Type and isinstance in Python

Python have a built-in method called as type which generally come in handy while figuring out the type of variable used in the program in the runtime.

If a single argument (object) is passed to type() built-in, it returns type of the given object. If three arguments (name, bases and dict) are passed, it returns a new type object.
Syntax:

type(object)
type(name, bases, dict)
      1. type() With a Single Object Parameter
        # Python code type() with a single object parameter
        x = 5
        s = "geeksforgeeks"
        y = [1,2,3]
        print(type(x))
        print(type(s))
        print(type(y))

        Output:

        class 'int'
        class 'str'
        class 'list'
        
      2. type() With a name, bases and dict Parameter
        # Python code for type() with a name,
        # bases and dict parameter
        o1 = type('X', (object,), dict(a='Foo', b=12))
        print(type(o1))
        print(vars(o1))
        class test:
              a = 'Foo'
          b = 12
          
        o2 = type('Y', (test,), dict(a='Foo', b=12))
        print(type(o2))
        print(vars(o2))

        Output:

        {'b': 12, 'a': 'Foo', '__dict__': , '__doc__': None, '__weakref__': }
        {'b': 12, 'a': 'Foo', '__doc__': None}
        

        If you need to check type of an object, it is recommended to use Python isinstance() function instead. It’s because isinstance() function also checks if the given object is an instance of the subclass.

isinstance()

The isinstance() function checks if the object (first argument) is an instance or subclass of classinfo class (second argument).

Syntax:

isinstance(object, classinfo) 

The isinstance() takes two parameters: object : object to be checked classinfo : class, type, or tuple of classes and types

Return Value :
true if the object is an instance or subclass of a class, or any element of the tuple falseotherwise. If class info is not a type or tuple of types, a TypeError exception is raised.

# Python code for  isinstance()
class Test:
    a = 5
  
TestInstance = Test()
print(isinstance(TestInstance, Test))
print(isinstance(TestInstance, (list, tuple)))
print(isinstance(TestInstance, (list, tuple, Test)))

Output:

True
False
True

type() vs. isinstance()

One elementary error people make is using the type() function where isinstance() would be more appropriate.

    • If you’re checking to see if an object has a certain type, you want isinstance() as it checks to see if the object passed in the first argument is of the type of any of the type objects passed in the second argument. Thus, it works as expected with subclassing and old-style classes, all of which have the legacy type object instance.
    • type(), on the other hand, simply returns the type object of an object and comparing what it returns to another type object will only yield True when you use the exact same type object on both sides.
      In Python, it’s preferable to use Duck Typing( type checking be deferred to run-time, and is implemented by means of dynamic typing or reflection) rather than inspecting the type of an object.

      #Python code to illustrate duck typing
      class User(object):
          def __init__(self, firstname):
              self.firstname = firstname
          @property
          def name(self):
              return self.firstname
      class Animal(object):
          pass
      class Fox(Animal):
          name = "Fox"
      class Bear(Animal):
          name = "Bear"
      # Use the .name attribute (or property) regardless of the type
      for a in [User("Geeksforgeeks"), Fox(), Bear()]:
          print(a.name)

      Output:

      Geeksforgeeks
      Fox
      Bear
    • The next reason not to use type() is the lack of support for inheritance.
      #python code to illustrate the lack of
      #support for inheritance in type()
      class MyDict(dict):
          """A normal dict, that is always created with an "initial" key"""
          def __init__(self):
              self["initial"] = "some data"
      d = MyDict()
      print(type(d) == dict)
      print(type(d) == MyDict)
      d = dict()
      print(type(d) == dict)
      print(type(d) == MyDict)

      Output:

      False
      True
      True
      False
      

      The MyDict class has all the properties of a dict, without any new methods. It will behave exactly like a dictionary. But type() will not return the expected result.
      Using isinstance() is preferable in this case because it will give the expected result:

      #python code to show isintance() support
      #inheritance
      class MyDict(dict):
          """A normal dict, that is always created with an "initial" key"""
          def __init__(self):
              self["initial"] = "some data"
      d = MyDict()
      print(isinstance(d, MyDict))
      print(isinstance(d, dict))
      d = dict()
      print(isinstance(d, MyDict))
      print(isinstance(d, dict))

      Output:

      True
      True
      False
      True

Disclaimer: This does not belong to TechCodeBit, its an article taken from the below
source and credits.
source and credits: http://www.geeksforgeeks.org
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rakesh

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