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

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.