python @abstractmethod decorator

Question

I have read python docs about abstract base classes:

From here:

abc.abstractmethod(function) A decorator indicating abstract methods.

Using this decorator requires that the class’s metaclass is ABCMeta or is derived from it. A class that has a metaclass derived from ABCMeta cannot be instantiated unless all of its abstract methods and properties are overridden.

And here

You can apply the @abstractmethod decorator to methods such as draw() that must be implemented; Python will then raise an exception for classes that don’t define the method. Note that the exception is only raised when you actually try to create an instance of a subclass lacking the method.

I've used this code to test that out:

import abc  class AbstractClass(object):   __metaclass__ = abc.ABCMeta    @abc.abstractmethod   def abstractMethod(self):     return  class ConcreteClass(AbstractClass):   def __init__(self):     self.me = "me"  c = ConcreteClass() c.abstractMethod() 

The code goes fine, so I don't get it. If I type c.abstractMethod I get:

<bound method ConcreteClass.abstractMethod of <__main__.ConcreteClass object at 0x7f694da1c3d0>> 

What I'm missing here? ConcreteClass must implement the abstract methods, but I get no exception.

Answer 1

Are you using python3 to run that code? If yes, you should know that declaring metaclass in python3 have changes you should do it like this instead:

import abc  class AbstractClass(metaclass=abc.ABCMeta):    @abc.abstractmethod   def abstractMethod(self):       return 

The full code and the explanation behind the answer is:

import abc  class AbstractClass(metaclass=abc.ABCMeta):      @abc.abstractmethod     def abstractMethod(self):         return  class ConcreteClass(AbstractClass):      def __init__(self):         self.me = "me"  # Will get a TypeError without the following two lines: #   def abstractMethod(self): #       return 0  c = ConcreteClass() c.abstractMethod() 

If abstractMethod is not defined for ConcreteClass, the following exception will be raised when running the above code: TypeError: Can't instantiate abstract class ConcreteClass with abstract methods abstractMethod

Answer 2

Import ABC from abc and make your own abstract class a child of ABC can help make the code look cleaner.

from abc import ABC, abstractmethod  class AbstractClass(ABC):    @abstractmethod   def abstractMethod(self):     return  class ConcreteClass(AbstractClass):   def __init__(self):     self.me = "me"  # The following would raise a TypeError complaining  # abstractMethod is not implemented c = ConcreteClass()   

Tested with Python 3.6