Testing if Python string variable holds number (int,float) or non-numeric str?

Question

If a Python string variable has had either an integer, floating point number or a non-numeric string placed in it, is there a way to easily test the "type" of that value?

The code below is real (and correct of course):

>>> strVar = "145"
>>> print type(strVar)
<type 'str'>
>>>

but is there a Python function or other method that will enable me to return 'int' from interrogating strVar set as above

Perhaps something like the nonsense code and results below ...

>>> print typeofvalue(strVar)
<type 'int'>

or more nonsense:

>>> print type(unquote(strVar))
<type 'int'>

Huh... Does var = "Hello 20" "contain a number"?

NullUserException
– NullUserException

2011-10-07 03:39:17 +00:00
Commented Oct 7, 2011 at 3:39 — NullUserException
– NullUserException, Commented Oct 7, 2011 at 3:39

JBernardo · Accepted Answer · 2011-10-07 03:55:32Z

12

import ast
def type_of_value(var):
    try:
       return type(ast.literal_eval(var))
    except Exception:
       return str

Or, if you only want to check for int, change the third line to block inside try with:

int(var)
return int

edited Oct 7, 2011 at 3:55

answered Oct 7, 2011 at 3:41

JBernardo

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4 Comments

Thomas Over a year ago

Just beat me, but this is probably better.

Sam Dolan Over a year ago

You should just catch ValueError otherwise you'll swallow the NameError when ast isn't imported. +1 nonetheless.

JBernardo Over a year ago

@sdolan The problem is that literal_eval may raise many different errors... I'll import ast :)

Demolishun Over a year ago

Like @sdolan pointed out, but also look at catching SyntaxError: "This function raises SyntaxError if the compiled source is invalid, and TypeError if the source contains null bytes." This is from the docs of compile() which is what this function calls.

Michael Hoffman · Accepted Answer · 2011-10-07 04:10:34Z

6

I'd do it like this:

def typeofvalue(text):
    try:
        int(text)
        return int
    except ValueError:
        pass

    try:
        float(text)
        return float
    except ValueError:
        pass

    return str

answered Oct 7, 2011 at 4:10

Michael Hoffman

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Comments

Rafael Ferreira · Accepted Answer · 2011-10-07 03:47:26Z

4

use .isdigit():

In [14]: a = '145'

In [15]: b = 'foo'

In [16]: a.isdigit()
Out[16]: True

In [17]: b.isdigit()
Out[17]: False

In [18]:

answered Oct 7, 2011 at 3:47

Rafael Ferreira

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2 Comments

PolyGeo Over a year ago

Thanks Rafael - have not tested because I did not recognise some of your syntax - but trust that it would work if I did. As it turns out my integer and floats could be negatives (latitude/longitude).

Wolf Over a year ago

My fav (for simple int vs. str cases). But maybe the Inand Out stuff does more harm than good.

Borealis · Accepted Answer · 2016-08-21 21:10:23Z

2

You can check if a variable is numeric using the built-in isinstance() function:

isinstance(x, (int, long, float, complex))

This also applies to string and unicode literal types:

isinstance(x, (str, unicode))

For example:

def checker(x):
    if isinstance(x, (int, long, float, complex)):
        print "numeric"
    elif isinstance(x, (str, unicode)):
        print "string"

>>> x = "145"
>>> checker(x)
string
>>> x = 145
>>> checker(x)
numeric

edited Aug 21, 2016 at 21:10

answered Aug 21, 2016 at 15:22

Borealis

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Comments

whitey04 · Accepted Answer · 2011-10-07 03:51:36Z

0

I'd use a regular expression

def instring (a):

  if re.match ('\d+', a):
    return int(a)
  elsif re.match ('\d+\.\d+', a):
    return float(a)
  else:
    return str(a)

answered Oct 7, 2011 at 3:51

whitey04

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4 Comments

JBernardo Over a year ago

you should add ^[+-]? and $ to your regexes... If OP wants scientific notation like 3e+12 you'll have to add ([eE][+-]\d+)?

whitey04 Over a year ago

@JBernardo: point taken... Also this sounds like a user input; so there should probably be more input validation leading to regexp than just "what type is it".

PolyGeo Over a year ago

Thanks whitey04 - have not tested because I did not recognise some of your syntax - but trust that it would work if I did.

Karl Knechtel Over a year ago

I can hardly imagine a less Pythonic approach to the task.

Ahmed Roshdy · Accepted Answer · 2017-09-10 19:32:01Z

0

Here is a short hand to do it without ast import

    try:
        if len(str(int(decdata))) == len(decdata): return 'int'
    except Exception:
        return 'not int'

Of course 's' is the string you want to evaluate

edited Sep 10, 2017 at 19:32

answered Sep 10, 2017 at 19:25

Ahmed Roshdy

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