如何在 with 语句中有多个虚拟文件编写器？

Question

我阅读 this answer of Is it possible to have an optional with/as statement in python? that you can have a dummy file writer with contextmanager。但是，我想在 with 语句上下文中打开多个虚拟文件编写器。

假设我创建了两个虚拟文件：touch a 和 touch b.

鉴于脚本的第一部分：

#!/usr/bin/python

from contextlib import contextmanager

# File names
fa="a"
fb="b"

# Dummy file handler
none_context = contextmanager(lambda: iter([None]))()

此添加适用于单个虚拟文件编写器（它打印 2）：

printing=False
with (open(fa) if printing else none_context) as writter:
    print 1 if printing else 2

这也有效，因为我们确实在读取文件（它打印 1）：

printing=True
with (open(fa, "r") if printing else none_context) as writter, \
    (open(fb, "r") if printing else none_context) as another_writter:
    print 1 if printing else 2

但是，如果我们使用两个虚拟文件编写器，它就不起作用:

printing=False
with (open(fa, "r") if printing else none_context) as writter, \
    (open(fb, "r") if printing else none_context) as another_writter:
    print 1 if printing else 2

显示错误：

Traceback (most recent call last):
  File "dummy_opener.py", line 23, in <module>
    with (open(fa, "r") if printing else none_context) as writter, \
  File "/usr/lib64/python2.7/contextlib.py", line 19, in __enter__
    raise RuntimeError("generator didn't yield")
RuntimeError: generator didn't yield

为什么会这样？还有：我怎样才能使这个多个 with open 命令与虚拟文件编写器一起工作？

Answer 1

您的代码失败，因为您已经在第一次调用时使用了迭代器，如果您在 with 块中调用 none_context()，原始代码将起作用：

none_context = contextmanager(lambda: iter([None]))
printing=False

with open(fa, "r") if printing else none_context() as writter, \
    open(fb, "r") if printing else none_context() as another_writter:
    print 1 if printing else 2

你可以看到使用你的原始代码，如果你为每个打开添加一个 None 那么代码将按预期工作：

none_context = contextmanager(lambda: iter([None,None,None]))()
printing=False

with open(fa, "r") if printing else none_context as writter, \
    open(fb, "r") if printing else none_context as another_writter,\
    open(fb, "r") if printing else none_context as another_writer3:
    print 1 if printing else 2

如何在 with 语句中有多个虚拟文件编写器？

How to have multiple dummy file writers in a with statement?

python

iterator

with-statement