[Python-Dev] PEP 343: confusing context terminology (original) (raw)

Phillip J. Eby pje at telecommunity.com
Tue Apr 18 17:59:17 CEST 2006

At 10:04 AM 4/18/2006 -0400, A.M. Kuchling wrote:

PEP 343 says:

This PEP proposes that the protocol used by the with statement be known as the "context management protocol", and that objects that implement that protocol be known as "context managers". The term "context" then encompasses all objects with a context() method that returns a context manager (this means that all context managers are contexts, but not all contexts are context managers). I read this paragraph as meaning: context = 'thing with context()' context manager = 'thing returned by context()' So in a 'with' statement: with A as B: ... A is a context; the context manager is internal and unavailable.

I believe this is backwards. But then again, I would think that, since I wrote the documentation that takes the opposite tack. :)

Partly, that's because I agree with your intuition about long-livedness of "manager", but mostly it's because the PEP is where the "contextmanager" decorator originated, and because it doesn't make any sense to call the method context() if it returns a context manager. :)

1) Am I misunderstanding the PEP? Is 'context = 'thing with context()' really the PEP's meaning?

2) If the answer to 1) is 'yes', can we change the PEP? My intuition is that a 'manager' keeps track of multiple entities being managed; to have a single 'context' produce multiple 'context managers' over time seems exactly backwards. I think I understand the logic behind the name -- that the context manager manages the entry to and exit from the block -- but think 'Manager' is the wrong term for this because the object is really encapsulating a procedure/algorithm, not managing a set of objects. The closest GoF pattern name might be 'Strategy', which is still a bit of a stretch.

I think we should correct the PEP.

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