我使用此

I use this context manager to capture output. It ultimately uses the same technique as some of the other answers by temporarily replacing sys.stdout. I prefer the context manager because it wraps all the bookkeeping into a single function, so I don't have to re-write any try-finally code, and I don't have to write setup and teardown functions just for this.

import sys
from contextlib import contextmanager
from StringIO import StringIO

@contextmanager
def captured_output():
    new_out, new_err = StringIO(), StringIO()
    old_out, old_err = sys.stdout, sys.stderr
    try:
        sys.stdout, sys.stderr = new_out, new_err
        yield sys.stdout, sys.stderr
    finally:
        sys.stdout, sys.stderr = old_out, old_err

像这样使用它:

with captured_output() as (out, err):
    foo()
# This can go inside or outside the `with` block
output = out.getvalue().strip()
self.assertEqual(output, 'hello world!')

此外，由于退出with块后将恢复原始输出状态，因此我们可以使用与第一个捕获块相同的功能来设置第二个捕获块，而使用设置和拆卸功能是不可能的，并且可以手动编写try-finally块时比较罗.当测试的目的是将两个函数的结果相互比较而不是与某个预先计算的值进行比较时，该功能就派上用场了.

Furthermore, since the original output state is restored upon exiting the with block, we can set up a second capture block in the same function as the first one, which isn't possible using setup and teardown functions, and gets wordy when writing try-finally blocks manually. That ability came in handy when the goal of a test was to compare the results of two functions relative to each other rather than to some precomputed value.