On 22 November 2017 at 16:38, Ivan Levkivskyi <levkivs...@gmail.com> wrote: > On 22 November 2017 at 17:16, Paul Moore <p.f.mo...@gmail.com> wrote: >> >> 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
Not in the docs, it isn't... The docs explicitly state that a new scope is involved. People may *intuitively understand it* like this, but it's not the definition. If it were the definition, I'd be fine, as people can reason about it and know that the conclusions they come to (no matter how unintuitive) are accurate. But if it's how to *understand* what a list comprehension does, then different rules apply - corner cases may work differently, but conversely the cases that work differently have to clearly be corner cases, otherwise it's no use as a mental model. > 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()) Again, not in the docs. > Although these two definitions are equivalent in simple cases (like having > `f(i)` instead of `yield i`) And if they don't imply list(...) is the same as [...] then that will come as a surprise to many people. > 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. Absolutely. And I'd like those semantics to be expressed in a way that doesn't need "except when await/async is involved" exceptions. Paul _______________________________________________ Python-Dev mailing list Python-Dev@python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
