How to avoid nested "with" statements when working with multiple files in Python

Question

When working with multiple files in Python code can get ugly when using the recommended style:

with open("foo.txt") as foo:
    with open("bar.txt", "w") as bar:
         with open("baz.txt", "w") as baz:
              # Read from foo, write different output to bar an baz

That’s three indentation levels just for working with files! The alternative would be this

foo = open("foo.txt")
bar = open("bar.txt", "w")
baz = open("baz.txt", "w")
# Read from foo, write different output to bar an baz
foo.close()
bar.close()
baz.close()

I have a feeling that either of these examples could be refactored to something more elegant. Any examples?

Asked By: chiborg

||

Source

Answer 1

Python 2.7 and up let you specify multiple context managers in one with statement:

with open("foo.txt") as foo, open("bar.txt", "w") as bar, open("baz.txt", "w") as baz:
    # Read from foo, write different output to bar an baz

The line does get long, and you cannot use parentheses to keep that below 80 characters. You can use backslash continuations however:

with open("foo.txt") as foo,
        open("bar.txt", "w") as bar,
        open("baz.txt", "w") as baz:
    # Read from foo, write different output to bar an baz

Or you could place newlines inside the parentheses of the ‘open()` calls:

with open(
    "foo.txt"
) as foo, open(
    "bar.txt", "w"
) as bar, open(
    "baz.txt", "w"
) as baz:
    # Read from foo, write different output to bar an baz

Starting Python 3.10, this has been made easier still with addition of parentheses support for multi-item context managers:

with (
    open("foo.txt") as foo,
    open("bar.txt", "w") as bar,
    open("baz.txt", "w") as baz,
):
    # Read from foo, write different output to bar an baz

Another option would be to use contextlib.ExitStack() context manager (only in Python 3.3 and up):

from contextlib import ExitStack

with ExitStack() as stack:
    foo = stack.enter_context(open("foo.txt"))
    bar = stack.enter_context(open("bar.txt"))
    baz = stack.enter_context(open("baz.txt"))

Answered By: Martijn Pieters

How to avoid nested "with" statements when working with multiple files in Python

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