Question

My Python application contains many abstract classes and implementations. For example:

import abc
import datetime

class MessageDisplay(object):
    __metaclass__ = abc.ABCMeta

    @abc.abstractproperty
    def display(self, message):
        pass

class FriendlyMessageDisplay(MessageDisplay):
    def greet(self):
        hour = datetime.datetime.now().timetuple().tm_hour

        if hour < 7:
            raise Exception("Cannot greet while asleep.")
        elif hour < 12:
            self.display("Good morning!")
        elif hour < 18:
            self.display("Good afternoon!")
        elif hour < 20:
            self.display("Good evening!")
        else:
            self.display("Good night.")

class FriendlyMessagePrinter(FriendlyMessageDisplay):
    def display(self, message):
        print(message)

FriendlyMessagePrinter is a concrete class that we can use…

FriendlyMessagePrinter().greet()

Good night.

…but MessageDisplay and FriendlyMessageDisplay are abstract classes and attempting to instantiate one would result in an error:

TypeError: Can't instantiate abstract class MessageDisplay with abstract methods say

How can I check if a given class object is an (uninstantiatable) abstract class?

Answer 1

import inspect
print(inspect.isabstract(object))                  # False
print(inspect.isabstract(MessageDisplay))          # True
print(inspect.isabstract(FriendlyMessageDisplay))  # True
print(inspect.isabstract(FriendlyMessagePrinter))  # False

This checks that the internal flag TPFLAGS_IS_ABSTRACT is set in the class object, so it can’t be fooled as easily as your implementation:

class Fake:
    __abstractmethods__ = 'bluh'

print(is_abstract(Fake), inspect.isabstract(Fake)) # True, False