[Python-Dev] PEP 479: Change StopIteration handling inside generators (original) (raw)

Chris Angelico rosuav at gmail.com
Sun Nov 23 01:20:27 CET 2014

On Sun, Nov 23, 2014 at 11:05 AM, Ron Adam <ron3200 at gmail.com> wrote:

Se we have these...

Tuple Comprehension (...) List Comprehension [...] Dict Comprehension {...} Colon make's it different from sets. Set Comprehension {...} I don't think there is any other way to create them. And they don't actually expand to any python code that is visible to a programmer. They are self contained literal expressions for creating these few objects.

Hmmm, there's no such thing as tuple comprehensions. Are you confusing comprehension syntax (which has a 'for' loop in it) with tuple/list/etc display, which doesn't?

lst = [1,2,3,4] # not a comprehension even = [n*2 for n in lst] # comprehension

Here is what I think(?) list comps do currently.

lst = [expr for items in itr if cond] Is the same as. lst = [] for x in itr: # StopIteration caught here. if cond: # StopIteration bubbles here. lst.append(expr) # StopIteration bubbles here.

Pretty much. It's done in a nested function (so x doesn't leak), but otherwise, yes.

And it would be changed to this...

def compexpression(): for x in itr: # StopIteration caught here. if cond: list.append(expr) # StopIteration from cond and expr caught here. lst = list(x for x in compexpression())

That wouldn't quite work, but this would:

def (): ret = [] try: for x in itr: if cond: ret.append(x) except StopIteration: pass return ret lst = ()

(And yes, the name's syntactically illegal, but if you look at a traceback, that's what is used.)

ChrisA

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