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