Determine if a Python class is an Abstract Base Class or Concrete

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