Struct usages in Python

[Arnaud Delobelle]

"Alex Gusarov" <alex.m.gusarov at gmail.com> writes:

>>  class Event(object):
>>
>> Always subclass object, unless you have a very compelling reason not to,
>> or you are subclassing something else.
>>
>
> I've thought that if I write
>
> class Event:
>     pass
>
> , it'll be subclass of object too, I was wrong?

You are wrong for Python 2.X, but right for Python 3 where old-style
classes are gone for good.

What you define with the statement

    class Event: pass

is an 'old-style' class.  Witness:

    >>> class Event: pass
    ... 
    >>> class NewEvent(object): pass
    ... 
    >>> type(Event)
    <type 'classobj'>
    >>> type(NewEvent)
    <type 'type'>
    >>> type(Event())
    <type 'instance'>
    del>>> type(NewEvent())
    <class '__main__.NewEvent'>

All old-style classes are actually objects of type 'classobj' (they
all have the same type!), all their instances are all of type 'instance'.

    >>> type(FooBar) == type(Event)
    True
    >>> type(FooBar()) == type(Event())
    True

Whereas instances of new-style classes are of type their class:

    >>> class NewFooBar(object): pass
    ... 
    >>> type(NewFooBar) == type(NewEvent)
    True
    >>> type(NewFooBar()) == type(NewEvent())
    False

However, in python 2.X (X > 2?), you can force all classes to of a
certain type by setting the global variable '__metaclass__'. So:

    >>> type(Event) # Event is an old-style class
    <type 'classobj'>
    >>> __metaclass__ = type
    >>> class Event: pass
    ... 
    >>> type(Event) # Now Event is new-style!
    <type 'type'>

HTH

-- 
Arnaud