From: Gabriel Genellina on
En Tue, 22 Jun 2010 23:33:55 -0300, Zac Burns <zac256(a)> escribi�:

> In the threading module there are several code bits following this
> convention:
> ###
> def Class(*args, **kwargs):
> return _Class(*args, **kwargs)
> class _Class(...
> ###
> This is true for Event, RLock, and others.
> Why do this? It seems to violate the rule of least astonishment
> (isinstance(Event(), Event) raises an exception). I assume there must be
> some trade-off that the designer intended to achieve. So, what is that
> trade-off and when should I follow this in my code?

Those classes are not meant to be subclassed (I don't know *why*). From
original module documentation (1998):

A factory function that returns a new primitive lock object...

A factory function that returns..."

Same for Condition, Semaphore, Event, but:

A class that represents a thread of control -- subclassable."

"Note that the classes marked as ''do not subclass'' are actually
implemented as factory functions; classes are
shown here as a way to structure the documentation only."

And those classes have a big "*** DO NOT SUBCLASS THIS CLASS ***" message.
The message never made into the documentation.

Gabriel Genellina