During test execution any output sent to stdout and stderr is captured. If a test or a setup method fails its according captured output will usually be shown along with the failure traceback.
In addition, stdin is set to a “null” object which will fail on attempts to read from it because it is rarely desired to wait for interactive input when running automated tests.
By default capturing is done by intercepting writes to low level file descriptors. This allows to capture output from simple print statements as well as output from a subprocess started by a test.
There are two ways in which py.test can perform capturing:
You can influence output capturing mechanisms from the command line:
py.test -s # disable all capturing py.test --capture=sys # replace sys.stdout/stderr with in-mem files py.test --capture=fd # also point filedescriptors 1 and 2 to temp file
One primary benefit of the default capturing of stdout/stderr output is that you can use print statements for debugging:
# content of test_module.py def setup_function(function): print ("setting up %s" % function) def test_func1(): assert True def test_func2(): assert False
and running this module will show you precisely the output of the failing function and hide the other one:
$ py.test =========================== test session starts ============================ platform linux2 -- Python 2.7.3 -- pytest-2.3.5 collected 2 items test_module.py .F ================================= FAILURES ================================= ________________________________ test_func2 ________________________________ def test_func2(): > assert False E assert False test_module.py:9: AssertionError ----------------------------- Captured stdout ------------------------------ setting up <function test_func2 at 0x2d79f50> ==================== 1 failed, 1 passed in 0.01 seconds ====================
The Fixtures as Function arguments (funcargs) allows test function a very easy way to access the captured output by simply using the names capsys or capfd in the test function signature. Here is an example test function that performs some output related checks:
def test_myoutput(capsys): # or use "capfd" for fd-level print ("hello") sys.stderr.write("world\n") out, err = capsys.readouterr() assert out == "hello\n" assert err == "world\n" print "next" out, err = capsys.readouterr() assert out == "next\n"
The readouterr() call snapshots the output so far - and capturing will be continued. After the test function finishes the original streams will be restored. Using capsys this way frees your test from having to care about setting/resetting output streams and also interacts well with py.test’s own per-test capturing.
If you want to capture on fd level you can use the capfd function argument which offers the exact same interface.