On 22 November 2017 at 17:16, Paul Moore
On 22 November 2017 at 16:08, Ivan Levkivskyi
wrote: On 22 November 2017 at 16:56, Yury Selivanov
wrote: On Wed, Nov 22, 2017 at 10:10 AM, Ivan Levkivskyi
wrote:
On 22 November 2017 at 15:47, Paul Moore
wrote: [...] I'm all for prohibiting using 'yield' expression in generator expressions or comprehensions. The semantics is way to hard to understand and hence be of any value. Making 'await' a SyntaxError is absolutely not an option. Async generator expressions are a shorthand syntax for defining asynchronous generators (PEP 525), and it's already being used in the wild.
OK, makes sense, so it looks like we may have the following plan:
- fix `yield` in comprehensions
I'm still not clear what "fix" would actually mean, but you propose clarifying the docs below, so I assume it means "according to whatever the updated docs say"...
I mean the initial proposal: make comprehensions equivalent to a for-loop
- update PEP 530 and docs re generator expressions vs comprehensions
Docs more importantly than PEP IMO. And are you implying that there's a difference between generator expressions and comprehensions? I thought both were intended to behave as if expanded to a function containing nested for loops? Nothing said in this thread so far (about semantics, as opposed to about current behaviour) implies there's a deliberate difference.
I think there may be a difference: comprehension `g = [(yield i) for i in range(3)]` is defined as this code: __result = [] __i = None try: for __i in range(3): __result.append(yield __i) g = __result finally: del __result, __i while `g = list((yield i) for i in range(3))` is defined as this code: def __gen(): for i in range(3): yield (yield i) g = list(__gen()) Although these two definitions are equivalent in simple cases (like having `f(i)` instead of `yield i`) But this is debatable, I think before we move to other points we need to agree on the clear definitions of semantics of generator expressions and comprehensions. -- Ivan