On Mon, 18 Nov 2019 at 13:12, Soni L. <fakedme...@gmail.com> wrote:
>
> On 2019-11-18 9:10 a.m., Oscar Benjamin wrote:
> > [snip]
> >
> > To me that seems clumsy and awkward compared to nested though:
> >
> > with nested(*map(open, filenames)) as files:
> > ...
> >
> > Ideally I would design nested to take an iterable rather than *args
> > and then it would be fine to do e.g.
> >
> > with nested(open(filename) for filename in filenames) as files:
> > ...
> >
> > Here nested could take advantage of the delayed evaluation in the
> > generator expression to invoke the __enter__ methods and call __exit__
> > on the opened files if any of the open calls fails. This would also
> > leave a "trap" though since using a list comprehension would suffer
> > the same problem as if nested took *args:
> >
> > with nested([open(filename) for filename in filenames]) as files:
> > ...
>
> If generator expressions (aka "(open(filename) for filename in
> filenames)") had __enter__ and __exit__ that deferred to inner __enter__
> and __exit__, this "trap" wouldn't exist:
>
> with (open(filename) for filename in filenames) as files:
> ... # fine
>
> with [open(filename) for filename in filenames] as files:
> ... # raises because list doesn't __enter__
>
> mainly because it wouldn't work with arbitrary iterators or iterables.
>
> (and if you need it to, "with (x for x in iterable)" would still be
> available)
Since generators already have a close method the obvious thing for
generator.__exit__ to do (if it existed) would be to call that. That
would make it possible to use patterns like:
def cat(filenames):
for filename in filenames:
with open(filename) as infile:
yield from infile
with cat(filenames) as lines:
for line in lines:
if key in line:
return line
Note that the return there stops iterating over the generator while it
is suspended. Making the generator a context manager whose __exit__
calls close ensures that the context manager inside the generator is
finalised.
--
Oscar
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