.. _`captures`:

Capturing of stdout/stderr output
=========================================================

By default ``stdout`` and ``stderr`` output is captured separately for
setup and test execution code.  If a test or a setup method fails its
according output will usually be shown along with the failure traceback.
In addition, ``stdin`` is set to a "null" object which will fail all
attempts to read from it.  This is important if some code paths in
test otherwise might lead to waiting for input - which is usually
not desired when running automated tests.

.. _printdebugging:

Using print statements for debugging
---------------------------------------------------

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 2.6.6 -- pytest-2.0.1
    collecting ... 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 0x2897d70>
    ==================== 1 failed, 1 passed in 0.02 seconds ====================

Setting capturing methods or disabling capturing
-------------------------------------------------

There are two ways in which ``py.test`` can perform capturing:

* ``fd`` level capturing (default): All writes going to the operating
  system file descriptors 1 and 2 will be captured, for example writes such
  as ``os.write(1, 'hello')``.  Capturing on ``fd``-level also includes
  **output from subprocesses**.

* ``sys`` level capturing: The ``sys.stdout`` and ``sys.stderr`` will
  will be replaced with in-memory files and the ``print`` builtin or
  output from code like ``sys.stderr.write(...)`` will be captured with
  this method.

.. _`disable 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


Accessing captured output from a test function
---------------------------------------------------

The :ref:`funcarg mechanism` 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.

.. include:: links.inc