How can I check if a string represents an int without using try except

Question

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

User · Answer

Here is a function that parses without raising errors. It handles obvious cases returns None on failure (handles up to 2000 '-/+' signs by default on CPython!):

#!/usr/bin/env python

def get_int(number):
    splits = number.split('.')
    if len(splits) > 2:
        # too many splits
        return None
    if len(splits) == 2 and splits[1]:
        # handle decimal part recursively :-)
        if get_int(splits[1]) != 0:
            return None

    int_part = splits[0].lstrip("+")
    if int_part.startswith('-'):
        # handle minus sign recursively :-)
        return get_int(int_part[1:]) * -1
    # successful 'and' returns last truth-y value (cast is always valid)
    return int_part.isdigit() and int(int_part)

Some tests:

tests = ["0", "0.0", "0.1", "1", "1.1", "1.0", "-1", "-1.1", "-1.0", "-0", "--0", "---3", '.3', '--3.', "+13", "+-1.00", "--+123", "-0.000"]

for t in tests:
    print "get_int(%s) = %s" % (t, get_int(str(t)))

Results:

get_int(0) = 0
get_int(0.0) = 0
get_int(0.1) = None
get_int(1) = 1
get_int(1.1) = None
get_int(1.0) = 1
get_int(-1) = -1
get_int(-1.1) = None
get_int(-1.0) = -1
get_int(-0) = 0
get_int(--0) = 0
get_int(---3) = -3
get_int(.3) = None
get_int(--3.) = 3
get_int(+13) = 13
get_int(+-1.00) = -1
get_int(--+123) = 123
get_int(-0.000) = 0

For your needs you can use:

def int_predicate(number):
     return get_int(number) is not None

User · Answer

The proper RegEx solution would combine the ideas of Greg Hewgill and Nowell  but not use a global variable  You can accomplish this by attaching an attribute to the method  Also  I know that it is frowned upon to put imports in a method  but what I m going for is a  lazy module  effect like http   peak telecommunity com DevCenter Importing lazy-imports  edit  My favorite technique so far is to use exclusively methods of the String object      usr bin env python    Uses exclusively methods of the String object def isInteger i       i   str i      return i   0  or  i if i find        gt  -1 else i lstrip  -    rstrip  0   rstrip       isdigit      Uses re module for regex def isIntegre i       import re     if not hasattr isIntegre    re            print  I compile only once  Remove this line when you are confident in that            isIntegre  re   re compile r  -    d    0           return isIntegre  re match str i   is not None    When executed directly run Unit Tests if   name         main         for obj in                     integers                 0  1  -1  1 0  -1 0                   0    0    0 0    1    -1     1    1 0    -1 0     1 0                     non-integers                 1 1  -1 1   1 1    -1 1     1 1                    1 1 1    1 1 0    1 0 1    1 0 0                    1 0     1  0    1                      0 0     0  0    0                      one   object     1 2 3    1 2 3     one   two                            Notice the integre uses  re   intended to be humorous          integer     an integer  if isInteger obj  else  NOT an integer           integre     an integre  if isIntegre obj  else  NOT an integre             Make strings look like strings in the output         if isinstance obj  str               obj       s      obj            print   30s is  14s is  14s     obj  integer  integre     And for the less adventurous members of the class  here is the output   I compile only once  Remove this line when you are confident in that                               0 is     an integer is     an integre                              1 is     an integer is     an integre                             -1 is     an integer is     an integre                            1 0 is     an integer is     an integre                           -1 0 is     an integer is     an integre                             0  is     an integer is     an integre                            0   is     an integer is     an integre                           0 0  is     an integer is     an integre                             1  is     an integer is     an integre                            -1  is     an integer is     an integre                             1  is     an integer is     an integre                           1 0  is     an integer is     an integre                          -1 0  is     an integer is     an integre                           1 0  is     an integer is     an integre                            1 1 is NOT an integer is NOT an integre                           -1 1 is NOT an integer is NOT an integre                           1 1  is NOT an integer is NOT an integre                          -1 1  is NOT an integer is NOT an integre                           1 1  is NOT an integer is NOT an integre                         1 1 1  is NOT an integer is NOT an integre                         1 1 0  is NOT an integer is NOT an integre                         1 0 1  is NOT an integer is NOT an integre                         1 0 0  is NOT an integer is NOT an integre                          1 0   is NOT an integer is NOT an integre                          1  0  is NOT an integer is NOT an integre                           1    is NOT an integer is NOT an integre                          0 0   is NOT an integer is NOT an integre                          0  0  is NOT an integer is NOT an integre                           0    is NOT an integer is NOT an integre                           one  is NOT an integer is NOT an integre  lt object object at 0x103b7d0a0 gt  is NOT an integer is NOT an integre                       1  2  3  is NOT an integer is NOT an integre                       1  2  3  is NOT an integer is NOT an integre                   one    two   is NOT an integer is NOT an integre

User · Answer

I do this all the time b c I have a mild but admittedly irrational aversion to using the try except pattern  I use this  all  xi in  1234567890  for xi in x    It doesn t accommodate negative numbers  so you could strip out all minus signs on the left side  and then check if the result comprises digits from 0-9  all  xi in  1234567890  for xi in x lstrip  -      You could also pass x to str   if you re not sure the input is a string  all  xi in  1234567890  for xi in str x  lstrip  -      There are some  edge   cases where this falls apart   It doesn t work for various scientific and or exponential notations  e g  1 2E3  10 3  etc   - both will return False  I don t think other answers accommodated this either  and even Python 3 8 has inconsistent opinions  since type 1E2  gives  lt class  float  gt  whereas type 10 2  gives  lt class  int  gt   An empty string input gives True  A leading plus sign  e g   quot  7 quot   gives False  Multiple minus signs are ignored so long as they re leading characters  This behavior is similar to the python interpreter  in that type ---1  returns  lt class int gt   However  it isn t completely consistent with the interpreter in that int  ---1   gives an error  but my solution returns True with the same input   So it won t work for every possible input  but if you can exclude those  it s an OK one-line check that returns False if x is not an integer and True if x is an integer  But if you really want behavior that exactly models the int   built-in  you re better off using try except  I don t know if it s pythonic  but it s one line  and it s relatively clear what the code does   I don t mean to say that the interpreter ignores leading minus signs  just that any number of leading minus signs does not change that the result is an integer  int --1  is actually interpreted as - -1   or 1  int ---1  is interpreted as - - -1    or -1  So an even number of leading minus signs gives a positive integer  and an odd number of minus signs gives a negative integer  but the result is always an integer

User · Answer

with positive integers you could use  isdigit    gt  gt  gt   16  isdigit   True   it doesn t work with negative integers though  suppose you could try the following    gt  gt  gt  s    -17   gt  gt  gt  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

User · Answer

I suggest the following   import ast  def is int s       return isinstance ast literal eval s   int    From the docs      Safely evaluate an expression node or a string containing a Python literal or container display  The string or node provided may only consist of the following Python literal structures  strings  bytes  numbers  tuples  lists  dicts  sets  booleans  and None    I should note that this will raise a ValueError exception when called against anything that does not constitute a Python literal  Since the question asked for a solution without try except  I have a Kobayashi-Maru type solution for that   from ast import literal eval from contextlib import suppress  def is int s       with suppress ValueError           return isinstance literal eval s   int      return False

User · Answer

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   gt  gt  gt  print RepresentsInt   123   True  gt  gt  gt  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

User · Answer

This is probably the most straightforward and pythonic way to approach it in my opinion  I didn t see this solution and it s basically the same as the regex one  but without the regex    def is int test       import string     return not  set test  - set string digits

User · Answer

gt  gt  gt   quot  7 quot  lstrip  quot -  quot   isdigit   True  gt  gt  gt   quot -7 quot  lstrip  quot -  quot   isdigit   True  gt  gt  gt   quot 7 quot  lstrip  quot -  quot   isdigit   True  gt  gt  gt   quot 13 4 quot  lstrip  quot -  quot   isdigit   False  So your function would be  def is int val      return val lstrip  quot -  quot   isdigit

User · Answer

The easiest way  which I use def is int item  str  - gt  bool      return item lstrip  -    isnumeric

User · Answer

def is int or float x       try              if int float x                if float x  is integer     return  int              else                       return  float      except ValueError   return False   for  i  in        -1    01    1 23     0011 002          print i  is int or float i        False   -1 int   01 int   1 23 float    0011 002 float

User · Answer

I have one possibility that doesn t use int at all  and should not raise an exception unless the string does not represent a number  float number   float number   1   It should work for any kind of string that float accepts  positive  negative  engineering notation

User · Answer

You know  I ve found  and I ve tested this over and over  that try except does not perform all that well  for whatever reason   I frequently try several ways of doing things  and I don t think I ve ever found a method that uses try except to perform the best of those tested  in fact it seems to me those methods have usually come out close to the worst  if not the worst   Not in every case  but in many cases   I know a lot of people say it s the  quot Pythonic quot  way  but that s one area where I part ways with them   To me  it s neither very performant nor very elegant  so  I tend to only use it for error trapping and reporting  I was going to gripe that PHP  perl  ruby  C  and even the freaking shell have simple functions for testing a string for integer-hood  but due diligence in verifying those assumptions tripped me up   Apparently this lack is a common sickness  Here s a quick and dirty edit of Bruno s post  import sys  time  re  g intRegex   re compile r quot     -   1-9  d  0   quot    testvals           integers     0  1  -1  1 0  -1 0       0    0    0 0    1    -1     1    1 0    -1 0     1 0    06         non-integers      abc 123       1 1  -1 1   1 1    -1 1     1 1        1 1 1    1 1 0    1 0 1    1 0 0        1 0     1  0    1          0 0     0  0    0          one   object     1 2 3    1 2 3     one   two          with spaces       0      0       0    01      def isInt try v       try      i   int v      except   return False     return True  def isInt str v       v   str v  strip       return v   0  or  v if v find        gt  -1 else v lstrip  -    rstrip  0   rstrip       isdigit    def isInt re v       import re     if not hasattr isInt re   intRegex            isInt re intRegex   re compile r quot     -   1-9  d  0   quot       return isInt re intRegex match str v  strip    is not None  def isInt re2 v       return g intRegex match str v  strip    is not None  def check int s       s   str s      if s 0  in   -                 return s 1   isdigit       return s isdigit         def timeFunc func  times       t1   time time       for n in range times           for v in testvals               r   func v      t2   time time       return t2 - t1  def testFuncs funcs       for func in funcs          sys stdout write   quot  t s t  quot    func   name        print       for v in testvals          if type v     type                  sys stdout write  quot   s  quot    v          else              sys stdout write  quot  s quot    str v           for func in funcs              sys stdout write   quot  t t s t  quot    func v           sys stdout write  quot  r n quot     if   name         main         print       print  quot tests   quot       testFuncs  isInt try  isInt str  isInt re  isInt re2  check int       print        print  quot timings   quot       print  quot isInt try     6 4f quot    timeFunc isInt try  10000       print  quot isInt str     6 4f quot    timeFunc isInt str  10000        print  quot isInt re      6 4f quot    timeFunc isInt re  10000       print  quot isInt re2     6 4f quot    timeFunc isInt re2  10000       print  quot check int     6 4f quot    timeFunc check int  10000    Here are the performance comparison results  timings   isInt try    0 6426 isInt str    0 7382 isInt re     1 1156 isInt re2    0 5344 check int    0 3452  A C method could scan it Once Through  and be done   A C method that scans the string once through would be the Right Thing to do  I think  EDIT  I ve updated the code above to work in Python 3 5  and to include the check int function from the currently most voted up answer  and to use the current most popular regex that I can find for testing for integer-hood  This regex rejects strings like  abc 123   I ve added  abc 123  as a test value  It is Very Interesting to me to note  at this point  that NONE of the functions tested  including the try method  the popular check int function  and the most popular regex for testing for integer-hood  return the correct answers for all of the test values  well  depending on what you think the correct answers are  see the test results below   The built-in int   function silently truncates the fractional part of a floating point number and returns the integer part before the decimal  unless the floating point number is first converted to a string  The check int   function returns false for values like 0 0 and 1 0  which technically are integers  and returns true for values like  06   Here are the current  Python 3 5  test results                isInt try         isInt str               isInt re                isInt re2           check int     0               True                    True                    True                    True            True      1               True                    True                    True                    True            True      -1              True                    True                    True                    True            True      1 0             True                    True                    False                   False           False     -1 0            True                    True                    False                   False           False      0              True                    True                    True                    True            True       0              False                   True                    False                   False           False      0 0            False                   True                    False                   False           False      1              True                    True                    True                    True            True       -1             True                    True                    True                    True            True        1             True                    True                    True                    True            True       1 0            False                   True                    False                   False           False      -1 0           False                   True                    False                   False           False       1 0           False                   True                    False                   False           False      06             True                    True                    False                   False           True       abc 123        False                   False                   False                   False           False     1 1             True                    False                   False                   False           False     -1 1            True                    False                   False                   False           False      1 1            False                   False                   False                   False           False      -1 1           False                   False                   False                   False           False       1 1           False                   False                   False                   False           False      1 1 1          False                   False                   False                   False           False      1 1 0          False                   False                   False                   False           False      1 0 1          False                   False                   False                   False           False      1 0 0          False                   False                   False                   False           False      1 0            False                   False                   False                   False           False      1  0           False                   False                   False                   False           False      1              False                   False                   False                   False           False      0 0            False                   False                   False                   False           False      0  0           False                   False                   False                   False           False      0              False                   False                   False                   False           False      one            False                   False                   False                   False           False      lt obj   gt          False                   False                   False                   False           False      1  2  3        False                   False                   False                   False           False      1  2  3        False                   False                   False                   False           False       one    two    False                   False                   False                   False           False       0             True                    True                    True                    True            False       0             False                   True                    False                   False           False        0            False                   False                   False                   False           False       01            False                   False                   False                   False           False      Just now I tried adding this function  def isInt float s       try          return float str s   is integer       except          return False  It performs almost as well as check int  0 3486  and it returns true for values like 1 0 and 0 0 and  1 0 and 0  and  0 and so on  But it also returns true for  06   so  Pick your poison  I guess

User · Answer

str isdigit   should do the trick   Examples   str isdigit  23      True str isdigit  abc      False str isdigit  23 4      False   EDIT  As  BuzzMoschetti pointed out  this way will fail for minus number  e g   -23    In case your input num can be less than 0  use re sub regex search regex replace contents  before applying str isdigit    For example    import re input num    -23  input num   re sub   -       input num         indicates to remove the first  -  only str isdigit input num     True

User · Answer

I really liked Shavais  post  but I added one more test case    amp  the built in isdigit   function    def isInt loop v       v   str v  strip         swapping  0123456789  for  9876543210  makes nominal difference  might have because  1  is toward the beginning of the string      numbers    0123456789      for i in v          if i not in numbers              return False     return True  def isInt Digit v       v   str v  strip       return v isdigit     and it significantly consistently beats the times of the rest   timings   isInt try    0 4628 isInt str    0 3556 isInt re     0 4889 isInt re2    0 2726 isInt loop    0 1842 isInt Digit    0 1577   using normal 2 7 python     python --version Python 2 7 10   Both the two test cases I added  isInt loop and isInt digit  pass the exact same test cases  they both only accept unsigned integers   but I thought that people could be more clever with modifying the string implementation  isInt loop  opposed to the built in isdigit   function  so I included it  even though there s a slight difference in execution time   and both methods beat everything else by a lot  but don t handle the extra stuff       -       Also  I did find it interesting to note that the regex  isInt re2 method  beat the string comparison in the same test that was performed by Shavais in 2012  currently 2018   Maybe the regex libraries have been improved

User · Answer

Uh   Try this    def int check a       if int a     a          return True     else          return False   This works if you don t put a string that s not a number   And also  I forgot to put the number check part     there is a function checking if the string is a number or not  It is str isdigit    Here s an example    a   2 a isdigit     If you call a isdigit    it will return True

User · Answer

I guess the question is related with speed since the try except has a time penalty     test data  First  I created a list of 200 strings  100 failing strings and 100 numeric strings   from random import shuffle numbers    u  1     100 nonumbers    u 1abc     100 testlist   numbers   nonumbers shuffle testlist  testlist   np array testlist      numpy solution  only works with arrays and unicode   np core defchararray isnumeric can also work with unicode strings np core defchararray isnumeric u  12   but it returns and array  So  it s a good solution if you have to do thousands of conversions and have missing data or non numeric data   import numpy as np  timeit np core defchararray isnumeric testlist  10000 loops  best of 3  27 9   s per loop   200 numbers per loop   try except  def check num s     try      int s      return True   except      return False  def check list l     return  check num e  for e in l    timeit check list testlist  1000 loops  best of 3  217   s per loop   200 numbers per loop   Seems that numpy solution is much faster

User · Answer

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

User · Answer

I think  s startswith  -   and s 1   isdigit     would be better to rewrite to   s replace  -       isdigit     because s 1   also creates a new string  But much better solution is  s lstrip   -   isdigit

User · Answer

If you want to accept lower-ascii digits only  here are tests to do so   Python 3 7    u isdecimal   and u isascii     Python  lt   3 6   u isdecimal   and u    str int u     Other answers suggest using  isdigit   or  isdecimal   but these both include some upper-unicode characters such as      u  u0662     u   u  u0662            u isdigit           True u isdecimal         True u isascii           False  Python 3 7  only  u    str int u      False

User · Answer

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

[python] How can I check if a string represents an int, without using try/except?

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