从Python 3.3开始，你可以使用contextlib模块中的ExitStack类。

它可以管理动态数量的上下文感知对象，这意味着如果您不知道要处理多少文件，它将特别有用。

文档中提到的规范用例是管理动态数量的文件。

with ExitStack() as stack:
    files = [stack.enter_context(open(fname)) for fname in filenames]
    # All opened files will automatically be closed at the end of
    # the with statement, even if attempts to open files later
    # in the list raise an exception

下面是一个通用的例子:

from contextlib import ExitStack

class X:
    num = 1

    def __init__(self):
        self.num = X.num
        X.num += 1

    def __repr__(self):
        cls = type(self)
        return '{cls.__name__}{self.num}'.format(cls=cls, self=self)

    def __enter__(self):
        print('enter {!r}'.format(self))
        return self.num

    def __exit__(self, exc_type, exc_value, traceback):
        print('exit {!r}'.format(self))
        return True

xs = [X() for _ in range(3)]

with ExitStack() as stack:
    print(stack._exit_callbacks)
    nums = [stack.enter_context(x) for x in xs]
    print(stack._exit_callbacks)
print(stack._exit_callbacks)
print(nums)

输出:

deque([])
enter X1
enter X2
enter X3
deque([<function ExitStack._push_cm_exit.<locals>._exit_wrapper at 0x7f5c95f86158>, <function ExitStack._push_cm_exit.<locals>._exit_wrapper at 0x7f5c95f861e0>, <function ExitStack._push_cm_exit.<locals>._exit_wrapper at 0x7f5c95f86268>])
exit X3
exit X2
exit X1
deque([])
[1, 2, 3]

这在Python 3 v3.1和Python 2.7中是可能的。新的with语法支持多个上下文管理器:

with A() as a, B() as b, C() as c:
    doSomething(a,b,c)

不像contextlib。嵌套的，这保证了a和b的__exit__()将被调用，即使C()或它的__enter__()方法引发异常。

你也可以在后面的定义中使用之前的变量(下面是h/t Ahmad):

with A() as a, B(a) as b, C(a, b) as c:
    doSomething(a, c)

从Python 3.10开始，你可以使用括号:

with (
    A() as a, 
    B(a) as b, 
    C(a, b) as c,
):
    doSomething(a, c)

从Python 3.3开始，你可以使用contextlib模块中的ExitStack类。

它可以管理动态数量的上下文感知对象，这意味着如果您不知道要处理多少文件，它将特别有用。

文档中提到的规范用例是管理动态数量的文件。

with ExitStack() as stack:
    files = [stack.enter_context(open(fname)) for fname in filenames]
    # All opened files will automatically be closed at the end of
    # the with statement, even if attempts to open files later
    # in the list raise an exception

下面是一个通用的例子:

from contextlib import ExitStack

class X:
    num = 1

    def __init__(self):
        self.num = X.num
        X.num += 1

    def __repr__(self):
        cls = type(self)
        return '{cls.__name__}{self.num}'.format(cls=cls, self=self)

    def __enter__(self):
        print('enter {!r}'.format(self))
        return self.num

    def __exit__(self, exc_type, exc_value, traceback):
        print('exit {!r}'.format(self))
        return True

xs = [X() for _ in range(3)]

with ExitStack() as stack:
    print(stack._exit_callbacks)
    nums = [stack.enter_context(x) for x in xs]
    print(stack._exit_callbacks)
print(stack._exit_callbacks)
print(nums)

输出:

deque([])
enter X1
enter X2
enter X3
deque([<function ExitStack._push_cm_exit.<locals>._exit_wrapper at 0x7f5c95f86158>, <function ExitStack._push_cm_exit.<locals>._exit_wrapper at 0x7f5c95f861e0>, <function ExitStack._push_cm_exit.<locals>._exit_wrapper at 0x7f5c95f86268>])
exit X3
exit X2
exit X1
deque([])
[1, 2, 3]

在Python 3.1+中，你可以指定多个上下文表达式，它们将被处理，就像多个with语句嵌套一样:

with A() as a, B() as b:
    suite

等于

with A() as a:
    with B() as b:
        suite

这也意味着你可以在第二个表达式中使用第一个表达式的别名(在使用db连接/游标时很有用):

with get_conn() as conn, conn.cursor() as cursor:
    cursor.execute(sql)

我认为你应该这样做:

from __future__ import with_statement

with open("out.txt","wt") as file_out:
    with open("in.txt") as file_in:
        for line in file_in:
            file_out.write(line)

contextlib。Nested支持:

import contextlib

with contextlib.nested(open("out.txt","wt"), open("in.txt")) as (file_out, file_in):

   ...

更新: 引用文档，关于contextlib.nested:

2.7版后已移除:with-statement现在支持此功能 直接使用功能(没有容易出错的令人困惑的怪癖)。

更多信息请参见rafawarsaw Dowgird的回答。