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?

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

Exactly the same language appears in the Python 3 language reference.

Answer 2

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

blah.txt

1
2
3
4
5

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:

1
2
3
4
5