python – 如何__enter__ n上下文管理器？

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使用with语句,我们可以只使用一个级别的缩进/嵌套来输入许多上下文处理程序：

>>> from contextlib import contextmanager
>>> @contextmanager
... def frobnicate(n):
...     print('frobbing {}'.format(n))
...     yield
... 
>>> frob1 = frobnicate(1)
>>> frob2 = frobnicate(2)
>>> with frob1, frob2:
...     pass
... 
frobbing 1
frobbing 2

但这似乎不起作用：

>>> frobs = [frobnicate(1), frobnicate(2)]
>>> with *frobs:
...     pass
# SyntaxError: invalid syntax

我们如何输入n个上下文管理器而不必手动写出每个上下文管理器？

解决方法:

python2.7让contextlib.nested做到了这一点,但由于容易出错的怪癖而被弃用.

This function has two major quirks that have led to it being deprecated. Firstly, as the context managers are all constructed before the function is invoked, the __new__() and __init__() methods of the inner context managers are not actually covered by the scope of the outer context managers. That means, for example, that using nested() to open two files is a programming error as the first file will not be closed promptly if an exception is thrown when opening the second file.

Secondly, if the __enter__() method of one of the inner context managers raises an exception that is caught and suppressed by the __exit__() method of one of the outer context managers, this construct will raise RuntimeError rather than skipping the body of the with statement.

python3.3使用contextlib.ExitStack做得更好,看起来像：

from contextlib import ExitStack

with ExitStack() as stack:
    contexts = [stack.enter_context(frobnicate(i)) for i in range(2)]
    ...

有关backport到python2.x代码,请参阅contextlib2.

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