[Python-Dev] PEP 479: Change StopIteration handling inside generators

[Ron Adam]

On 11/22/2014 06:20 PM, Chris Angelico wrote:
> 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.

Just didn't think it through quite well enough.  But you are correct, that 
would be a generator expression.

One less case to worry about. :-)


> 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 comp_expression():
>> >         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 comp_expression())

> That wouldn't quite work, but this would:

Right, the list.append() should be a yield(expr).


> def <listcomp>():
>      ret = []
>      try:
>          for x in itr:
>              if cond:
>                  ret.append(x)
>      except StopIteration:
>          pass
>      return ret
> lst = <listcomp>()
>
> (And yes, the name's syntactically illegal, but if you look at a
> traceback, that's what is used.)

That's fine too.

The real question is how much breakage would such a change make?  That will 
probably need a patch in order to test it out.

Cheers,
    Ron