Is it possible to make abstract classes in Python

Question

How can I make a class or method abstract in Python   I tried redefining   new     like so   class F      def   new   cls           raise Exception  Unable to create an instance of abstract class  s   cls    but now if I create a class G that inherits from F like so   class G F       pass   then I can t instantiate G either  since it calls its super class s   new   method   Is there a better way to define an abstract class

User · Answer

I find the accepted answer  and all the others strange  since they pass self to an abstract class   An abstract class is not instantiated so can t have a self    So try this  it works   from abc import ABCMeta  abstractmethod   class Abstract metaclass ABCMeta        staticmethod      abstractmethod     def foo               An abstract method  No need to write pass      class Derived Abstract       def foo self           print  Hooray      FOO   Derived   FOO foo

User · Answer

As explained in the other answers  yes you can use abstract classes in Python using the abc module  Below I give an actual example using abstract  classmethod   property and  abstractmethod  using Python 3 6    For me it is usually easier to start off with examples I can easily copy amp paste  I hope this answer is also useful for others  Let s first create a base class called Base  from abc import ABC  abstractmethod  class Base ABC         classmethod      abstractmethod     def from dict cls  d           pass           property      abstractmethod     def prop1 self           pass       property      abstractmethod     def prop2 self           pass       prop2 setter      abstractmethod     def prop2 self  val           pass       abstractmethod     def do stuff self           pass  Our Base class will always have a from dict classmethod  a property prop1  which is read-only  and a property prop2  which can also be set  as well as a function called do stuff  Whatever class is now built based on Base will have to implement all of these four methods properties  Please note that for a method to be abstract  two decorators are required - classmethod and abstract property  Now we could create a class A like this  class A Base       def   init   self  name  val1  val2           self name   name         self   val1   val1         self  val2   val2       classmethod     def from dict cls  d           name   d  name           val1   d  val1           val2   d  val2            return cls name  val1  val2        property     def prop1 self           return self   val1       property     def prop2 self           return self  val2       prop2 setter     def prop2 self  value           self  val2   value      def do stuff self           print  juhu         def i am not abstract self           print  I can be customized    All required methods properties are implemented and we can - of course - also add additional functions that are not part of Base  here  i am not abstract   Now we can do  a1   A  dummy   10   stuff   a2   A from dict   name    from d    val1   20   val2    stuff     a1 prop1   prints 10  a1 prop2   prints  stuff   As desired  we cannot set prop1  a prop1   100  will return  AttributeError  can t set attribute  Also our from dict method works fine  a2 prop1   prints 20  If we now defined a second class B like this  class B Base       def   init   self  name           self name   name       property     def prop1 self           return self name  and tried to instantiate an object like this  b   B  iwillfail    we will get an error  TypeError  Can t instantiate abstract class B with abstract methods do stuff  from dict  prop2  listing all the things defined in Base which we did not implement in B

User · Answer

also this works and is simple   class A abstract object        def   init   self             quite simple  old-school way          if self   class     name       A abstract                raise NotImplementedError  You can t instantiate this abstract class  Derive it  please     class B A abstract            pass  b   B      here an exception is raised  a   A abstract

User · Answer

Most of the answers inherit the base class to define the abstract methods  But this is not always useful  What if you want to define an abstract method at runtime  For example in java we can do this class UserClass        BaseClass f   new BaseClass         public void method             system out println   quot this is a test quot                 So what to do if we need to implement that  so in that case class BaseClass       def   init   self  func            self function   func      def abstract function self            if not self function              raise NotImplementedError  quot function not implemented quot            else              return self function        def run self            self abstract function    def func        print  this is a test    bc   BaseClass  func   bc run    should work

User · Answer

Here s a very easy way without having to deal with the ABC module   In the   init   method of the class that you want to be an abstract class  you can check the  type  of self   If the type of self is the base class  then the caller is trying to instantiate the base class  so raise an exception   Here s a simple example   class Base        def   init   self           if type self  is Base              raise Exception  Base is an abstract class and cannot be instantiated directly             Any initialization code         print  In the   init    method of the Base class    class Sub Base       def   init   self           print  In the   init   method of the Sub class before calling   init   of the Base class           super     init             print  In the   init   method of the Sub class after calling   init   of the Base class    subObj   Sub   baseObj   Base     When run  it produces   In the   init   method of the Sub class before calling   init   of the Base class In the   init    method of the Base class In the   init   method of the Sub class after calling   init   of the Base class Traceback  most recent call last     File   Users irvkalb Desktop Demo files Abstract py   line 16  in  lt module gt      baseObj   Base     File   Users irvkalb Desktop Demo files Abstract py   line 4  in   init       raise Exception  Base is an abstract class and cannot be instantiated directly   Exception  Base is an abstract class and cannot be instantiated directly   This shows that you can instantiate a subclass that inherits from a base class  but you cannot instantiate the base class directly

User · Answer

The old-school  pre-PEP 3119  way to do this is just to raise NotImplementedError in the abstract class when an abstract method is called   class Abstract object       def foo self           raise NotImplementedError  subclasses must override foo       class Derived Abstract       def foo self           print  Hooray     gt  gt  gt  d   Derived    gt  gt  gt  d foo   Hooray   gt  gt  gt  a   Abstract    gt  gt  gt  a foo   Traceback  most recent call last           This doesn t have the same nice properties as using the abc module does  You can still instantiate the abstract base class itself  and you won t find your mistake until you call the abstract method at runtime    But if you re dealing with a small set of simple classes  maybe with just a few abstract methods  this approach is a little easier than trying to wade through the abc documentation

User · Answer

You can also harness the   new   method to your advantage  You just forgot something  The   new   method always returns the new object so you must return its superclass  new method  Do as follows   class F      def   new   cls           if cls is F              raise TypeError  Cannot create an instance of abstract class       format cls   name             return super     new   cls    When using the new method  you have to return the object  not the None keyword  That s all you missed

User · Answer

from abc import ABCMeta  abstractmethod    Abstract class and abstract method declaration  class Jungle metaclass ABCMeta         constructor with default values      def   init   self  name  Unknown         self visitorName   name       def welcomeMessage self            print  Hello  s   Welcome to the Jungle    self visitorName          abstract method is compulsory to defined in child-class       abstractmethod      def scarySound self            pass

User · Answer

Use the abc module to create abstract classes  Use the abstractmethod decorator to declare a method abstract  and declare a class abstract using one of three ways  depending upon your Python version   In Python 3 4 and above  you can inherit from ABC  In earlier versions of Python  you need to specify your class s metaclass as ABCMeta  Specifying the metaclass has different syntax in Python 3 and Python 2  The three possibilities are shown below     Python 3 4  from abc import ABC  abstractmethod class Abstract ABC        abstractmethod     def foo self           pass       Python 3 0  from abc import ABCMeta  abstractmethod class Abstract metaclass ABCMeta        abstractmethod     def foo self           pass       Python 2 from abc import ABCMeta  abstractmethod class Abstract        metaclass     ABCMeta       abstractmethod     def foo self           pass   Whichever way you use  you won t be able to instantiate an abstract class that has abstract methods  but will be able to instantiate a subclass that provides concrete definitions of those methods    gt  gt  gt  Abstract   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  Can t instantiate abstract class Abstract with abstract methods foo  gt  gt  gt  class StillAbstract Abstract           pass       gt  gt  gt  StillAbstract   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  Can t instantiate abstract class StillAbstract with abstract methods foo  gt  gt  gt  class Concrete Abstract           def foo self               print  Hello  World         gt  gt  gt  Concrete    lt   main   Concrete object at 0x7fc935d28898 gt

User · Answer

This one will be working in python 3   from abc import ABCMeta  abstractmethod  class Abstract metaclass ABCMeta         abstractmethod     def foo self           pass  Abstract    gt  gt  gt  TypeError  Can not instantiate abstract class Abstract with abstract methods foo

User · Answer

Just a quick addition to  TimGilbert s old-school answer   you can make your abstract base class s init   method throw an exception and that would prevent it from being instantiated  no    gt  gt  gt  class Abstract object           def   init   self               raise NotImplementedError  You can t instantiate this class         gt  gt  gt  a   Abstract   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt    File   lt stdin gt    line 3  in   init   NotImplementedError  You can t instantiate this class

User · Answer

In your code snippet  you could also resolve this by providing an implementation for the   new   method in the subclass  likewise   def G F       def   new   cls             do something here   But this is a hack and I advise you against it  unless you know what you are doing  For nearly all cases I advise you to use the abc module  that others before me have suggested   Also when you create a new  base  class  make it subclass object  like this  class MyBaseClass object    I don t know if it is that much significant anymore  but it helps retain style consistency on your code

User · Answer

Most Previous answers were correct but here is the answer and example for Python 3 7  Yes  you can create an abstract class and method  Just as a reminder sometimes a class should define a method which logically belongs to a class  but that class cannot specify how to implement the method  For example  in the below Parents and Babies classes they both eat but the implementation will be different for each because babies and parents eat a different kind of food and the number of times they eat is different  So  eat method subclasses overrides AbstractClass eat    from abc import ABC  abstractmethod  class AbstractClass ABC        def   init   self  value           self value   value         super     init           abstractmethod     def eat self           pass  class Parents AbstractClass       def eat self           return  eat solid food    str self value      times each day   class Babies AbstractClass       def eat self           return  Milk only    str self value      times or more each day   food   3     mom   Parents food  print  moms ----------   print mom eat     infant   Babies food  print  infants ----------   print infant eat      OUTPUT   moms ---------- eat solid food 3 times each day infants ---------- Milk only 3 times or more each day

[python] Is it possible to make abstract classes in Python?

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