[Python-Dev] PEP 343 update (with statement context terminology) (original) (raw)

Phillip J. Eby pje at telecommunity.com
Mon Apr 24 22:40:21 CEST 2006

At 01:19 PM 4/24/2006 -0700, Aahz wrote:

What is EXPRESSION, then? Not the value it returns, but EXPRESSION itself -- does it have a name? What about the kinds of things we use for EXPRESSION?

I read "EXPRESSION returns a value" as simply meaning that "value = EXPRESSION", i.e. that the result of computing EXPRESSION is the value. That's what it usually means when we talk about expressions returning a value -- that computing the expression produces a value.

I still don't see a third thing here. "EXPRESSION returns a value" (Thing 1). That value is "used to create a context" (by calling context). This context (Thing 2) "is used to execute a block" (by calling enter and exit).

I don't get how you can have a difference between "EXPRESSION" and "value it returns" unless you're bringing functions into play. In everything else in Python, an expression is the value it returns. How could it be otherwise?

Maybe you meant to write an explanation that included three objects, but what you wrote is actually a precise and accurate description of how things works. The value produced by EXPRESSION is used to create a context, and the context is used to execute the block. I don't know how you could explain it any more simply than that -- certainly not by adding a mysterious third gunman on the grassy knoll. :)

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