How to assert output with nosetest/unittest in python?

Question

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

def foo():
    print \'hello world!\'

So when I want to test this function the code will b

Answer 1

If you really want to do this, you can reassign sys.stdout for the duration of the test.

def test_foo():
    import sys
    from foomodule import foo
    from StringIO import StringIO

    saved_stdout = sys.stdout
    try:
        out = StringIO()
        sys.stdout = out
        foo()
        output = out.getvalue().strip()
        assert output == 'hello world!'
    finally:
        sys.stdout = saved_stdout

If I were writing this code, however, I would prefer to pass an optional out parameter to the foo function.

def foo(out=sys.stdout):
    out.write("hello, world!")

Then the test is much simpler:

def test_foo():
    from foomodule import foo
    from StringIO import StringIO

    out = StringIO()
    foo(out=out)
    output = out.getvalue().strip()
    assert output == 'hello world!'