Difference between abstract class and interface in Python

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What is the difference between abstract class and interface in Python

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In general  interfaces are used only in languages that use the single-inheritance class model  In these single-inheritance languages  interfaces are typically used if any class could use a particular method or set of methods  Also in these single-inheritance languages  abstract classes are used to either have defined class variables in addition to none or more methods  or to exploit the single-inheritance model to limit the range of classes that could use a set of methods     Languages that support the multiple-inheritance model tend to use only classes or abstract base classes and not interfaces   Since Python supports multiple inheritance  it does not use interfaces and you would want to use base classes or abstract base classes   http   docs python org library abc html

User · Answer

In general  interfaces are used only in languages that use the single-inheritance class model  In these single-inheritance languages  interfaces are typically used if any class could use a particular method or set of methods  Also in these single-inheritance languages  abstract classes are used to either have defined class variables in addition to none or more methods  or to exploit the single-inheritance model to limit the range of classes that could use a set of methods     Languages that support the multiple-inheritance model tend to use only classes or abstract base classes and not interfaces   Since Python supports multiple inheritance  it does not use interfaces and you would want to use base classes or abstract base classes   http   docs python org library abc html

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In a more basic way to explain  An interface is sort of like an empty muffin pan  It s a class file with a set of method definitions that have no code   An abstract class is the same thing  but not all functions need to be empty  Some can have code  It s not strictly empty   Why differentiate  There s not much practical difference in Python  but on the planning level for a large project  it could be more common to talk about interfaces  since there s no code  Especially if you re working with Java programmers who are accustomed to the term

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Python doesn t really have either concept   It uses duck typing  which removed the need for interfaces  at least for the computer  -    Python  lt   2 5  Base classes obviously exist  but there is no explicit way to mark a method as  pure virtual   so the class isn t really abstract   Python    2 6  Abstract base classes do exist  http   docs python org library abc html   And allow you to specify methods that must be implemented in subclasses  I don t much like the syntax  but the feature is there  Most of the time it s probably better to use duck typing from the  using  client side

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What is the difference between abstract class and interface in Python    An interface  for an object  is a set of methods and attributes on that object   In Python  we can use an abstract base class to define and enforce an interface   Using an Abstract Base Class  For example  say we want to use one of the abstract base classes from the collections module   import collections class MySet collections Set       pass   If we try to use it  we get an TypeError because the class we created does not support the expected behavior of sets    gt  gt  gt  MySet   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  Can t instantiate abstract class MySet with abstract methods   contains      iter      len     So we are required to implement at least   contains      iter    and    len    Let s use this implementation example from the documentation   class ListBasedSet collections Set          Alternate set implementation favoring space over speed     and not requiring the set elements to be hashable               def   init   self  iterable           self elements   lst              for value in iterable              if value not in lst                  lst append value      def   iter   self           return iter self elements      def   contains   self  value           return value in self elements     def   len   self           return len self elements   s1   ListBasedSet  abcdef   s2   ListBasedSet  defghi   overlap   s1  amp  s2   Implementation  Creating an Abstract Base Class  We can create our own Abstract Base Class by setting the metaclass to abc ABCMeta and using the abc abstractmethod decorator on relevant methods  The metaclass will be add the decorated functions to the   abstractmethods   attribute  preventing instantiation until those are defined   import abc   For example   effable  is defined as something that can be expressed in words  Say we wanted to define an abstract base class that is effable  in Python 2   class Effable object         metaclass     abc ABCMeta      abc abstractmethod     def   str   self           raise NotImplementedError  users must define   str   to use this base class     Or in Python 3  with the slight change in metaclass declaration   class Effable object  metaclass abc ABCMeta        abc abstractmethod     def   str   self           raise NotImplementedError  users must define   str   to use this base class     Now if we try to create an effable object without implementing the interface   class MyEffable Effable        pass   and attempt to instantiate it    gt  gt  gt  MyEffable   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  Can t instantiate abstract class MyEffable with abstract methods   str     We are told that we haven t finished the job   Now if we comply by providing the expected interface   class MyEffable Effable        def   str   self           return  expressable     we are then able to use the concrete version of the class derived from the abstract one    gt  gt  gt  me   MyEffable    gt  gt  gt  print me  expressable    There are other things we could do with this  like register virtual subclasses that already implement these interfaces  but I think that is beyond the scope of this question  The other methods demonstrated here would have to adapt this method using the abc module to do so  however   Conclusion  We have demonstrated that the creation of an Abstract Base Class defines interfaces for custom objects in Python

User · Answer

In general  interfaces are used only in languages that use the single-inheritance class model  In these single-inheritance languages  interfaces are typically used if any class could use a particular method or set of methods  Also in these single-inheritance languages  abstract classes are used to either have defined class variables in addition to none or more methods  or to exploit the single-inheritance model to limit the range of classes that could use a set of methods     Languages that support the multiple-inheritance model tend to use only classes or abstract base classes and not interfaces   Since Python supports multiple inheritance  it does not use interfaces and you would want to use base classes or abstract base classes   http   docs python org library abc html

User · Answer

What is the difference between abstract class and interface in Python    An interface  for an object  is a set of methods and attributes on that object   In Python  we can use an abstract base class to define and enforce an interface   Using an Abstract Base Class  For example  say we want to use one of the abstract base classes from the collections module   import collections class MySet collections Set       pass   If we try to use it  we get an TypeError because the class we created does not support the expected behavior of sets    gt  gt  gt  MySet   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  Can t instantiate abstract class MySet with abstract methods   contains      iter      len     So we are required to implement at least   contains      iter    and    len    Let s use this implementation example from the documentation   class ListBasedSet collections Set          Alternate set implementation favoring space over speed     and not requiring the set elements to be hashable               def   init   self  iterable           self elements   lst              for value in iterable              if value not in lst                  lst append value      def   iter   self           return iter self elements      def   contains   self  value           return value in self elements     def   len   self           return len self elements   s1   ListBasedSet  abcdef   s2   ListBasedSet  defghi   overlap   s1  amp  s2   Implementation  Creating an Abstract Base Class  We can create our own Abstract Base Class by setting the metaclass to abc ABCMeta and using the abc abstractmethod decorator on relevant methods  The metaclass will be add the decorated functions to the   abstractmethods   attribute  preventing instantiation until those are defined   import abc   For example   effable  is defined as something that can be expressed in words  Say we wanted to define an abstract base class that is effable  in Python 2   class Effable object         metaclass     abc ABCMeta      abc abstractmethod     def   str   self           raise NotImplementedError  users must define   str   to use this base class     Or in Python 3  with the slight change in metaclass declaration   class Effable object  metaclass abc ABCMeta        abc abstractmethod     def   str   self           raise NotImplementedError  users must define   str   to use this base class     Now if we try to create an effable object without implementing the interface   class MyEffable Effable        pass   and attempt to instantiate it    gt  gt  gt  MyEffable   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  Can t instantiate abstract class MyEffable with abstract methods   str     We are told that we haven t finished the job   Now if we comply by providing the expected interface   class MyEffable Effable        def   str   self           return  expressable     we are then able to use the concrete version of the class derived from the abstract one    gt  gt  gt  me   MyEffable    gt  gt  gt  print me  expressable    There are other things we could do with this  like register virtual subclasses that already implement these interfaces  but I think that is beyond the scope of this question  The other methods demonstrated here would have to adapt this method using the abc module to do so  however   Conclusion  We have demonstrated that the creation of an Abstract Base Class defines interfaces for custom objects in Python

User · Answer

Python doesn t really have either concept   It uses duck typing  which removed the need for interfaces  at least for the computer  -    Python  lt   2 5  Base classes obviously exist  but there is no explicit way to mark a method as  pure virtual   so the class isn t really abstract   Python    2 6  Abstract base classes do exist  http   docs python org library abc html   And allow you to specify methods that must be implemented in subclasses  I don t much like the syntax  but the feature is there  Most of the time it s probably better to use duck typing from the  using  client side

User · Answer

Python doesn t really have either concept   It uses duck typing  which removed the need for interfaces  at least for the computer  -    Python  lt   2 5  Base classes obviously exist  but there is no explicit way to mark a method as  pure virtual   so the class isn t really abstract   Python    2 6  Abstract base classes do exist  http   docs python org library abc html   And allow you to specify methods that must be implemented in subclasses  I don t much like the syntax  but the feature is there  Most of the time it s probably better to use duck typing from the  using  client side

User · Answer

Python    2 6 has Abstract Base Classes      Abstract Base Classes  abbreviated   ABCs  complement duck-typing by   providing a way to define interfaces   when other techniques like hasattr     would be clumsy  Python comes with   many builtin ABCs for data structures    in the collections module   numbers    in the numbers module   and streams    in the io module   You can create   your own ABC with the abc module    There is also the Zope Interface module  which is used by projects outside of zope  like twisted  I m not really familiar with it  but there s a wiki page here that might help   In general  you don t need the concept of abstract classes  or interfaces in python  edited - see S Lott s answer for details

User · Answer

In a more basic way to explain  An interface is sort of like an empty muffin pan  It s a class file with a set of method definitions that have no code   An abstract class is the same thing  but not all functions need to be empty  Some can have code  It s not strictly empty   Why differentiate  There s not much practical difference in Python  but on the planning level for a large project  it could be more common to talk about interfaces  since there s no code  Especially if you re working with Java programmers who are accustomed to the term

User · Answer

What you ll see sometimes is the following   class Abstract1  object           Some description that tells you it s abstract      often listing the methods you re expected to supply         def aMethod  self            raise NotImplementedError   Should have implemented this      Because Python doesn t have  and doesn t need  a formal Interface contract  the Java-style distinction between abstraction and interface doesn t exist   If someone goes through the effort to define a formal interface  it will also be an abstract class   The only differences would be in the stated intent in the docstring     And the difference between abstract and interface is a hairsplitting thing when you have duck typing   Java uses interfaces because it doesn t have multiple inheritance   Because Python has multiple inheritance  you may also see something like this  class SomeAbstraction  object        pass   lots of stuff - but missing something  class Mixin1  object        def something  self            pass   one implementation  class Mixin2  object        def something  self            pass   another  class Concrete1  SomeAbstraction  Mixin1        pass  class Concrete2  SomeAbstraction  Mixin2        pass   This uses a kind of abstract superclass with mixins to create concrete subclasses that are disjoint

User · Answer

In general  interfaces are used only in languages that use the single-inheritance class model  In these single-inheritance languages  interfaces are typically used if any class could use a particular method or set of methods  Also in these single-inheritance languages  abstract classes are used to either have defined class variables in addition to none or more methods  or to exploit the single-inheritance model to limit the range of classes that could use a set of methods     Languages that support the multiple-inheritance model tend to use only classes or abstract base classes and not interfaces   Since Python supports multiple inheritance  it does not use interfaces and you would want to use base classes or abstract base classes   http   docs python org library abc html

User · Answer

What you ll see sometimes is the following   class Abstract1  object           Some description that tells you it s abstract      often listing the methods you re expected to supply         def aMethod  self            raise NotImplementedError   Should have implemented this      Because Python doesn t have  and doesn t need  a formal Interface contract  the Java-style distinction between abstraction and interface doesn t exist   If someone goes through the effort to define a formal interface  it will also be an abstract class   The only differences would be in the stated intent in the docstring     And the difference between abstract and interface is a hairsplitting thing when you have duck typing   Java uses interfaces because it doesn t have multiple inheritance   Because Python has multiple inheritance  you may also see something like this  class SomeAbstraction  object        pass   lots of stuff - but missing something  class Mixin1  object        def something  self            pass   one implementation  class Mixin2  object        def something  self            pass   another  class Concrete1  SomeAbstraction  Mixin1        pass  class Concrete2  SomeAbstraction  Mixin2        pass   This uses a kind of abstract superclass with mixins to create concrete subclasses that are disjoint

User · Answer

For completeness  we should mention PEP3119 where ABC was introduced and compared with interfaces  and original Talin s comment   The abstract class is not perfect interface    belongs to the inheritance hierarchy is mutable   But if you consider writing it your own way   def some function self        raise NotImplementedError    interface   type       your interface    object          extra func   some function          slots       extra func                         instancecheck     your instance checker          subclasscheck     your subclass checker                   ok  rather as a class or as a metaclass and fighting with python to achieve the immutable object and doing refactoring       you ll quite fast realize that you re inventing the wheel to eventually achieve abc ABCMeta  abc ABCMeta was proposed as a useful addition of the missing interface functionality  and that s fair enough in a language like python   Certainly  it was able to be enhanced better whilst writing version 3  and adding new syntax and immutable interface concept      Conclusion   The abc ABCMeta IS  pythonic  interface in python

User · Answer

Abstract classes are classes that contain one or more abstract methods  Along with abstract methods  Abstract classes can have static  class and instance methods  But in case of interface  it will only have abstract methods not other  Hence it is not compulsory to inherit abstract class but it is compulsory to inherit interface

User · Answer

Python doesn t really have either concept   It uses duck typing  which removed the need for interfaces  at least for the computer  -    Python  lt   2 5  Base classes obviously exist  but there is no explicit way to mark a method as  pure virtual   so the class isn t really abstract   Python    2 6  Abstract base classes do exist  http   docs python org library abc html   And allow you to specify methods that must be implemented in subclasses  I don t much like the syntax  but the feature is there  Most of the time it s probably better to use duck typing from the  using  client side

User · Answer

Python    2 6 has Abstract Base Classes      Abstract Base Classes  abbreviated   ABCs  complement duck-typing by   providing a way to define interfaces   when other techniques like hasattr     would be clumsy  Python comes with   many builtin ABCs for data structures    in the collections module   numbers    in the numbers module   and streams    in the io module   You can create   your own ABC with the abc module    There is also the Zope Interface module  which is used by projects outside of zope  like twisted  I m not really familiar with it  but there s a wiki page here that might help   In general  you don t need the concept of abstract classes  or interfaces in python  edited - see S Lott s answer for details

User · Answer

What you ll see sometimes is the following   class Abstract1  object           Some description that tells you it s abstract      often listing the methods you re expected to supply         def aMethod  self            raise NotImplementedError   Should have implemented this      Because Python doesn t have  and doesn t need  a formal Interface contract  the Java-style distinction between abstraction and interface doesn t exist   If someone goes through the effort to define a formal interface  it will also be an abstract class   The only differences would be in the stated intent in the docstring     And the difference between abstract and interface is a hairsplitting thing when you have duck typing   Java uses interfaces because it doesn t have multiple inheritance   Because Python has multiple inheritance  you may also see something like this  class SomeAbstraction  object        pass   lots of stuff - but missing something  class Mixin1  object        def something  self            pass   one implementation  class Mixin2  object        def something  self            pass   another  class Concrete1  SomeAbstraction  Mixin1        pass  class Concrete2  SomeAbstraction  Mixin2        pass   This uses a kind of abstract superclass with mixins to create concrete subclasses that are disjoint

User · Answer

Abstract classes are classes that contain one or more abstract methods  Along with abstract methods  Abstract classes can have static  class and instance methods  But in case of interface  it will only have abstract methods not other  Hence it is not compulsory to inherit abstract class but it is compulsory to inherit interface

User · Answer

Python    2 6 has Abstract Base Classes      Abstract Base Classes  abbreviated   ABCs  complement duck-typing by   providing a way to define interfaces   when other techniques like hasattr     would be clumsy  Python comes with   many builtin ABCs for data structures    in the collections module   numbers    in the numbers module   and streams    in the io module   You can create   your own ABC with the abc module    There is also the Zope Interface module  which is used by projects outside of zope  like twisted  I m not really familiar with it  but there s a wiki page here that might help   In general  you don t need the concept of abstract classes  or interfaces in python  edited - see S Lott s answer for details

User · Answer

For completeness  we should mention PEP3119 where ABC was introduced and compared with interfaces  and original Talin s comment   The abstract class is not perfect interface    belongs to the inheritance hierarchy is mutable   But if you consider writing it your own way   def some function self        raise NotImplementedError    interface   type       your interface    object          extra func   some function          slots       extra func                         instancecheck     your instance checker          subclasscheck     your subclass checker                   ok  rather as a class or as a metaclass and fighting with python to achieve the immutable object and doing refactoring       you ll quite fast realize that you re inventing the wheel to eventually achieve abc ABCMeta  abc ABCMeta was proposed as a useful addition of the missing interface functionality  and that s fair enough in a language like python   Certainly  it was able to be enhanced better whilst writing version 3  and adding new syntax and immutable interface concept      Conclusion   The abc ABCMeta IS  pythonic  interface in python

User · Answer

What you ll see sometimes is the following   class Abstract1  object           Some description that tells you it s abstract      often listing the methods you re expected to supply         def aMethod  self            raise NotImplementedError   Should have implemented this      Because Python doesn t have  and doesn t need  a formal Interface contract  the Java-style distinction between abstraction and interface doesn t exist   If someone goes through the effort to define a formal interface  it will also be an abstract class   The only differences would be in the stated intent in the docstring     And the difference between abstract and interface is a hairsplitting thing when you have duck typing   Java uses interfaces because it doesn t have multiple inheritance   Because Python has multiple inheritance  you may also see something like this  class SomeAbstraction  object        pass   lots of stuff - but missing something  class Mixin1  object        def something  self            pass   one implementation  class Mixin2  object        def something  self            pass   another  class Concrete1  SomeAbstraction  Mixin1        pass  class Concrete2  SomeAbstraction  Mixin2        pass   This uses a kind of abstract superclass with mixins to create concrete subclasses that are disjoint

User · Answer

Python    2 6 has Abstract Base Classes      Abstract Base Classes  abbreviated   ABCs  complement duck-typing by   providing a way to define interfaces   when other techniques like hasattr     would be clumsy  Python comes with   many builtin ABCs for data structures    in the collections module   numbers    in the numbers module   and streams    in the io module   You can create   your own ABC with the abc module    There is also the Zope Interface module  which is used by projects outside of zope  like twisted  I m not really familiar with it  but there s a wiki page here that might help   In general  you don t need the concept of abstract classes  or interfaces in python  edited - see S Lott s answer for details

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