[Python-Dev] PEP 343 update (with statement context terminology) (original) (raw)
Jim Jewett jimjjewett at gmail.com
Tue Apr 25 22:51:55 CEST 2006
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So things like decimal.Context get left trying to find a sane name for what their context method returns. decimal.Context.context() returns a . . . context? What? Wasn't it already a context? Oh, so it actually returns a "with statement context object".
[I was OK with "context specifiers", but here is another attempt.]
With statements execute in a temporary context.
with with_expression as temporary_context_name: stmt1 stmt2
The with_expression must return an object implementing the Temporary Context protocol. The Temporary Context's enter method runs before the statement block, and its exit method runs after the block.
Temporary Contexts can represent arbitrary changes to the execution environment. Often, they just ensure that a file is closed in a timely fashion.
A more advanced example is Decimal.Context, which controls the precision of certain math calculations. Decimal.Context also implements the Temporary Context protocol, so that you can increase the precision of a few calculations without slowing down the rest of the program.
-jJ
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