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 :

from contextlib import 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 .

---

Incidentally, the reason you can't do something like

with (
        ThingA() as a,
        ThingB() as b):
    ...

is because a ( can also be the first token of the expression for a context manager, and CPython's current parser wouldn't be able to tell what rule it's supposed to be parsing when it sees the first ( . This is one of the motivating examples for PEP 617 , which introduces a much more powerful new parser, so the syntax you wanted may soon exist.

Answer 2

This seems tidiest to me:

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

Answer 3

Python 3.9+ only:

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

Python ≤ 3.8:

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

Unfortunately, comments are not possible with this syntax.

---

Here's proof that it works:

Python 3.9.0a6 (default, Jun 20 2020, 14:52:53) 
Type 'copyright', 'credits' or 'license' for more information
IPython 7.15.0 -- An enhanced Interactive Python. Type '?' for help.

In [3]: with (open('x') 
   ...:        as f): 
   ...:     pass 
   ...:                                                                                                                                                                                                 
---------------------------------------------------------------------------
FileNotFoundError                         Traceback (most recent call last)
<ipython-input-3-47d5a51db08b> in <module>
----> 1 with (open('x')
      2        as f):
      3     pass
      4 

FileNotFoundError: [Errno 2] No such file or directory: 'x'

In [4]:                                                                                                                                                                                                 
Do you really want to exit ([y]/n)? y

wPython 3.8.2 (default, May  8 2020, 20:08:31) 
Type 'copyright', 'credits' or 'license' for more information
IPython 7.15.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: with (open('x') 
   ...:       as f): 
   ...:     pass                                                                                                                                                                                        
  File "<ipython-input-1-e538abd13934>", line 2
    as f):
    ^
SyntaxError: invalid syntax

Answer 4

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 5

I would keep things simple and readable by adding the comment before the with statement, or on the line itself:

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

Answer 6

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.