What s the canonical way to check for type in Python

Question

What is the best way to check whether a given object is of a given type  How about checking whether the object inherits from a given type   Let s say I have an object o  How do I check whether it s a str

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To Hugo   You probably mean list rather than array  but that points to the whole problem with type checking - you don t want to know if the object in question is a list  you want to know if it s some kind of sequence or if it s a single object  So try to use it like a sequence   Say you want to add the object to an existing sequence  or if it s a sequence of objects  add them all  try     my sequence extend o  except TypeError    my sequence append o    One trick with this is if you are working with strings and or sequences of strings - that s tricky  as a string is often thought of as a single object  but it s also a sequence of characters  Worse than that  as it s really a sequence of single-length strings   I usually choose to design my API so that it only accepts either a single value or a sequence - it makes things easier  It s not hard to put a     around your single value when you pass it in if need be    Though this can cause errors with strings  as they do look like  are  sequences

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Here is an example why duck typing is evil without knowing when it is dangerous  For instance  Here is the Python code  possibly omitting proper indenting   note that this  situation is avoidable by taking care of isinstance and issubclassof functions to make sure that when you really need a duck  you don t get a bomb   class Bomb      def   init   self                   def talk self           self explode        def explode self           print  BOOM   The bomb explodes    class Duck      def   init   self                  def talk self           print  I am a duck  I will not blow up if you ask me to talk        class Kid      kids duck   None      def   init   self           print  Kid comes around a corner and asks you for money so he could buy a duck        def takeDuck self  duck           self kids duck   duck         print  The kid accepts the duck  and happily skips along       def doYourThing self           print  The kid tries to get the duck to talk          self kids duck talk    myKid   Kid   myBomb   Bomb   myKid takeDuck myBomb  myKid doYourThing

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For more complex type validations I like typeguard s approach of validating based on python type hint annotations   from typeguard import check type from typing import List  try      check type  mylist    1  2   List int   except TypeError as e      print e    You can perform very complex validations in very clean and readable fashion   check type  foo    1  3 14   List Union int  float      vs isinstance foo  list  and all isinstance a   int  float   for a in foo

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A simple way to check type is to compare it with something whose type you know    gt  gt  gt  a    1  gt  gt  gt  type a     type 1  True  gt  gt  gt  b    abc   gt  gt  gt  type b     type     True

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To check if o is an instance of str or any subclass of str  use isinstance  this would be the  canonical  way    if isinstance o  str     To check if the type of o is exactly str  exclude subclasses    if type o  is str    The following also works  and can be useful in some cases   if issubclass type o   str     See Built-in Functions in the Python Library Reference for relevant information   One more note  in this case  if you re using Python 2  you may actually want to use   if isinstance o  basestring     because this will also catch Unicode strings  unicode is not a subclass of str  both str and unicode are subclasses of basestring   Note that basestring no longer exists in Python 3  where there s a strict separation of strings  str  and binary data  bytes    Alternatively  isinstance accepts a tuple of classes  This will return True if o is an instance of any subclass of any of  str  unicode    if isinstance o   str  unicode

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I think the best way is to typing well your variables  You can do this by using the  typing  library   Example   from typing import NewType UserId   NewType   UserId   int  some id   UserId  524313    See https   docs python org 3 library typing html

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After the question was asked and answered  type hints were added to Python  Type hints in Python allow types to be checked but in a very different way from statically typed languages  Type hints in Python associate the expected types of arguments with functions as runtime accessible data associated with functions and this allows for types to be checked  Example of type hint syntax   def foo i  int       return i  foo 5  foo  oops     In this case we want an error to be triggered for foo  oops   since the annotated type of the argument is int  The added type hint does not cause an error to occur when the script is run normally  However  it adds attributes to the function describing the expected types that other programs can query and use to check for type errors   One of these other programs that can be used to find the type error is mypy   mypy script py script py 12  error  Argument 1 to  foo  has incompatible type  str   expected  int     You might need to install mypy from your package manager  I don t think it comes with CPython but seems to have some level of  officialness     Type checking this way is different from type checking in statically typed compiled languages  Because types are dynamic in Python  type checking must be done at runtime  which imposes a cost -- even on correct programs -- if we insist that it happen at every chance  Explicit type checks may also be more restrictive than needed and cause unnecessary errors  e g  does the argument really need to be of exactly list type or is anything iterable sufficient     The upside of explicit type checking is that it can catch errors earlier and give clearer error messages than duck typing  The exact requirements of a duck type can only be expressed with external documentation  hopefully it s thorough and accurate  and errors from incompatible types can occur far from where they originate   Python s type hints are meant to offer a compromise where types can be specified and checked but there is no additional cost during usual code execution   The typing package offers type variables that can be used in type hints to express needed behaviors without requiring particular types  For example  it includes variables such as Iterable and Callable for hints to specify the need for any type with those behaviors   While type hints are the most Pythonic way to check types  it s often even more Pythonic to not check types at all and rely on duck typing  Type hints are relatively new and the jury is still out on when they re the most Pythonic solution  A relatively uncontroversial but very general comparison  Type hints provide a form of documentation that can be enforced  allow code to generate earlier and easier to understand errors  can catch errors that duck typing can t  and can be checked statically  in an unusual sense but it s still outside of runtime   On the other hand  duck typing has been the Pythonic way for a long time  doesn t impose the cognitive overhead of static typing  is less verbose  and will accept all viable types and then some

User · Answer

The most Pythonic way to check the type of an object is    not to check it   Since Python encourages Duck Typing  you should just try   except to use the object s methods the way you want to use them   So if your function is looking for a writable file object  don t check that it s a subclass of file  just try to use its  write   method   Of course  sometimes these nice abstractions break down and isinstance obj  cls  is what you need   But use sparingly

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isinstance o  str  will return True if o is an str or is of a type that inherits from str   type o  is str will return True if and only if o is a str  It will return False if o is of a type that inherits from str

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I think the cool thing about using a dynamic language like Python is you really shouldn t have to check something like that   I would just call the required methods on your object and catch an AttributeError  Later on this will allow you to call your methods with other  seemingly unrelated  objects to accomplish different tasks  such as mocking an object for testing   I ve used this a lot when getting data off the web with urllib2 urlopen   which returns a file like object  This can in turn can be passed to almost any method that reads from a file  because it implements the same read   method as a real file   But I m sure there is a time and place for using isinstance    otherwise it probably wouldn t be there

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You can check for type of a variable using   name   of a type   Ex    gt  gt  gt  a    1 2 3 4     gt  gt  gt  b   1    gt  gt  gt  type a    name    list   gt  gt  gt  type a    name       list  True  gt  gt  gt  type b    name       list  False  gt  gt  gt  type b    name    int

User · Answer

isinstance o  str    Link to docs

[python] What's the canonical way to check for type in Python?

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