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)
[snip]
Oscar
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