On Fri, Nov 15, 2019 at 9:41 PM Jonathan Fine <jfine2...@gmail.com> wrote:
>
> Hi All
>
> The original poster wanted, inside 'with' context management, to open several
> files. Context management is, roughly speaking, deferred execution wrapped in
> a try ... except .... statement.
>
> The lambda construction also provides for deferred execution. Perhaps
> something like
> >>> with helper(lambda: open('a'), open('b'), open('c')) as a, b c:
> >>> pass
> would do what the original poster wanted, for a suitable value of helper. It
> certainly gives the parentheses the OP wanted, without introducing new syntax.
>
> However, map also provides for deferred execution. It returns an iterable
> (not a list).
> >>> x = map(open, ['a', 'b', 'c'])
> >>> x
> <map object at 0x7f8cb5653b70>
> >>> next(x)
> FileNotFoundError: [Errno 2] No such file or directory: 'a'
>
> So the OP could even write:
> >>> with helper(map(open, ['a', 'b', 'c'])) as a, b, c:
> >>> pass
> for a (different) suitable value of helper.
>
> I hope this helps.
I don't understand your point about deferred execution here. But a
vitally important part of multi-with semantics is that, if something
goes wrong while opening "c", the files "b" and "a" need to be
properly closed. Trying to write a helper() that is capable of doing
this would be extremely difficult, possibly impossible (and would need
to do its own mapping, otherwise it IS impossible).
ChrisA
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