Python multi-line with statement

Question

What is a clean way to create a multi-line with in python? I want to open up several files inside a single with, but it's far enough to the right that I want it on multiple lines. Like this:

class Dummy:
    def __enter__(self): pass
    def __exit__(self, type, value, traceback): pass

with Dummy() as a, Dummy() as b,
     Dummy() as c:
    pass

Unfortunately, that is a SyntaxError. So I tried this:

with (Dummy() as a, Dummy() as b,
      Dummy() as c):
    pass

Also a syntax error. However, this worked:

with Dummy() as a, Dummy() as b,\
     Dummy() as c:
    pass

But what if I wanted to place a comment? This does not work:

with Dummy() as a, Dummy() as b,\
     # my comment explaining why I wanted Dummy() as c\
     Dummy() as c:
    pass

Nor does any obvious variation on the placement of the \s.

Is there a clean way to create a multi-line with statement that allows comments inside it?

Answer 1

Given that you've tagged this Python 3, if you need to intersperse comments with your context managers, I would use a contextlib.ExitStack:

with ExitStack() as stack:
    a = stack.enter_context(Dummy()) # Relevant comment
    b = stack.enter_context(Dummy()) # Comment about b
    c = stack.enter_context(Dummy()) # Further information

This is equivalent to

with Dummy() as a, Dummy() as b, Dummy() as c:

This has the benefit that you can generate your context managers in a loop instead of needing to separately list each one. The documentation gives the example that if you want to open a bunch of files, and you have the filenames in a list, you can do

with ExitStack() as stack:
    files = [stack.enter_context(open(fname)) for fname in filenames]

If your context managers take so much screen space that you want to put comments between them, you probably have enough to want to use some sort of loop.

---

As Mr. Deathless mentions in the comments, there's a contextlib backport on PyPI under the name contextlib2. If you're on Python 2, you can use the backport's implementation of ExitStack.

Answer 2

This seems tidiest to me:

with open('firstfile', 'r') as (f1 # first
  ), open('secondfile', 'r') as (f2 # second
  ):
    pass

Answer 3

This isn't exactly clean, but you could do this:

with Dummy() as a, Dummy() as b, (
     #my comment
     Dummy()) as c:
    pass

There are no syntax errors, but it's not the cleanest. You could also do this:

with Dummy() as a, Dummy() as b, Dummy(
     #my comment
     ) as c:
    pass

Consider finding a way of doing this without using the comments in the middle of the with.

Answer 4

Like TigerhawkT3's answer, but with indenting that doesn't trigger pycodestyle's error E124:

with (
        open('firstfile', 'r')) as f1, (  # first
        open('secondfile', 'r')) as f2:  # second
    pass

IMO it's still ugly, but at least it passes the linter.