Python nested context manager on multiple lines

Question:

In Python 2.6, we used to format our nested context manager that way:

with nested(
    context1,
    context2
) as a, b:
    pass

From Python 2.7 and on, nested is deprecated. I’ve seen lots of example of multiple context manager on a single line, but I can’t find a syntax that allow them on multiple lines. How would you do this?

# That's working fine
with context1 as a, context2 as b:
    pass

# But how do we make it multine?
# These are not working
with (
    context1,
    context2
) as a, b:
    pass

with context1 as a,
    context2 as b:
    pass
Asked By: Simon Boudrias

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Source

Answers:

Python 3.10 and newer

Starting from Python 3.10, parentheses are allowed, and you can finally do this:

with (
    context1 as a,
    context2 as b
):
    pass

Backslash characters

Two or more physical lines may be joined into logical lines using
backslash characters ()

(citing the Explicit line joining section)

If you want put context managers on different lines, you can make that work by ending lines with backslashes:

with context1 as a,
     context2 as b:
    pass

contextlib.ExitStack

contextlib.ExitStack is a

context manager that is designed to make it easy to programmatically
combine other context managers and cleanup functions, especially those
that are optional or otherwise driven by input data.

It’s available in Python 3.3 and newer, and allows to enter a variable number of context managers easily. For just two context managers the usage looks like this:

from contextlib import ExitStack

with ExitStack() as es:
    a = es.enter_context(context1)
    b = es.enter_context(context2)

Nesting

It’s possible to split a context expression across several nested with statements:

With more than one item, the context managers are processed as if
multiple with statements were nested:

with A() as a, B() as b:

suite is equivalent to

with A() as a:
    with B() as b:
        suite

(from The with statement)

Answered By: vaultah
with context1 as a, 
context2 as b:
pass

Like any line-break, backslash provides the solution

Answered By: Sudeep Juvekar

Your example is equivalent to:

with context1 as a:
    with context2 as b:
        pass

which looks nice on two lines.

Reference: https://docs.python.org/2.7/reference/compound_stmts.html#the-with-statement

Answered By: Robᵩ

There is a way to creatively use the parentheses and avoid the backslash: parenthesize the expression before as. Not very sightly, though:

with (
  open('/etc/passwd')) as foo, (
  open('/tmp/bar')) as bar:
  pass  # this parses correctly

It’s easy to nest more and more if needed.

Answered By: 9000