116 lines
3.9 KiB
Plaintext
116 lines
3.9 KiB
Plaintext
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.. _`captures`:
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Capturing of the stdout/stderr output
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=========================================================
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Default stdout/stderr/stdin capturing behaviour
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---------------------------------------------------------
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During test execution any output sent to ``stdout`` and ``stderr`` is
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captured. If a test or a setup method fails its according captured
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output will usually be shown along with the failure traceback.
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In addition, ``stdin`` is set to a "null" object which will
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fail on attempts to read from it because it is rarely desired
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to wait for interactive input when running automated tests.
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By default capturing is done by intercepting writes to low level
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file descriptors. This allows to capture output from simple
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print statements as well as output from a subprocess started by
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a test.
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Setting capturing methods or disabling capturing
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-------------------------------------------------
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There are two ways in which ``py.test`` can perform capturing:
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* file descriptor (FD) level capturing (default): All writes going to the
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operating system file descriptors 1 and 2 will be captured.
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* ``sys`` level capturing: Only writes to Python files ``sys.stdout``
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and ``sys.stderr`` will be captured. No capturing of writes to
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filedescriptors is performed.
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.. _`disable capturing`:
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You can influence output capturing mechanisms from the command line::
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py.test -s # disable all capturing
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py.test --capture=sys # replace sys.stdout/stderr with in-mem files
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py.test --capture=fd # also point filedescriptors 1 and 2 to temp file
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.. _printdebugging:
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Using print statements for debugging
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---------------------------------------------------
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One primary benefit of the default capturing of stdout/stderr output
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is that you can use print statements for debugging::
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# content of test_module.py
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def setup_function(function):
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print ("setting up %s" % function)
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def test_func1():
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assert True
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def test_func2():
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assert False
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and running this module will show you precisely the output
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of the failing function and hide the other one::
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$ py.test
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=========================== test session starts ============================
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platform linux2 -- Python 2.7.3 -- pytest-2.5.0
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collected 2 items
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test_module.py .F
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================================= FAILURES =================================
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________________________________ test_func2 ________________________________
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def test_func2():
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> assert False
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E assert False
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test_module.py:9: AssertionError
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----------------------------- Captured stdout ------------------------------
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setting up <function test_func2 at 0x29437d0>
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==================== 1 failed, 1 passed in 0.01 seconds ====================
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Accessing captured output from a test function
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---------------------------------------------------
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The :ref:`funcarg mechanism` allows test function a very easy
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way to access the captured output by simply using the names
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``capsys`` or ``capfd`` in the test function signature. Here
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is an example test function that performs some output related
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checks::
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def test_myoutput(capsys): # or use "capfd" for fd-level
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print ("hello")
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sys.stderr.write("world\n")
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out, err = capsys.readouterr()
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assert out == "hello\n"
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assert err == "world\n"
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print "next"
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out, err = capsys.readouterr()
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assert out == "next\n"
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The ``readouterr()`` call snapshots the output so far -
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and capturing will be continued. After the test
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function finishes the original streams will
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be restored. Using ``capsys`` this way frees your
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test from having to care about setting/resetting
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output streams and also interacts well with py.test's
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own per-test capturing.
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If you want to capture on ``fd`` level you can use
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the ``capfd`` function argument which offers the exact
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same interface.
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.. include:: links.inc
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