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

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. :)