CODEWITHSUNDEEP
pythonfiledesign-patternsrefactoring
chiborg
chiborg

Reputation: 28104

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

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?

Upvotes: 11

Views: 5300

Answers (1)

Martijn Pieters
Martijn Pieters

Reputation: 1123700

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"))

Upvotes: 23

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