python @abstractmethod decorator
-
12-01-2021 - |
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 fromABCMeta
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.
Solution
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