python @abstractmethod decorator

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.

2 Answers

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

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