314 lines
10 KiB
ReStructuredText
314 lines
10 KiB
ReStructuredText
.. _`writinghooks`:
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Writing hook functions
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======================
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.. _validation:
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hook function validation and execution
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--------------------------------------
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pytest calls hook functions from registered plugins for any
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given hook specification. Let's look at a typical hook function
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for the ``pytest_collection_modifyitems(session, config,
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items)`` hook which pytest calls after collection of all test items is
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completed.
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When we implement a ``pytest_collection_modifyitems`` function in our plugin
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pytest will during registration verify that you use argument
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names which match the specification and bail out if not.
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Let's look at a possible implementation:
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.. code-block:: python
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def pytest_collection_modifyitems(config, items):
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# called after collection is completed
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# you can modify the ``items`` list
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...
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Here, ``pytest`` will pass in ``config`` (the pytest config object)
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and ``items`` (the list of collected test items) but will not pass
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in the ``session`` argument because we didn't list it in the function
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signature. This dynamic "pruning" of arguments allows ``pytest`` to
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be "future-compatible": we can introduce new hook named parameters without
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breaking the signatures of existing hook implementations. It is one of
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the reasons for the general long-lived compatibility of pytest plugins.
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Note that hook functions other than ``pytest_runtest_*`` are not
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allowed to raise exceptions. Doing so will break the pytest run.
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.. _firstresult:
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firstresult: stop at first non-None result
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-------------------------------------------
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Most calls to ``pytest`` hooks result in a **list of results** which contains
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all non-None results of the called hook functions.
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Some hook specifications use the ``firstresult=True`` option so that the hook
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call only executes until the first of N registered functions returns a
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non-None result which is then taken as result of the overall hook call.
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The remaining hook functions will not be called in this case.
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.. _`hookwrapper`:
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hookwrapper: executing around other hooks
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-------------------------------------------------
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.. currentmodule:: _pytest.core
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pytest plugins can implement hook wrappers which wrap the execution
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of other hook implementations. A hook wrapper is a generator function
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which yields exactly once. When pytest invokes hooks it first executes
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hook wrappers and passes the same arguments as to the regular hooks.
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At the yield point of the hook wrapper pytest will execute the next hook
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implementations and return their result to the yield point in the form of
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a :py:class:`Result <pluggy._Result>` instance which encapsulates a result or
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exception info. The yield point itself will thus typically not raise
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exceptions (unless there are bugs).
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Here is an example definition of a hook wrapper:
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.. code-block:: python
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import pytest
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@pytest.hookimpl(hookwrapper=True)
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def pytest_pyfunc_call(pyfuncitem):
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do_something_before_next_hook_executes()
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outcome = yield
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# outcome.excinfo may be None or a (cls, val, tb) tuple
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res = outcome.get_result() # will raise if outcome was exception
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post_process_result(res)
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outcome.force_result(new_res) # to override the return value to the plugin system
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Note that hook wrappers don't return results themselves, they merely
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perform tracing or other side effects around the actual hook implementations.
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If the result of the underlying hook is a mutable object, they may modify
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that result but it's probably better to avoid it.
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For more information, consult the
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:ref:`pluggy documentation about hookwrappers <pluggy:hookwrappers>`.
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.. _plugin-hookorder:
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Hook function ordering / call example
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-------------------------------------
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For any given hook specification there may be more than one
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implementation and we thus generally view ``hook`` execution as a
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``1:N`` function call where ``N`` is the number of registered functions.
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There are ways to influence if a hook implementation comes before or
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after others, i.e. the position in the ``N``-sized list of functions:
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.. code-block:: python
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# Plugin 1
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@pytest.hookimpl(tryfirst=True)
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def pytest_collection_modifyitems(items):
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# will execute as early as possible
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...
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# Plugin 2
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@pytest.hookimpl(trylast=True)
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def pytest_collection_modifyitems(items):
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# will execute as late as possible
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...
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# Plugin 3
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@pytest.hookimpl(hookwrapper=True)
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def pytest_collection_modifyitems(items):
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# will execute even before the tryfirst one above!
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outcome = yield
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# will execute after all non-hookwrappers executed
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Here is the order of execution:
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1. Plugin3's pytest_collection_modifyitems called until the yield point
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because it is a hook wrapper.
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2. Plugin1's pytest_collection_modifyitems is called because it is marked
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with ``tryfirst=True``.
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3. Plugin2's pytest_collection_modifyitems is called because it is marked
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with ``trylast=True`` (but even without this mark it would come after
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Plugin1).
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4. Plugin3's pytest_collection_modifyitems then executing the code after the yield
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point. The yield receives a :py:class:`Result <pluggy._Result>` instance which encapsulates
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the result from calling the non-wrappers. Wrappers shall not modify the result.
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It's possible to use ``tryfirst`` and ``trylast`` also in conjunction with
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``hookwrapper=True`` in which case it will influence the ordering of hookwrappers
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among each other.
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Declaring new hooks
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------------------------
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.. note::
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This is a quick overview on how to add new hooks and how they work in general, but a more complete
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overview can be found in `the pluggy documentation <https://pluggy.readthedocs.io/en/latest/>`__.
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.. currentmodule:: _pytest.hookspec
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Plugins and ``conftest.py`` files may declare new hooks that can then be
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implemented by other plugins in order to alter behaviour or interact with
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the new plugin:
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.. autofunction:: pytest_addhooks
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:noindex:
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Hooks are usually declared as do-nothing functions that contain only
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documentation describing when the hook will be called and what return values
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are expected. The names of the functions must start with `pytest_` otherwise pytest won't recognize them.
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Here's an example. Let's assume this code is in the ``sample_hook.py`` module.
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.. code-block:: python
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def pytest_my_hook(config):
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"""
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Receives the pytest config and does things with it
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"""
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To register the hooks with pytest they need to be structured in their own module or class. This
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class or module can then be passed to the ``pluginmanager`` using the ``pytest_addhooks`` function
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(which itself is a hook exposed by pytest).
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.. code-block:: python
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def pytest_addhooks(pluginmanager):
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""" This example assumes the hooks are grouped in the 'sample_hook' module. """
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from my_app.tests import sample_hook
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pluginmanager.add_hookspecs(sample_hook)
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For a real world example, see `newhooks.py`_ from `xdist <https://github.com/pytest-dev/pytest-xdist>`_.
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.. _`newhooks.py`: https://github.com/pytest-dev/pytest-xdist/blob/974bd566c599dc6a9ea291838c6f226197208b46/xdist/newhooks.py
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Hooks may be called both from fixtures or from other hooks. In both cases, hooks are called
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through the ``hook`` object, available in the ``config`` object. Most hooks receive a
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``config`` object directly, while fixtures may use the ``pytestconfig`` fixture which provides the same object.
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.. code-block:: python
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@pytest.fixture()
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def my_fixture(pytestconfig):
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# call the hook called "pytest_my_hook"
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# 'result' will be a list of return values from all registered functions.
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result = pytestconfig.hook.pytest_my_hook(config=pytestconfig)
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.. note::
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Hooks receive parameters using only keyword arguments.
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Now your hook is ready to be used. To register a function at the hook, other plugins or users must
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now simply define the function ``pytest_my_hook`` with the correct signature in their ``conftest.py``.
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Example:
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.. code-block:: python
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def pytest_my_hook(config):
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"""
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Print all active hooks to the screen.
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"""
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print(config.hook)
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.. _`addoptionhooks`:
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Using hooks in pytest_addoption
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-------------------------------
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Occasionally, it is necessary to change the way in which command line options
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are defined by one plugin based on hooks in another plugin. For example,
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a plugin may expose a command line option for which another plugin needs
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to define the default value. The pluginmanager can be used to install and
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use hooks to accomplish this. The plugin would define and add the hooks
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and use pytest_addoption as follows:
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.. code-block:: python
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# contents of hooks.py
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# Use firstresult=True because we only want one plugin to define this
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# default value
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@hookspec(firstresult=True)
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def pytest_config_file_default_value():
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""" Return the default value for the config file command line option. """
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# contents of myplugin.py
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def pytest_addhooks(pluginmanager):
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""" This example assumes the hooks are grouped in the 'hooks' module. """
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from . import hook
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pluginmanager.add_hookspecs(hook)
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def pytest_addoption(parser, pluginmanager):
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default_value = pluginmanager.hook.pytest_config_file_default_value()
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parser.addoption(
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"--config-file",
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help="Config file to use, defaults to %(default)s",
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default=default_value,
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)
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The conftest.py that is using myplugin would simply define the hook as follows:
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.. code-block:: python
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def pytest_config_file_default_value():
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return "config.yaml"
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Optionally using hooks from 3rd party plugins
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---------------------------------------------
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Using new hooks from plugins as explained above might be a little tricky
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because of the standard :ref:`validation mechanism <validation>`:
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if you depend on a plugin that is not installed, validation will fail and
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the error message will not make much sense to your users.
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One approach is to defer the hook implementation to a new plugin instead of
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declaring the hook functions directly in your plugin module, for example:
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.. code-block:: python
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# contents of myplugin.py
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class DeferPlugin:
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"""Simple plugin to defer pytest-xdist hook functions."""
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def pytest_testnodedown(self, node, error):
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"""standard xdist hook function."""
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def pytest_configure(config):
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if config.pluginmanager.hasplugin("xdist"):
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config.pluginmanager.register(DeferPlugin())
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This has the added benefit of allowing you to conditionally install hooks
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depending on which plugins are installed.
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