Double-underscore names and methods are special to Python. Developers are
prohibited from creating their own (although the language doesn't enforce
that prohibition). From PEP 0008, written by Guido himself:
__double_leading_and_trailing_underscore__: "magic" objects or
attributes that live in user-controlled namespaces. E.g. __init__,
__import__ or __file__. Never invent such names; only use them
as documented.
http://www.python.org/dev/peps/pep-0008/
But then there are modules like doctest, which uses the special double-
underscore name __test__.
There are times where I would like to create my own protocol, like the
early sequence protocol: if an object has a __getitem__ method, it can be
used with for loops. Python calls obj.__getitem__(i) with i starting at 0
and increasing by one each time until it gets an IndexError exception.
This sequence protocol has been made partly obsolete by the iterator
protocol, but you see the point: in the spirit of duck typing, having the
ability to create a protocol is a Very Good Thing, and double leading and
trailing underscore names are the accepted Python style for such special
methods.
But the style guide says not to do that.
So I find myself conflicted. I'm aware that the style guide also says,
know when to break the rules, but the examples given don't seem to apply
to this case. The prohibition against inventing new double-underscore
names like __parrot__ seems much stronger than the other style guides.
So what do folks think? I believe the protocol idiom ("look for a method
called __parrot__ and then do something with it") is too useful and
powerful to be ignored, but then if __parrot__ is reserved by Python,
what to do?
--
Steven
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