Python: Check if a string represents an int, Without using Try/Except?

Question

Hi,

Is there any way to tell whether a string represents an integer (e.g., '3', '-17' but not '3.14' or 'asfasfas') Without using a try/except mechanism?

is_int('3.14') = False
is_int('-7')   = True

Thanks,

Adam

Answer 1

If you only deal with numeric values, one method could be to compare the int() and float() casts, but this will break when testing a string. (not to mention this can be expensive on large sets)

def isinteger(x):
    return int(x) == float(x)

Answer 2

with positive integers you could use .isdigit:

>>> '16'.isdigit()
True

it doesn't work with negative integers though. suppose you could try the following:

>>> s = '-17'
>>> s.startswith('-') and s[1:].isdigit()
True

it won't work with '16.0' format, which is similar to int casting in this sense.

edit:

def check_int(s):
    if s[0] in ('-', '+'):
     return s[1:].isdigit()
    return s.isdigit()

Answer 3

Use a regular expression:

import re
def RepresentsInt(s):
    return re.match(r"[-+]?\d+$", s) is not None

If you must accept decimal fractions also:

def RepresentsInt(s):
    return re.match(r"[-+]?\d+(\.0*)?$", s) is not None

For improved performance if you're doing this often, compile the regular expression only once using re.compile().

Answer 4

Greg Hewgill's approach was missing a few components: the leading "^" to only match the start of the string, and compiling the re beforehand. But this approach will allow you to avoid a try: exept:

import re
INT_RE = re.compile(r"^[-]?\d+$")
def RepresentsInt(s):
    return INT_RE.match(str(s)) is not None

I would be interested why you are trying to avoid try: except?

Answer 5

If you're really just annoyed at using try/excepts all over the place, please just write a helper function:

def RepresentsInt(s):
    try: 
        int(s)
        return True
    except ValueError:
        return False

>>> print RepresentsInt("+123")
True
>>> print RepresentsInt("10.0")
False

It's going to be WAY more code to exactly cover all the strings that Python considers integers. I say just be pythonic on this one.