pythonsubclassfactorymetaclass

how do i subclass threading.Event?


In Python 2.7.5:

from threading import Event

class State(Event):
    def __init__(self, name):
        super(Event, self).__init__()
        self.name = name

    def __repr__(self):
        return self.name + ' / ' + self.is_set()

I get:

TypeError: Error when calling the metaclass bases
function() argument 1 must be code, not str

Why?

Everything I know about threading.Event I learned from: http://docs.python.org/2/library/threading.html?highlight=threading#event-objects

What does it mean when it says that threading.Event() is a factory function for the class threading.Event ??? (Uhh... just looks like plain old instanciation to me).


Solution

  • threading.Event is not a class, it's function in threading.py

    def Event(*args, **kwargs):
        """A factory function that returns a new event.
    
        Events manage a flag that can be set to true with the set() method and reset
        to false with the clear() method. The wait() method blocks until the flag is
        true.
    
        """
        return _Event(*args, **kwargs)
    

    Sinse this function returns _Event instance, you can subclass _Event (although it's never a good idea to import and use underscored names):

    from threading import _Event
    
    class State(_Event):
        def __init__(self, name):
            super(Event, self).__init__()
            self.name = name
    
        def __repr__(self):
            return self.name + ' / ' + self.is_set()