如何在python中使用鼻子测试/单元测试来断言输出? [英] How to assert output with nosetest/unittest in python?

问题描述

我正在为下一个函数编写测试:

I'm writing tests for a function like next one:

def foo():
    print 'hello world!'

因此，当我要测试此功能时，代码将如下所示:

So when I want to test this function the code will be like this:

import sys
from foomodule import foo
def test_foo():
    foo()
    output = sys.stdout.getline().strip() # because stdout is an StringIO instance
    assert output == 'hello world!'

但是，如果我使用-s参数运行鼻子测试，则测试会崩溃.如何使用unittest或鼻子模块捕获输出?

But if I run nosetests with -s parameter the test crashes. How can I catch the output with unittest or nose module?

推荐答案

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.

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