[Python-Dev] Multiline with statement line continuation

[Steven D'Aprano]

On Tue, Aug 12, 2014 at 10:28:14AM +1000, Nick Coghlan wrote:
> On 12 Aug 2014 09:09, "Allen Li" <cyberdupo56 at gmail.com> wrote:
> >
> > This is a problem I sometimes run into when working with a lot of files
> > simultaneously, where I need three or more `with` statements:
> >
> >     with open('foo') as foo:
> >         with open('bar') as bar:
> >             with open('baz') as baz:
> >                 pass
> >
> > Thankfully, support for multiple items was added in 3.1:
> >
> >     with open('foo') as foo, open('bar') as bar, open('baz') as baz:
> >         pass
> >
> > However, this begs the need for a multiline form, especially when
> > working with three or more items:
> >
> >     with open('foo') as foo, \
> >          open('bar') as bar, \
> >          open('baz') as baz, \
> >          open('spam') as spam \
> >          open('eggs') as eggs:
> >         pass
> 
> I generally see this kind of construct as a sign that refactoring is
> needed. For example, contextlib.ExitStack offers a number of ways to manage
> multiple context managers dynamically rather than statically.

I don't think that ExitStack is the right solution for when you have a 
small number of context managers known at edit-time. The extra effort of 
writing your code, and reading it, in a dynamic manner is not justified. 
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

I prefer the first, even with the long line.


-- 
Steven