171 lines
5.8 KiB
ReStructuredText
171 lines
5.8 KiB
ReStructuredText
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.. _`captures`:
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How to capture 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. (this
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behavior can be configured by the ``--show-capture`` command-line option).
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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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.. _capture-method:
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Setting capturing methods or disabling capturing
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-------------------------------------------------
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There are three ways in which ``pytest`` can perform capturing:
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* ``fd`` (file descriptor) 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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* ``tee-sys`` capturing: Python writes to ``sys.stdout`` and ``sys.stderr``
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will be captured, however the writes will also be passed-through to
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the actual ``sys.stdout`` and ``sys.stderr``. This allows output to be
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'live printed' and captured for plugin use, such as junitxml (new in pytest 5.4).
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.. _`disable capturing`:
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You can influence output capturing mechanisms from the command line:
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.. code-block:: bash
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pytest -s # disable all capturing
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pytest --capture=sys # replace sys.stdout/stderr with in-mem files
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pytest --capture=fd # also point filedescriptors 1 and 2 to temp file
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pytest --capture=tee-sys # combines 'sys' and '-s', capturing sys.stdout/stderr
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# and passing it along to the actual sys.stdout/stderr
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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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.. code-block:: python
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# content of test_module.py
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def setup_function(function):
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print("setting up", 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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.. code-block:: pytest
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$ pytest
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=========================== test session starts ============================
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platform linux -- Python 3.x.y, pytest-7.x.y, pluggy-1.x.y
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rootdir: /home/sweet/project
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collected 2 items
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test_module.py .F [100%]
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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:12: AssertionError
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-------------------------- Captured stdout setup ---------------------------
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setting up <function test_func2 at 0xdeadbeef0001>
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========================= short test summary info ==========================
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FAILED test_module.py::test_func2 - assert False
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======================= 1 failed, 1 passed in 0.12s ========================
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Accessing captured output from a test function
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---------------------------------------------------
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The ``capsys``, ``capsysbinary``, ``capfd``, and ``capfdbinary`` fixtures
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allow access to stdout/stderr output created during test execution. Here is
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an example test function that performs some output related checks:
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.. code-block:: python
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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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captured = capsys.readouterr()
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assert captured.out == "hello\n"
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assert captured.err == "world\n"
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print("next")
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captured = capsys.readouterr()
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assert captured.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 pytest's
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own per-test capturing.
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If you want to capture on filedescriptor level you can use
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the ``capfd`` fixture which offers the exact
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same interface but allows to also capture output from
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libraries or subprocesses that directly write to operating
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system level output streams (FD1 and FD2).
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The return value from ``readouterr`` changed to a ``namedtuple`` with two attributes, ``out`` and ``err``.
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If the code under test writes non-textual data, you can capture this using
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the ``capsysbinary`` fixture which instead returns ``bytes`` from
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the ``readouterr`` method.
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If the code under test writes non-textual data, you can capture this using
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the ``capfdbinary`` fixture which instead returns ``bytes`` from
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the ``readouterr`` method. The ``capfdbinary`` fixture operates on the
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filedescriptor level.
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To temporarily disable capture within a test, both ``capsys``
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and ``capfd`` have a ``disabled()`` method that can be used
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as a context manager, disabling capture inside the ``with`` block:
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.. code-block:: python
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def test_disabling_capturing(capsys):
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print("this output is captured")
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with capsys.disabled():
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print("output not captured, going directly to sys.stdout")
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print("this output is also captured")
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