On 30 July 2018 at 20:15, Rudy Matela <r...@matela.com.br> wrote:
> Hello,
Hi Rudy,
> Do you think it would be nice to allow with statements inside genexps or
> list comprehensions? The functions __enter__ and __exit__ would be
> automatically called as iterables are traversed. I am thinking of
> drafting a PEP about this. Examples:
>
>
> This
>
> g = (f.read() for fn in filenames with open(fn) as f)
>
> would be equivalent to the following use of a generator function:
>
> def __gen():
> for fn in filenames:
> with open(fn) as f:
> yield f.read()
> g = __gen()
Yielding from a with block should be discouraged rather than given
special syntax. There is essentially a contradiction between the
meaning/purpose of yield (suspend indefinitely) and with (definitely
call __exit__).
If I partially iterate over g as in
for line in g:
break
then at this point g is suspended and f.__exit__ has not been called,
so the file is not closed. I may choose to iterate over g later or
not, so it has to remain in suspension just in case. In practice if
you do this in CPython then f.__exit__ will *probably* be invoked
indirectly by g.__del__ if/when the gc collects g. This defeats the
main point of using with-open though which is to avoid depending on
the gc for closing files.
--
Oscar
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