如何从Python函数调用中捕获标准输出？ [英] How to capture stdout output from a Python function call?

问题描述

我正在使用对对象做某事的Python库

I'm using a Python library that does something to an object

do_something(my_object)

并进行更改。这样做时，它会向stdout打印一些统计信息，我希望掌握这些信息。正确的解决方案是更改 do_something（）以返回相关信息，

and changes it. While doing so, it prints some statistics to stdout, and I'd like to get a grip on this information. The proper solution would be to change do_something() to return the relevant information,

out = do_something(my_object)

，但是<$的开发者还需要一段时间c $ c> do_something（）解决此问题。作为一种解决方法，我考虑过解析任何 do_something（）写到stdout的内容。

but it will be a while before the devs of do_something() get to this issue. As a workaround, I thought about parsing whatever do_something() writes to stdout.

如何捕获stdout输出在代码的两点之间，例如

How can I capture stdout output between two points in the code, e.g.,

start_capturing()
do_something(my_object)
out = end_capturing()

？

推荐答案

尝试使用此上下文管理器：

Try this context manager:

from io import StringIO 
import sys

class Capturing(list):
    def __enter__(self):
        self._stdout = sys.stdout
        sys.stdout = self._stringio = StringIO()
        return self
    def __exit__(self, *args):
        self.extend(self._stringio.getvalue().splitlines())
        del self._stringio    # free up some memory
        sys.stdout = self._stdout

用法：

with Capturing() as output:
    do_something(my_object)

输出现在是一个包含函数调用打印的行的列表。

output is now a list containing the lines printed by the function call.

高级用法：

可能不明显的是，此操作可以执行多次，并且将结果连接在一起：

What may not be obvious is that this can be done more than once and the results concatenated:

with Capturing() as output:
    print('hello world')

print('displays on screen')

with Capturing(output) as output:  # note the constructor argument
    print('hello world2')

print('done')
print('output:', output)

输出：

displays on screen                     
done                                   
output: ['hello world', 'hello world2']

更新：他们在redirect_stdout（） python.3.4中的docs.python.org/3/library/contextlib.html#contextlib.redirect_stdout rel = noreferrer> contextlib 以及 redirect_stderr（））。因此，您可以使用 io.StringIO 来达到类似的结果（尽管 Captureing 是一个列表以及一个上下文管理器可以说更方便）。

Update: They added redirect_stdout() to contextlib in Python 3.4 (along with redirect_stderr()). So you could use io.StringIO with that to achieve a similar result (though Capturing being a list as well as a context manager is arguably more convenient).

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