On Aug 13, 2014 7:04 PM, "Akira Li" <4kir4.1i at gmail.com> wrote:
>
> Nick Coghlan <ncoghlan at gmail.com> writes:
>
> > On 12 August 2014 22:15, Steven D'Aprano <steve at pearwood.info> wrote:
> >> Compare the natural way of writing this:
> >>
> >> with open("spam") as spam, open("eggs", "w") as eggs,
frobulate("cheese") as cheese:
> >> # do stuff with spam, eggs, cheese
> >>
> >> versus the dynamic way:
> >>
> >> with ExitStack() as stack:
> >> spam, eggs = [stack.enter_context(open(fname), mode) for fname,
mode in
> >> zip(("spam", "eggs"), ("r", "w")]
> >> cheese = stack.enter_context(frobulate("cheese"))
> >> # do stuff with spam, eggs, cheese
> >
> > You wouldn't necessarily switch at three. At only three, you have lots
> > of options, including multiple nested with statements:
> >
> > with open("spam") as spam:
> > with open("eggs", "w") as eggs:
> > with frobulate("cheese") as cheese:
> > # do stuff with spam, eggs, cheese
> >
> > The "multiple context managers in one with statement" form is there
> > *solely* to save indentation levels, and overuse can often be a sign
> > that you may have a custom context manager trying to get out:
> >
> > @contextlib.contextmanager
> > def dish(spam_file, egg_file, topping):
> > with open(spam_file), open(egg_file, 'w'), frobulate(topping):
> > yield
> >
> > with dish("spam", "eggs", "cheese") as spam, eggs, cheese:
> > # do stuff with spam, eggs & cheese
> >
> > ExitStack is mostly useful as a tool for writing flexible custom
> > context managers, and for dealing with context managers in cases where
> > lexical scoping doesn't necessarily work, rather than being something
> > you'd regularly use for inline code.
> >
> > "Why do I have so many contexts open at once in this function?" is a
> > question developers should ask themselves in the same way its worth
> > asking "why do I have so many local variables in this function?"
>
> Multiline with-statement can be useful even with *two* context
> managers. Two is not many.
>
> Saving indentations levels along is a worthy goal. It can affect
> readability and the perceived complexity of the code.
>
> Here's how I'd like the code to look like:
>
> with (open('input filename') as input_file,
> open('output filename', 'w') as output_file):
> # code with list comprehensions to transform input file into output
file
>
> Even one additional unnecessary indentation level may force to split
> list comprehensions into several lines (less readable) and/or use
> shorter names (less readable). Or it may force to move the inline code
> into a separate named function prematurely, solely to preserve the
> indentation level (also may be less readable) i.e.,
>
> with ... as input_file:
> with ... as output_file:
> ... #XXX indentation level is lost for no reason
>
> with ... as infile, ... as outfile: #XXX shorter names
> ...
>
> with ... as input_file:
> with ... as output_file:
> transform(input_file, output_file) #XXX unnecessary function
>
> And (nested() can be implemented using ExitStack):
>
> with nested(open(..),
> open(..)) as (input_file, output_file):
> ... #XXX less readable
>
> Here's an example where nested() won't help:
>
> def get_integers(filename):
> with (open(filename, 'rb', 0) as file,
> mmap.mmap(file.fileno(), 0, access=mmap.ACCESS_READ) as
mmapped_file):
> for match in re.finditer(br'\d+', mmapped_file):
> yield int(match.group())
>
> Here's another:
>
> with (open('log'+'some expression that generates filename', 'a') as
logfile,
> redirect_stdout(logfile)):
> ...
>
Just a thought, would it bit wierd that:
with (a as b, c as d): "works"
with (a, c): "boom"
with(a as b, c): ?
>
> --
> Akira
>
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