[Python-ideas] Change how Generator Expressions handle StopIteration

[Guido van Rossum]

On Sun, Nov 2, 2014 at 1:00 PM, Terry Reedy <tjreedy at udel.edu> wrote:

> On 11/2/2014 2:50 PM, Andrew Barnert wrote:
>
>> This is related to the fact that, although the docs imply otherwise,
>> [COMP] isn't exactly equivalent to list(COMP),
>>
>
> That purported equivalence is a common meme, which I may have helped
> spread.


I may have started it. I was aware of the non-equivalence (only
mostly-equivalence) in Python 2 and I wanted to make then identical in
Python 3 -- having one construct being exactly equivalent to another reduce
the amount of explaining needed. Unfortunately, people had started to
depend on the (in my *current* opinion deplorable) behavior of generator
expressions in the face of StopIteration thrown by arbitrary parts of the
expression or condition, and the equivalence is still imperfect. At least
the variable leakage has been fixed.

I know that when we first introduced generators (not generator expressions)
I was in favor of interpreting arbitrary things that raise StopIteration in
a generator to cause the generator to terminate just as if it had decided
to stop (i.e. 'return' or falling of the end), because I thought there were
some useful patterns that could be written more compactly this way -- in
particular, the pattern where a generator iterates over another iterator by
calling next() on it, does some processing on the value thus produced, and
then yielding the processed value (or not), and where the logical response
to a StopIteration from the inner iterator is to exit the generator. For
example:

  def only_positive(it):
      while True:
          x = next(it)
          if x > 0: yield x

This *particular* example is much better written as:

  def only_positive(x):
      for x in it:
          if x > 0: yield x

but the idea was that there might be variants where being constrained by a
single for-loop would make the code less elegant if you had to catch the
StopIteration and explicitly exit the generator.

However, I don't think this idea has panned out. I haven't done a survey,
but I have a feeling that in most cases where an explicit next() call is
used (as opposed to a for-loop) there's a try/except Stopiteration around
it, and a fair amount if time is wasted debugging situations where a
StopIteration unexpectedly escapes and silently interrupts some loop over
an unrelated generator (instead of loudly bubbling up to the top and
causing a traceback, which would be more debuggable). And the use case of
raising StopIteration from a condition used in a generator expression is
iffy at best (it makes the condition function hard to use in other
contexts, and it calls to attention the difference between generators and
comprehensions).

So I will go out on a limb here and say that this was a mistake and if we
can think of easing the transitional pain it would be a good thing to fix
this eventually.

-- 
--Guido van Rossum (python.org/~guido)
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