Modern Python Standard Library Cookbook

How to do it...

contextlib.ExitStack serves various purposes, one of which is to allow us to apply a variable number of context managers to a block.

For example, we might want to apply both context managers only when we are printing an even number in a loop:

from contextlib import ExitStack

for n in range(5):
    with ExitStack() as stack:
        stack.enter_context(first())
        if n % 2 == 0:
            stack.enter_context(second())
        print('NUMBER: {}'.format(n))

The result will be that the second is only added to the context, and thus invoked for even numbers:

First
Second
NUMBER: 0
First
NUMBER: 1
First
Second
NUMBER: 2
First
NUMBER: 3
First
Second
NUMBER: 4

As you can see, for 1 and 3, only First is printed.

Of course, when exiting the context declared through ...