test_ok1/py/test/plugin/pytest_iocapture.py

"""
'capsys' and 'capfd' funcargs for capturing stdout/stderror either
by intercepting sys.stdout/stderr or File Descriptors 1/2. 

Calling the reset() method of the capture funcargs gives 
a out/err tuple of strings representing the captured streams. 
You can call reset() multiple times each time getting
the chunk of output that was captured between the invocations. 

"""
import py

def pytest_funcarg__capsys(request):
    """ capture writes to sys.stdout/sys.stderr. """ 
    capture = Capture(py.io.StdCapture)
    request.addfinalizer(capture.finalize)
    return capture 

def pytest_funcarg__capfd(request):
    """ capture writes to filedescriptors 1 and 2"""
    capture = Capture(py.io.StdCaptureFD)
    request.addfinalizer(capture.finalize)
    return capture 

class Capture:
    def __init__(self, captureclass):
        self._captureclass = captureclass
        self._capture = self._captureclass()

    def finalize(self):
        self._capture.reset()

    def reset(self):
        res = self._capture.reset()
        self._capture = self._captureclass()
        return res 

def test_generic(plugintester):
    plugintester.hookcheck()

class TestCapture:
    def test_std_functional(self, testdir):        
        reprec = testdir.inline_runsource("""
            def test_hello(capsys):
                print 42
                out, err = capsys.reset()
                assert out.startswith("42")
        """)
        reprec.assertoutcome(passed=1)
        
    def test_stdfd_functional(self, testdir):        
        reprec = testdir.inline_runsource("""
            def test_hello(capfd):
                import os
                os.write(1, "42")
                out, err = capfd.reset()
                assert out.startswith("42")
        """)
        reprec.assertoutcome(passed=1)