Are multiple `with` statements on one line equivalent to nested `with` statements, in python?

Question

Are these two statements equivalent?

with A() as a, B() as b:
  # do something

with A() as a:
  with B() as b:
    # do something

I ask because both a and b alter global variables (tensorflow here) and b depends on changes made by a. So I know the 2nd form is safe to use, but is it equivalent to shorten it to the 1st form?

Asked By: David Parks

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Source

Answer 1

Yes, listing multiple with statements on one line is exactly the same as nesting them, according to the Python 2.7 language reference:

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

Similar language appears in the Python 3 language reference.

Update for 3.10+

Changed in version 3.10: Support for using grouping parentheses to break the statement in multiple lines.
with (
   A() as a,
   B() as b,
):
   SUITE

Answered By: Kevin

Answer 2

Absolutely identical. Just depends on your personal preference.

Answered By: Batman

Answer 3

As others have said, it’s the same result. Here’s a more detailed example of how this syntax might be used:

blah.txt

I can open one file and write its contents to another file in a succinct manner:

with open('blah.txt', 'r') as infile, open('foo.txt', 'w+') as outfile:
    for line in infile:
        outfile.write(str(line))

foo.txt now contains:

Answered By: blacksite

Are multiple `with` statements on one line equivalent to nested `with` statements, in python?

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