[Python-Dev] PEP 343 update (with statement context terminology) (original) (raw)
Nick Coghlan ncoghlan at gmail.com
Tue Apr 25 05:22:18 CEST 2006
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Paul Moore wrote:
In the documentation of contextmanager, consider the examples:
@contextmanager def tag(name): ... class Tag: ... @contextmanager def context(self): ... Now, tag should be a function which returns a context manager (a1 definition - object with a context method) ("with" statement, item 1 of the definition). On the other hand, Tag.context is a context manager's context method, which according to item 2 of the definition of the "with" statement, should be a function which returns a context object (a1 definition - object with enter and exit methods). These are two different types of function.
Paraphrasing:
def iterseq(seq): for x in range(len(seq)): yield x[i]
class Container: ... def iter(self): for x in self.contents: yield x
Now, iterseq should be a function which returns an iterable (object with an iter() method that is passed to for statements). On the other hand, Tag.iter is an iterable's iter method, which should be a function which returns an iterator (an object with iter() and next() methods).
These are two different types of function.
Why yes, yes they are. When iterators were introduced, a deliberate design decision was taken to make the iterator protocol a superset of the iterable protocol, precisely to allow for loops not to care whether they were being given an iterable or an iterator.
Iterator's can be standalone, or they can be intended specifically for use as a particular iterable's native iterator. The terminology is the same either way.
PEP 343 made a deliberate, conscious design decision to copy the semantics of iterators by making the context management protocol a superset of the context protocol (or rather, the context specification protocol in alpha 2).
What I don't understand is why you and Phillip are so keen on breaking this deliberate symmetry with an existing part of the language design. Of course it isn't necessary, but then neither was it necessary to make the iterator protocol a superset of the iterable protocol.
Cheers, Nick.
-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia
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