En Tue, 22 Jun 2010 23:33:55 -0300, Zac Burns <zac...@gmail.com> escribió:
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):
http://svn.python.org/view/python/trunk/Lib/threading_api.py?view=markup&pathrev=10430
"Lock()
A factory function that returns a new primitive lock object...
RLock()
A factory function that returns..."
Same for Condition, Semaphore, Event, but:
"Thread
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
--
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