Abstract methods in Python

Question

I am having trouble in using inheritance with Python  While the concept seems too easy for me in Java yet up till now I have been unable to understand in Python which is surprising to me at least   I have a prototype which follow   class Shape       def   init   self  shape name          self shape   shape name  class Rectangle Shape      def   init   self  name          self shape   name   In the above code how can I make an abstract method that would need to be implemented for all the subclasses

User · Accepted Answer

Something along these lines  using ABC  import abc  class Shape object         metaclass     abc ABCMeta       abc abstractmethod     def method to implement self  input              Method documentation            return   Also read this good tutorial  http   www doughellmann com PyMOTW abc   You can also check out zope interface which was used prior to introduction of ABC in python      http   pypi python org pypi zope interface http   wiki zope org Interfaces FrontPage

User · Answer

You have to implement an abstract base class  ABC    Check python docs

User · Answer

Abstract base classes are deep magic   Periodically I implement something using them and am amazed at my own cleverness  very shortly afterwards I find myself completely confused by my own cleverness  this may well just be a personal limitation though    Another way of doing this  should be in the python std libs if you ask me  is to make a decorator   def abstractmethod method               An  abstractmethod member fn decorator       put this in some library somewhere for reuse                def default abstract method  args    kwargs           raise NotImplementedError  call to abstract method                                        repr method       default abstract method   name     method   name           return default abstract method   class Shape object        def   init   self  shape name          self shape   shape name       abstractmethod     def foo self           print  bar          return  class Rectangle Shape         note you don t need to do the constructor twice either     pass    r   Rectangle  x   r foo     I didn t write the decorator   It just occurred to me someone would have   You can find it here  http   code activestate com recipes 577666-abstract-method-decorator   Good one jimmy2times   Note the discussion at the bottom of that page r e  type safety of the decorator    That could be fixed with the inspect module if anyone was so inclined

User · Answer

You can t  with language primitives  As has been called out  the abc package provides this functionality in Python 2 6 and later  but there are no options for Python 2 5 and earlier  The abc package is not a new feature of Python  instead  it adds functionality by adding explicit  does this class say it does this   checks  with manually-implemented consistency checks to cause an error during initialization if such declarations are made falsely   Python is a militantly dynamically-typed language  It does not specify language primitives to allow you to prevent a program from compiling because an object does not match type requirements  this can only be discovered at run time  If you require that a subclass implement a method  document that  and then just call the method in the blind hope that it will be there   If it s there  fantastic  it simply works  this is called duck typing  and your object has quacked enough like a duck to satisfy the interface  This works just fine even if self is the object you re calling such a method on  for the purposes of mandatory overrides due to base methods that need specific implementations of features  generic functions   because self is a convention  not anything actually special   The exception is in   init    because when your initializer is being called  the derived type s initializer hasn t  so it hasn t had the opportunity to staple its own methods onto the object yet   If the method was t implemented  you ll get an AttributeError  if it s not there at all  or a TypeError  if something by that name is there but it s not a function or it didn t have that signature   It s up to you how you handle that- either call it programmer error and let it crash the program  and it  should  be obvious to a python developer what causes that kind of error there- an unmet duck interface   or catch and handle those exceptions when you discover that your object didn t support what you wish it did  Catching AttributeError and TypeError is important in a lot of situations  actually

User · Answer

You can use six and abc to construct a class for both python2 and python3 efficiently as follows    import six import abc   six add metaclass abc ABCMeta  class MyClass object               documentation               abc abstractmethod     def initialize self  para None                       documentation                     raise NotImplementedError   This is an awesome document of it

User · Answer

See the abc module  Basically  you define   metaclass     abc ABCMeta on the class  then decorate each abstract method with  abc abstractmethod  Classes derived from this class cannot then be instantiated unless all abstract methods have been overridden   If your class is already using a metaclass  derive it from ABCMeta rather than type and you can continue to use your own metaclass   A cheap alternative  and the best practice before the abc module was introduced  would be to have all your abstract methods just raise an exception  NotImplementedError is a good one  so that classes derived from it would have to override that method to be useful    However  the abc solution is better because it keeps such classes from being instantiated at all  i e   it  fails faster    and also because you can provide a default or base implementation of each method that can be reached using the super   function in derived classes

User · Answer

Before abc was introduced you would see this frequently   class Base object       def go self           raise NotImplementedError  Please Implement this method     class Specialized Base       def go self           print  Consider me implemented

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