如何避免嵌套的"with"?在Python中处理多个文件时的语句 [英] How to avoid nested "with" statements when working with multiple files in Python

问题描述

在Python代码中使用多个文件时，使用推荐的样式可能会很丑陋:

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

这是三个缩进级别，仅用于处理文件！替代方法是这样

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()

我觉得可以将这两个示例中的任何一个都重构为更优雅的形式.有例子吗?

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

推荐答案

Python 2.7及更高版本允许您在一个 with 语句中指定多个上下文管理器:

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

行确实变长，并且不能使用括号将其保持在80个字符以下.您可以使用 \ 反斜杠继续:

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

另一种选择是使用 contextlib.ExitStack()上下文管理器(仅在Python 3.3及更高版本中):

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"))

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