What is the difference between old style and new style classes in Python

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What is the difference between old style and new style classes in Python   When should I use one or the other

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New-style classes inherit from object and must be written as such in Python 2 2 onwards  i e  class Classname object   instead of class Classname    The core change is to unify types and classes  and the nice side-effect of this is that it allows you to inherit from built-in types   Read descrintro for more details

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Guido has written The Inside Story on New-Style Classes  a really great article about new-style and old-style class in Python   Python 3 has only new-style class  Even if you write an  old-style class   it is implicitly derived from object   New-style classes have some advanced features lacking in old-style classes  such as super  the new C3 mro  some magical methods  etc

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Here s a very practical  true false difference  The only difference between the two versions of the following code is that in the second version Person inherits from object  Other than that  the two versions are identical  but with different results    Old-style classes  class Person         names cache          def   init   self name           self name   name     def   new   cls name           return cls  names cache setdefault name object   new   cls name    ahmed1   Person  Ahmed   ahmed2   Person  Ahmed   print ahmed1 is ahmed2 print ahmed1 print ahmed2    gt  gt  gt  False  lt   main   Person instance at 0xb74acf8c gt   lt   main   Person instance at 0xb74ac6cc gt   gt  gt  gt    New-style classes  class Person object        names cache          def   init   self name           self name   name     def   new   cls name           return cls  names cache setdefault name object   new   cls name    ahmed1   Person  Ahmed   ahmed2   Person  Ahmed   print ahmed2 is ahmed1 print ahmed1 print ahmed2   gt  gt  gt  True  lt   main   Person object at 0xb74ac66c gt   lt   main   Person object at 0xb74ac66c gt   gt  gt  gt

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Declaration-wise   New-style classes inherit from object  or from another new-style class   class NewStyleClass object       pass  class AnotherNewStyleClass NewStyleClass       pass   Old-style classes don t   class OldStyleClass        pass   Python 3 Note   Python 3 doesn t support old style classes  so either form noted above results in a new-style class

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Old style classes are still marginally faster for attribute lookup  This is not usually important  but it may be useful in performance-sensitive Python 2 x code    In  3   class A              def   init   self                   self a    hi there           In  4   class B object               def   init   self                   self a    hi there           In  6   aobj   A   In  7   bobj   B    In  8    timeit aobj a 10000000 loops  best of 3  78 7 ns per loop  In  10    timeit bobj a 10000000 loops  best of 3  86 9 ns per loop

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From New-style and classic classes      Up to Python 2 1  old-style classes were the only flavour available to the user       The concept of  old-style  class is unrelated to the concept of type    if x is an instance of an old-style class  then x   class     designates the class of x  but type x  is always  lt type    instance  gt         This reflects the fact that all old-style instances  independently of   their class  are implemented with a single built-in type  called   instance       New-style classes were introduced in Python 2 2 to unify the concepts of class and type     A new-style class is simply a user-defined type  no more  no less       If x is an instance of a new-style class  then type x  is typically   the same as x   class    although this is not guaranteed     a   new-style class instance is permitted to override the value returned   for x   class          The major motivation for introducing new-style classes is to provide a unified object model with a full meta-model        It also has a number of immediate benefits  like the ability to   subclass most built-in types  or the introduction of  descriptors     which enable computed properties       For compatibility reasons  classes are still old-style by default        New-style classes are created by specifying another new-style class    i e  a type  as a parent class  or the  top-level type  object if no   other parent is needed        The behaviour of new-style classes differs from that of old-style   classes in a number of important details in addition to what type   returns        Some of these changes are fundamental to the new object model  like   the way special methods are invoked  Others are  fixes  that could not   be implemented before for compatibility concerns  like the method   resolution order in case of multiple inheritance       Python 3 only has new-style classes        No matter if you subclass from object or not  classes are new-style   in Python 3

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New style classes may use super Foo  self  where Foo is a class and self is the instance   super type   object-or-type   Return a proxy object that delegates method calls to a parent or sibling class of type  This is useful for accessing inherited methods that have been overridden in a class  The search order is same as that used by getattr   except that the type itself is skipped   And in Python 3 x you can simply use super   inside a class without any parameters

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Important behavior changes between old and new style classes   super added MRO changed  explained below  descriptors added new style class objects cannot be raised unless derived from Exception  example below    slots   added   MRO  Method Resolution Order  changed  It was mentioned in other answers  but here goes a concrete example of the difference between classic MRO and C3 MRO  used in new style classes    The question is the order in which attributes  which include methods and member variables  are searched for in multiple inheritance   Classic classes do a depth-first search from left to right  Stop on the first match  They do not have the   mro   attribute   class C  i   0 class C1 C   pass class C2 C   i   2 class C12 C1  C2   pass class C21 C2  C1   pass  assert C12   i    0 assert C21   i    2  try      C12   mro   except AttributeError      pass else      assert False   New-style classes MRO is more complicated to synthesize in a single English sentence  It is explained in detail here  One of its properties is that a base class is only searched for once all its derived classes have been  They have the   mro   attribute which shows the search order   class C object   i   0 class C1 C   pass class C2 C   i   2 class C12 C1  C2   pass class C21 C2  C1   pass  assert C12   i    2 assert C21   i    2  assert C12   mro       C12  C1  C2  C  object  assert C21   mro       C21  C2  C1  C  object    New style class objects cannot be raised unless derived from Exception  Around Python 2 5 many classes could be raised  and around Python 2 6 this was removed  On Python 2 7 3     OK  old  class Old  pass try      raise Old   except Old      pass else      assert False    TypeError  new not derived from  Exception   class New object   pass try      raise New   except TypeError      pass else      assert False    OK  derived from  Exception   class New Exception   pass try      raise New   except New      pass else      assert False      str   is a new style object  so you can t raise it  try      raise  str  except TypeError      pass else      assert False

[python] What is the difference between old style and new style classes in Python?

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