Checking whether a variable is an integer or not

Question

How do I check whether a variable is an integer

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Consider the case x   n   1 0 m   where n 10  5  m 5   In Python  x will be 10 000000000000002  which is only not integer because of floating point arithmetic operations   So I d check  if str float x   endswith   0    print  It s an integer     I ve tested it with this code   for a in range 2 100       for b in range 2 100           x    a  b    1 0 b          print a b  x  str float x   endswith   0     It outputs True for all a and b

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Here s a summary of the different methods mentioned here    int x     x try x   operator index x  isinstance x  int  isinstance x  numbers Integral    and here s how they apply to a variety of numerical types that have integer value       You can see they aren t 100  consistent   Fraction and Rational are conceptually the same  but one supplies a  index   method and the other doesn t   Complex types don t like to convert to int even if the real part is integral and imaginary part is 0    np int8 16 32 64 5  means that np int8 5   np int32 5   etc  all behave identically

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If you just need the value  operator index    index   special method  is the way to go in my opinion  Since it should work for all types that can be safely cast to an integer  I e  floats fail  integers  even fancy integer classes that do not implement the Integral abstract class work by duck typing   operator index is what is used for list indexing  etc  And in my opinion it should be used for much more promoted   In fact I would argue it is the only correct way to get integer values if you want to be certain that floating points  due to truncating problems  etc  are rejected and it works with all integral types  i e  numpy  etc   even if they may not  yet  support the abstract class    This is what   index   was introduced for

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If the variable is entered like a string  e g   2010      if variable and variable isdigit        return variable  or whatever you want to do with it   else       return  Error   or whatever you want to do with it    Before using this I worked it out with try except and checking for  int variable    but it was longer code  I wonder if there s any difference in use of resources or speed

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I can check if the number is integer include number like 7 0 def is int x       if x - round x     0           return True     else          return False

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Why not try something like   if x 1    0

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it s really astounding to see such a heated discussion coming up when such a basic  valid and  i believe  mundane question is being asked    some people have pointed out that type-checking against int  and long  might loose cases where a big decimal number is encountered  quite right   some people have pointed out that you should  just do x   1 and see whether that fails  well  for one thing  this works on floats too  and  on the other hand  it s easy to construct a class that is definitely not very numeric  yet defines the   operator in some way   i am at odds with many posts vigorously declaring that you should not check for types  well  GvR once said something to the effect that in pure theory  that may be right  but in practice  isinstance often serves a useful purpose  that s a while ago  don t have the link  you can read what GvR says about related issues in posts like this one    what is funny is how many people seem to assume that the OP s intent was to check whether the type of a given x is a numerical integer type   what i understood is what i normally mean when using the OP s words  whether x represents an integer number  and this can be very important  like ask someone how many items they d want to pick  you may want to check you get a non-negative integer number back  use cases like this abound    it s also  in my opinion  important to see that  1  type checking is but ONE   and often quite coarse   measure of program correctness  because  2  it is often bounded values that make sense  and out-of-bounds values that make nonsense  sometimes just some intermittent values make sense   like considering all numbers  only those real  non-complex   integer numbers might be possible in a given case    funny non-one seems to mention checking for x    math floor  x    if that should give an error with some big decimal class  well  then maybe it s time to re-think OOP paradigms  there is also PEP 357 that considers how to use not-so-obviously-int-but-certainly-integer-like values to be used as list indices  not sure whether i like the solution

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A simple method I use in all my software is this  It checks whether the variable is made up of numbers   test   input  Enter some text here     if test isdigit      True     print  This is a number    else     print  This is not a number

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Testing  if object is a string  works with Python 2   and Python 3      text   get text    try      text   text    except      return  Not a string   do something text

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There is another option to do the type check    For example     n   14   if type n   int    return  this is an int

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If you need to do this  do isinstance  lt var gt   int   unless you are in Python 2 x in which case you want isinstance  lt var gt    int  long    Do not use type  It is almost never the right answer in Python  since it blocks all the flexibility of polymorphism  For instance  if you subclass int  your new class should register as an int  which type will not do  class Spam int   pass x   Spam 0  type x     int   False isinstance x  int    True  This adheres to Python s strong polymorphism  you should allow any object that behaves like an int  instead of mandating that it be one  BUT The classical Python mentality  though  is that it s easier to ask forgiveness than permission  In other words  don t check whether x is an integer  assume that it is and catch the exception results if it isn t  try      x    1 except TypeError           This mentality is slowly being overtaken by the use of abstract base classes  which let you register exactly what properties your object should have  adding  multiplying  doubling   by making it inherit from a specially-constructed class  That would be the best solution  since it will permit exactly those objects with the necessary and sufficient attributes  but you will have to read the docs on how to use it

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Found a related question here on SO itself   Python developers prefer to not check types but do a type specific operation and catch a TypeError exception  But if you don t know the type then you have the following    gt  gt  gt  i   12345  gt  gt  gt  type i   lt type  int  gt   gt  gt  gt  type i  is int True

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A more general approach that will attempt to check for both integers and integers given as strings will be  def isInt anyNumberOrString       try          int anyNumberOrString   to check float and int use  float anyNumberOrString           return True     except ValueError           return False  isInt  A    False isInt  5    True isInt 8   True isInt  5 88    False  see comment above on how to make this True

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If you are reading from a file and you have an array or dictionary with values of multiple datatypes  the following will be useful  Just check whether the variable can be type casted to int or any other datatype you want to enforce  or not    try       int a        Variable a is int except ValueError          Variable a is not an int

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I ve had this problem before  if your type to use it in a if statement  and let s just say you wanted it to return true  you would enter this into a line   The bottom line in all that is really needed to be looked at     In  1   test   1  In  2   test2   1 0  In  3   type test     int Out 3   True  In  4   type test2     int Out 4   False  In  5   if type x     int is True    You can do the same thing to check if it s a float  if it s true or false  and use to assign a name   like x if you know what I mean

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If you have not int you can do just this   var   15 4 if var - int var     0       print  Value is not integer

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you can do this by   name    Bob  if type name     str      print  this works  else      print  this does not work    and it will return  this works     but if you change name to int 1  then it will return  this does not work  because it is now a string    you can also try   name   int 5  if type name     int      print  this works  else      print  this does not work    and the same thing will happen

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import numpy as np  if  np floor x -x    0     return  this is an int

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usr bin env python  import re  def is int x        if isinstance x  int long              return True     matchObj   re match r  -  d     d    str x            if matchObj           x   matchObj group 1           if int x -0  0               return True       return False  print is int 6   print is int 1 0   print is int 1 1   print is int 0 1   print is int -956 0

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gt  gt  gt  isinstance 3  int  True   See here for more   Note that this does not help if you re looking for int-like attributes  In this case you may also want to check for long    gt  gt  gt  isinstance 3L   long  int   True   I ve seen checks of this kind against an array index type in the Python source  but I don t think that s visible outside of C   Token SO reply  Are you sure you should be checking its type  Either don t pass a type you can t handle  or don t try to outsmart your potential code reusers  they may have a good reason not to pass an int to your function

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Never   Check   Types   Do this   Always   try      some operation that  requires  an integer except TypeError  e      it wasn t an integer  fail

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You can use this function   def is int x           if type x     int         return True     return False   Test   print is int  7 0     False print is int 7 0    False print is int 7 5    False print is int -1    True

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use the int function to help  intchecker   float input  Please enter a integer      intcheck   0 while intcheck    1      if intchecker - int intchecker   gt  0          intchecker   float input  You didn t enter a integer                                      Please enter a integer          else          intcheck   1 print  you have entered a integer

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In the presence of numpy check like     isinstance var  numbers Integral        slow  or     isinstance var   int  long  np integer        in order to match all type variants like np int8  np uint16        Drop long in PY3   Recognizing ANY integer-like object from anywhere is a tricky guessing game  Checking   var  amp  0    0    for truth and non-exception may be a good bet  Similarly  checking for signed integer type exclusively   var   -1     -var - 1

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I was writing a program to check if a number was square and I encountered this issue  the  code I used was   import math print   this program will tell you if a number is square   print   enter an integer   num   float input    if num  gt  0      print   ok        num    math sqrt num       inter   int num      if num    inter              print   It s a square number  and its root is               print  num      else              print   It s not a square number  but its root is               print  num  else      print   That s not a positive number      To tell if the number was an integer I converted the float number you get from square rooting the user input to a rounded integer  stored as the value    if those two numbers were equal then the first number must have been an integer  allowing the program to respond  This may not be the shortest way of doing this but it worked for me

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You can also use str isdigit  Try looking up help str isdigit   def is digit str         return str isdigit

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If you want to check with no regard for Python version  2 x vs 3 x   use six  PyPI  and it s integer types attribute   import six  if isinstance obj  six integer types       print  obj is an integer      Within six  a very light-weight single-file module   it s simply doing this   import sys PY3   sys version info 0     3  if PY3      integer types   int  else      integer types    int  long

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All proposed answers so far seem to miss the fact that a double  floats in python are actually doubles  can also be an integer  if it has nothing after the decimal point    I use the built-in is integer   method on doubles to check this   Example  to do something every xth time in a for loop    for index in range y          do something     if  index x   is integer              do something special   Edit   You can always convert to a float before calling this method  The three possibilities    gt  gt  gt  float 5  is integer   True  gt  gt  gt  float 5 1  is integer   False  gt  gt  gt  float 5 0  is integer   True   Otherwise  you could check if it is an int first like Agostino said   def is int val       if type val     int          return True     else          if val is integer                return True         else              return False

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If you really need to check then it s better to use abstract base classes rather than concrete classes  For an integer that would mean    gt  gt  gt  import numbers  gt  gt  gt  isinstance 3  numbers Integral  True   This doesn t restrict the check to just int  or just int and long  but also allows other user-defined types that behave as integers to work

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If you want to check that a string consists of only digits  but converting to an int won t help  you can always just use regex   import re x    01234  match   re search    d     x  try  x   match group 0  except AttributeError  print  not a valid number    Result  x     01234    In this case  if x were  hello   converting it to a numeric type would throw a ValueError  but data would also be lost in the process  Using a regex and catching an AttributeError would allow you to confirm numeric characters in a string with  for instance  leading 0 s   If you didn t want it to throw an AttributeError  but instead just wanted to look for more specific problems  you could vary the regex and just check the match   import re x    h01234  match   re search   D   x  if not match      print  x is a number   else      print  encountered a problem at character    match group 0    Result   encountered a problem at character  h    That actually shows you where the problem occurred without the use of exceptions  Again  this is not for testing the type  but rather the characters themselves  This gives you much more flexibility than simply checking for types  especially when converting between types can lose important string data  like leading 0 s

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why not just check if the value you want to check is equal to itself cast as an integer as shown below   def isInt val       return val    int val

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You can do this       if type x  is int

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If you want to write a Python 2-3 compatible code To test whether a value is an integer  of any kind   you can to do this     Python 2 and 3   import sys if sys version info  lt   3        integer types    int  long   else      integer types    int     gt  gt  gt  isinstance 1  integer types  True    Python 2 only  if isinstance x   int  long                Python 3 only  if isinstance x  int            source   http   python3porting com differences html

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A simple way to do this is to directly check if the remainder on division by 1 is 0 or not   if this variable   1    0      list append this variable  else      print  Not an Integer

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Here is a simple example how you can determine an integer  def is int x       print round x       if x    round x           print  True       else          print  False   is int 7 0      True is int 7 5      False is int -1       True

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It is very simple to check in python  You can do like this   Suppose you want to check a variable is integer or not      For checking a variable is integer or not in python  if type variable  is int       print  This line will be executed   else       print  Not an integer

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Value Is Int                                         def value is int value       try          tempVal   int value          return True     except          return False   Call this function   if value is int value       print  Integer  else      print  Not integer

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Rather than over complicate things  why not just a simple  if type var  is int

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The simplest way is   if n  int n       --do something--       Where n is the variable

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if type input  enter        int       print  Entered number is an Integer  else       print  Entered number isn t an Integer    This ll work to check out whether number is an integer or not

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val 3  gt  gt  gt  isinstance val int    True   will work

[python] Checking whether a variable is an integer or not

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