On Tue, Nov 17, 2020 at 10:16 PM Greg Ewing <greg.ew...@canterbury.ac.nz> wrote:
> On 18/11/20 4:36 pm, Larry Hastings wrote: > > > > But, > > the thinking went, you'd never want to examine the last value from a > > list generator, so it was more convenient if it behaved as if it had its > > own scope. > > List comprehensions used to leak, but apparently that was considered > surprising by enough people that it was changed. > > Generator expressions are a bit different -- the only sane way to > implement them was to make the body implicitly a separate function, > and non-leaking behaviour naturally fell out of that. Making them > leak would have taken extra work just to get something that nobody > really had a good use case for. > > The desire to make list comprehensions and generator expressions > behave consistently may have contributed to the decision to change > list comprehensions to be non-leaking. > It did if I remember correctly. For me, generator expressions are small functions like you say (see, people did get the equivalent of multi-line lambdas, just in a very specific format 😉), and all the comprehensions are just passing a generator expression to the appropriate constructor, e.g. list(), set(), and dict(). In that regard the scoping is consistent to me. -Brett > > So yes, it's all a bit messy, for reasons that are partly historical > and partly pragmatic, but things seem to work out okay in practice > most of the time. > > If there's anything I would change, it would be to have the for > statement create a new binding on each iteration, so that capturing > it with a def or lambda inside the loop works as expected. I even > came up with a way to do that while still allowing the last-bound > value to be seen afterwards, but the idea didn't gain any traction. > > -- > Greg > _______________________________________________ > Python-Dev mailing list -- python-dev@python.org > To unsubscribe send an email to python-dev-le...@python.org > https://mail.python.org/mailman3/lists/python-dev.python.org/ > Message archived at > https://mail.python.org/archives/list/python-dev@python.org/message/FC7WYLN7C54ZK2FRQSLH5GFA4P2OHBBC/ > Code of Conduct: http://python.org/psf/codeofconduct/ >
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