Multiple variables in a “with” statement?

StackOverflow

Is it possible to declare more than one variable using a with statement in Python?

Something like:

from __future__ import with_statement

with open("out.txt";"wt"), open("in.txt") as file_out, file_in:
    for line in file_in:
        file_out.write(line)

... or is cleaning up two resources at the same time the problem?

Answer rating: 793

It is possible in Python 3 since v3.1 and Python 2.7. The new with syntax supports multiple context managers:

with A() as a, B() as b, C() as c:
    doSomething(a,b,c)

Unlike the contextlib.nested, this guarantees that a and b will have their __exit__()"s called even if C() or it"s __enter__() method raises an exception.

You can also use earlier variables in later definitions (h/t Ahmad below):

with A() as a, B(a) as b, C(a, b) as c:
    doSomething(a, c)

Answer rating: 63

Note that if you split the variables into lines, you must use backslashes to wrap the newlines.

with A() as a, 
     B() as b, 
     C() as c:
    doSomething(a,b,c)

Parentheses don"t work, since Python creates a tuple instead.

with (A(),
      B(),
      C()):
    doSomething(a,b,c)

Since tuples lack a __enter__ attribute, you get an error (undescriptive and does not identify class type):

AttributeError: __enter__

If you try to use as within parentheses, Python catches the mistake at parse time:

with (A() as a,
      B() as b,
      C() as c):
    doSomething(a,b,c)
SyntaxError: invalid syntax



When will this be fixed?

This issue is tracked in https://bugs.python.org/issue12782.

Recently, Python announced in PEP 617 that they"ll be replacing the current parser with a new one. Because Python"s current parser is LL(1), it cannot distinguish between "multiple context managers" with (A(), B()): and "tuple of values" with (A(), B())[0]:.

The new parser can properly parse multiple context managers surrounded by parentheses. The new parser has been enabled in 3.9, but this syntax will still be rejected until the old parser is removed in Python 3.10.





Get Solution for free from DataCamp guru