How to compare type of an object in Python

Question

Basically I want to do this   obj    str  type   obj      string   I tried   type   obj      type   string     and it didn t work   Also  what about the other types  For example  I couldn t replicate NoneType

User · Answer

Use str instead of string   type   obj      str   Explanation    gt  gt  gt  a    Hello   gt  gt  gt  type a   str True  gt  gt  gt  type a   lt type  str  gt   gt  gt  gt

User · Answer

You re very close  string is a module  not a type  You probably want to compare the type of obj against the type object for strings  namely str   type obj     str    this works because str is already a type   Alternatively   type obj     type       Note  in Python 2  if obj is a unicode type  then neither of the above will work  Nor will isinstance    See John s comments to this post for how to get around this    I ve been trying to remember it for about 10 minutes now  but was having a memory block

User · Answer

You can compare classes for check level      usr bin env python  coding utf8  class A object       def t self           print  A      def r self           print  rA           self t    class B A       def t self           print  B   class C A       def t self           print  C   class D B  C       def t self           print  D           super D  self  t    class E C  B       pass  d   D   d t   d r    e   E   e t   e r    print isinstance e  D    False print isinstance e  E    True print isinstance e  C    True print isinstance e  B    True print isinstance e   A      True print e   class    gt   A   False print e   class    lt   C   False print e   class    lt   E   False print e   class    lt   E   True

User · Answer

isinstance     In your case  isinstance  this is a string   str  will return True   You may also want to read this  http   www canonical org  kragen isinstance

User · Answer

It is because you have to write  s  hello  type s     type       type accepts an instance and returns its type  In this case you have to compare two instances  types   If you need to do preemptive checking  it is better if you check for a supported interface than the type    The type does not really tell you much  apart of the fact that your code want an instance of a specific type  regardless of the fact that you could have another instance of a completely different type which would be perfectly fine because it implements the same interface   For example  suppose you have this code  def firstElement parameter       return parameter 0    Now  suppose you say  I want this code to accept only a tuple   import types  def firstElement parameter       if type parameter     types TupleType           raise TypeError  function accepts only a tuple       return parameter 0    This is reducing the reusability of this routine  It won t work if you pass a list  or a string  or a numpy array  Something better would be   def firstElement parameter       if not  hasattr parameter     getitem     and callable getattr parameter    getitem                raise TypeError  interface violation       return parameter 0    but there s no point in doing it  parameter 0  will raise an exception if the protocol is not satisfied anyway    this of course unless you want to prevent side effects or having to recover from calls that you could invoke before failing   Stupid  example  just to make the point   def firstElement parameter       if not  hasattr parameter     getitem     and callable getattr parameter    getitem                raise TypeError  interface violation       os system  rm file       return parameter 0    in this case  your code will raise an exception before running the system   call  Without interface checks  you would have removed the file  and then raised the exception

User · Answer

i think this should do it  if isinstance obj  str

User · Answer

To get the type  use the   class   member  as in unknown thing   class    Talk of duck-typing is useless here because it doesn t answer a perfectly good question  In my application code I never need to know the type of something  but it s still useful to have a way to learn an object s type  Sometimes I need to get the actual class to validate a unit test  Duck typing gets in the way there because all possible objects have the same API  but only one is correct  Also  sometimes I m maintaining somebody else s code  and I have no idea what kind of object I ve been passed  This is my biggest problem with dynamically typed languages like Python  Version 1 is very easy and quick to develop  Version 2 is a pain in the buns  especially if you didn t write version 1  So sometimes  when I m working with a function I didn t write  I need to know the type of a parameter  just so I know what methods I can call on it    That s where the   class   parameter comes in handy  That  as far as I can tell  is the best way  maybe the only way  to get an object s type

User · Answer

isinstance works   if isinstance obj  MyClass   do foo obj    but  keep in mind  if it looks like a duck  and if it sounds like a duck  it is a duck   EDIT   For the None type  you can simply do   if obj is None  obj   MyClass

User · Answer

First  avoid all type comparisons   They re very  very rarely necessary   Sometimes  they help to check parameter types in a function -- even that s rare   Wrong type data will raise an exception  and that s all you ll ever need   All of the basic conversion functions will map as equal to the type function   type 9  is int type 2 5  is float type  x   is str type u x   is unicode type 2 3j  is complex   There are a few other cases   isinstance   x   basestring   isinstance  u u   basestring   isinstance  9  int   isinstance  2 5  float   isinstance   2 3j   complex     None  BTW  never needs any of this kind of type checking   None is the only instance of NoneType   The None object is a Singleton   Just check for None  variable is None   BTW  do not use the above in general   Use ordinary exceptions and Python s own natural polymorphism

User · Answer

For other types  check out the types module    gt  gt  gt  import types  gt  gt  gt  x    mystring   gt  gt  gt  isinstance x  types StringType  True  gt  gt  gt  x   5  gt  gt  gt  isinstance x  types IntType  True  gt  gt  gt  x   None  gt  gt  gt  isinstance x  types NoneType  True   P S  Typechecking is a bad idea

User · Answer

Type doesn t work on certain classes  If you re not sure of the object s type use the   class   method  as so    gt  gt  gt obj    a string   gt  gt  gt obj   class      str True   Also see this article - http   www siafoo net article 56

User · Answer

I use type x     type y   For instance  if I want to check something is an array   type  x      type         string check   type  x      type       or type  x      type  u       If you want to check against None  use is  x is None

User · Answer

You can always use the type x     type y  trick  where y is something with known type     check if x is a regular string type x     type       check if x is an integer type x     type 1    check if x is a NoneType type x     type None    Often there are better ways of doing that  particularly with any recent python   But if you only want to remember one thing  you can remember that   In this case  the better ways would be     check if x is a regular string type x     str   check if x is either a regular string or a unicode string type x  in  str  unicode    alternatively  isinstance x  basestring    check if x is an integer type x     int   check if x is a NoneType x is None   Note the last case  there is only one instance of NoneType in python  and that is None   You ll see NoneType a lot in exceptions  TypeError   NoneType  object is unsubscriptable -- happens to me all the time    but you ll hardly ever need to refer to it in code   Finally  as fengshaun points out  type checking in python is not always a good idea   It s more pythonic to just use the value as though it is the type you expect  and catch  or allow to propagate  exceptions that result from it

User · Answer

Use isinstance object  type   As above this is easy to use if you know the correct type  e g     isinstance  dog   str     gives bool True   But for more esoteric objects  this can be difficult to use   For example    import numpy as np  a   np array  1 2 3    isinstance a np array     breaks   but you can do this trick    y   type np array  1    isinstance a y     gives bool True    So I recommend instantiating a variable  y in this case  with a type of the object you want to check  e g   type np array      then using isinstance

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