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