[Python-Dev] Tricky way of of creating a generator via a comprehension expression

[Chris Angelico]

On Thu, Nov 23, 2017 at 3:36 PM, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
> Paul Moore wrote:
>>
>> 3. List comprehensions are the same as list(the equivalent generator
>> expression).
>
>
> I don't think that's ever been quite true -- there have
> always been odd cases such as what happens if you
> raise StopIteration in list(generator_expression).

You mean if the genexp leaks one? That's basically an error either way
- the genexp will raise RuntimeError, but it's still an exception.

>>> from __future__ import generator_stop
>>> def boom(): raise StopIteration
...
>>> [x if x < 3 else boom() for x in range(5)]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in <listcomp>
  File "<stdin>", line 1, in boom
StopIteration
>>> list(x if x < 3 else boom() for x in range(5))
Traceback (most recent call last):
  File "<stdin>", line 1, in <genexpr>
  File "<stdin>", line 1, in boom
StopIteration

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: generator raised StopIteration
>>>

So that's _one_ difference removed (mostly).

ChrisA