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 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.
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