[Python-Dev] accumulator display syntax

[Phillip J. Eby]

At 01:57 PM 10/17/03 -0500, Skip Montanaro wrote:

>     >> But, indexing does stretch quite far in the current Python syntax and
>     >> semantics (in Python's *pragmatics* you're supposed to use it far
>     >> more restrainedly).
>
>     Guido> Which is why I didn't like the 'sum[x for x in S]' notation much.
>     Guido> Let's look for an in-line generator notation instead.  I like
>
>     Guido>   sum((yield x for x in S))
>
>     Guido> but perhaps we can make this work:
>
>     Guido>   sum(x for x in S)
>
>Forgive my extreme density on this matter, but I don't understand what
>
>     (yield x for x in S)
>
>is supposed to do.  Is it supposed to return a generator function which I
>can assign to a variable (or pass to the builtin function sum() as in your
>example) and call later, or is it supposed to turn the current function into
>a generator function (so that each executed yield statement returns a value
>to the caller of the current function)?

Neither.  It returns an *iterator*, conceptually equivalent to:

def temp():
     for x in S:
         yield x

temp = temp()

Except of course without creating a 'temp' name.  I suppose you could also 
think of it as:

(lambda: for x in S: yield x)()

except of course that you can't make a generator lambda.

If you look at it this way, then you can consider [x for x in S] to be 
shorthand syntax for list(x for x in S), as they would both produce the 
same result.  However, IIRC, the current listcomp implementation actually 
binds 'x' in the current local namespace, whereas the generator version 
would not.  (And the listcomp version might be faster.)