'with'语句中有多个变量？

Question

是否可以使用withPython中的语句声明多个变量？

就像是:

from __future__ import with_statement

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

......还是正在清理两个资源同时出现问题？

Answer 1

从v3.1和 Python 2.7 开始,它可以在Python 3中实现.新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)

不同的是contextlib.nested,这保证了a和b将他们__exit__()的调用,即使C()或者它的__enter__()方法抛出一个异常.

Answer 2

contextlib.nested 支持这个:

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:

从版本2.7开始不推荐使用:with语句现在直接支持此功能(没有令人困惑的容易出错的怪癖).

有关更多信息,请参阅RafałFowgird的答案.

Answer 3

请注意,如果将变量拆分为行,则必须使用反斜杠来换行换行符.

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

括号不起作用,因为Python会创建一个元组.

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

由于元组缺少__enter__属性,因此会出现错误(无法识别并且不识别类类型):

AttributeError: __enter__

Answer 4

我想你想这样做:

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)

Answer 5

因为Python 3.3,你可以使用类ExitStack从contextlib模块.

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

这是一个通用的例子:

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]

Answer 6

从 Python 3.10 开始，有一个新功能括号上下文管理器，它允许以下语法：

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