8

The isdigit method of the str type returns True iff the given string is nothing but one or more digits. If it's not, you know the string should be treated as just a string.

answered May 10 '13 at 18:08

jwodder

54,758
12
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2

isdigit() retunrs False for negative numbers – Mawg says reinstate Monica Apr 14 '16 at 14:48

score 2 · Answer 4 · edited May 23 '17 at 12:02

2

Depending on your definition of shortly, you could use one of the following options:

try: int(your_input); except ValueError: # ...
your_input.isdigit()
use a regex
use parse which is kind of the opposite of format

edited May 23 '17 at 12:02

Community

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answered May 10 '13 at 18:16

Alexei Sholik

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score 1 · Answer 5 · answered May 10 '13 at 18:12

1

Don't check. Go ahead and assume that it is the right input, and catch an exception if it isn't.

intresult = None
while intresult is None:
    input = raw_input()
    try: intresult = int(input)
    except ValueError: pass

answered May 10 '13 at 18:12

Marcin

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How to check if a variable is an integer or a string?

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