[Python-Dev] Multiline with statement line continuation (original) (raw)

Steven D'Aprano steve at pearwood.info
Tue Aug 12 14:15:41 CEST 2014

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

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