[Python-ideas] for line in input with open(path) as input...

[Nick Coghlan]

On Sun, Feb 3, 2013 at 1:47 AM, Yuval Greenfield <ubershmekel at gmail.com> wrote:
> I really like Golang's solution (defer) which Nick sort of emulates with
> ExitStack.
>
> http://docs.python.org/3/library/contextlib.html#contextlib.ExitStack
>
> If ExitStack ever became a language feature

Why would it ever become a language feature? It works just fine as a
context manager, and the need for it isn't frequent enough to justify
special syntax.

> we could write stuff like:
>
>
>     def f():
>         fhand = local open(path)
>         process(fhand)
>         ghand = local open(path2)
>         process(ghand)

Why would you leave fhand open longer than necessary? The above would
be better written as:

    def f():
        with open(path) as fhand:
            process(fhand)
        with open(path2) as ghand:
            process(ghand)

If you need both files open at the same time, you can use a nested
context manager:

    def f():
        with open(path) as fhand:
            with open(path2) as ghand:
                process(fhand, ghand)

Or the nesting behaviour built into with statements themselves:

    def f():
        with open(path) as fhand, open(path2) as ghand:
            process(fhand, ghand)

It's only when the number of paths you need to open is dynamic that
ExitStack comes into play (this is actually very close to the example
in ExitStack's docstring, as handling a variable number of
simultaneously open files was the use case that highlighted the fatal
flaw in the way the old contextlib.nested design handled context
managers that acquired the resource in __init__ rather than
__enter__):

    def f(*paths):
        with contextlib.ExitStack() as stack:
            files = [stack.enter_context(open(path)) for path in paths]
            process(files)

Function and class definitions control name scope (amongst other
things), with statements control deterministic cleanup, loops control
iteration. That's what I mean by "separation of concerns" in relation
to these aspects of the language design and it's a *good* thing (and
one of the key reasons with statements behave like PEP 343, rather
than being closer to Guido's original looping idea that is described
in PEP 340).

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia