python: create a "with" block on several context managers

Question

Suppose you have three objects you acquire via context manager, for instance A lock, a db connection and an ip socket. You can acquire them by:

with lock:
   with db_con:
       with socket:
            #do stuff

But is there a way to do it in one block? something like

with lock,db_con,socket:
   #do stuff

Furthermore, is it possible, given an array of unknown length of objects that have context managers, is it possible to somehow do:

a=[lock1, lock2, lock3, db_con1, socket, db_con2]
with a as res:
    #now all objects in array are acquired

If the answer is "no", is it because the need for such a feature implies bad design, or maybe I should suggest it in a pep? :-P

Answer 1

The first part of your question is possible in Python 3.1.

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

Changed in version 3.1: Support for multiple context expressions

Answer 2

You can use contextlib.nested:

from contextlib import nested

with nested(A(), B(), C()) as (X, Y, Z):
    do_something()

is equivalent to:

m1, m2, m3 = A(), B(), C()
with m1 as X:
    with m2 as Y:
        with m3 as Z:
            do_something()

Note that this isn't exactly the same as normally using nested with, because A(), B(), and C() will all be called initially, before entering the context managers. But usually this makes no difference.

In Python 2.7 and 3.1, syntax has been added for this, and contextlib.nested has been deprecated:

with A() as X, B() as Y, C() as Z:
    do_something()