Writing plugins¶
It is easy to implement local conftest plugins for your own project or pip-installable plugins that can be used throughout many projects, including third party projects. Please refer to How to install and use plugins if you only want to use but not write plugins.
A plugin contains one or multiple hook functions. Writing hooks
explains the basics and details of how you can write a hook function yourself.
pytest
implements all aspects of configuration, collection, running and
reporting by calling well specified hooks of the following plugins:
builtin plugins: loaded from pytest’s internal
_pytest
directory.external plugins: installed third-party modules discovered through entry points in their packaging metadata
conftest.py plugins: modules auto-discovered in test directories
In principle, each hook call is a 1:N
Python function call where N
is the
number of registered implementation functions for a given specification.
All specifications and implementations follow the pytest_
prefix
naming convention, making them easy to distinguish and find.
Plugin discovery order at tool startup¶
pytest
loads plugin modules at tool startup in the following way:
by scanning the command line for the
-p no:name
option and blocking that plugin from being loaded (even builtin plugins can be blocked this way). This happens before normal command-line parsing.by loading all builtin plugins.
by scanning the command line for the
-p name
option and loading the specified plugin. This happens before normal command-line parsing.by loading all plugins registered through installed third-party package entry points, unless the
PYTEST_DISABLE_PLUGIN_AUTOLOAD
environment variable is set.by loading all plugins specified through the
PYTEST_PLUGINS
environment variable.by loading all “initial “
conftest.py
files:determine the test paths: specified on the command line, otherwise in
testpaths
if defined and running from the rootdir, otherwise the current dirfor each test path, load
conftest.py
andtest*/conftest.py
relative to the directory part of the test path, if exist. Before aconftest.py
file is loaded, loadconftest.py
files in all of its parent directories. After aconftest.py
file is loaded, recursively load all plugins specified in itspytest_plugins
variable if present.
conftest.py: local per-directory plugins¶
Local conftest.py
plugins contain directory-specific hook
implementations. Hook Session and test running activities will
invoke all hooks defined in conftest.py
files closer to the
root of the filesystem. Example of implementing the
pytest_runtest_setup
hook so that is called for tests in the a
sub directory but not for other directories:
a/conftest.py:
def pytest_runtest_setup(item):
# called for running each test in 'a' directory
print("setting up", item)
a/test_sub.py:
def test_sub():
pass
test_flat.py:
def test_flat():
pass
Here is how you might run it:
pytest test_flat.py --capture=no # will not show "setting up"
pytest a/test_sub.py --capture=no # will show "setting up"
Note
If you have conftest.py
files which do not reside in a
python package directory (i.e. one containing an __init__.py
) then
“import conftest” can be ambiguous because there might be other
conftest.py
files as well on your PYTHONPATH
or sys.path
.
It is thus good practice for projects to either put conftest.py
under a package scope or to never import anything from a
conftest.py
file.
Note
Some hooks cannot be implemented in conftest.py files which are not initial due to how pytest discovers plugins during startup. See the documentation of each hook for details.
Writing your own plugin¶
If you want to write a plugin, there are many real-life examples you can copy from:
a custom collection example plugin: A basic example for specifying tests in Yaml files
builtin plugins which provide pytest’s own functionality
many external plugins providing additional features
All of these plugins implement hooks and/or fixtures to extend and add functionality.
Note
Make sure to check out the excellent cookiecutter-pytest-plugin project, which is a cookiecutter template for authoring plugins.
The template provides an excellent starting point with a working plugin, tests running with tox, a comprehensive README file as well as a pre-configured entry-point.
Also consider contributing your plugin to pytest-dev once it has some happy users other than yourself.
Making your plugin installable by others¶
If you want to make your plugin externally available, you
may define a so-called entry point for your distribution so
that pytest
finds your plugin module. Entry points are
a feature that is provided by packaging tools.
pytest looks up the pytest11
entrypoint to discover its
plugins, thus you can make your plugin available by defining
it in your pyproject.toml
file.
# sample ./pyproject.toml file
[build-system]
requires = ["hatchling"]
build-backend = "hatchling.build"
[project]
name = "myproject"
classifiers = [
"Framework :: Pytest",
]
[project.entry-points.pytest11]
myproject = "myproject.pluginmodule"
If a package is installed this way, pytest
will load
myproject.pluginmodule
as a plugin which can define
hooks. Confirm registration with pytest --trace-config
Note
Make sure to include Framework :: Pytest
in your list of
PyPI classifiers
to make it easy for users to find your plugin.
Assertion Rewriting¶
One of the main features of pytest
is the use of plain assert
statements and the detailed introspection of expressions upon
assertion failures. This is provided by “assertion rewriting” which
modifies the parsed AST before it gets compiled to bytecode. This is
done via a PEP 302 import hook which gets installed early on when
pytest
starts up and will perform this rewriting when modules get
imported. However, since we do not want to test different bytecode
from what you will run in production, this hook only rewrites test modules
themselves (as defined by the python_files
configuration option),
and any modules which are part of plugins.
Any other imported module will not be rewritten and normal assertion behaviour
will happen.
If you have assertion helpers in other modules where you would need
assertion rewriting to be enabled you need to ask pytest
explicitly to rewrite this module before it gets imported.
- register_assert_rewrite(*names)[source]
Register one or more module names to be rewritten on import.
This function will make sure that this module or all modules inside the package will get their assert statements rewritten. Thus you should make sure to call this before the module is actually imported, usually in your __init__.py if you are a plugin using a package.
- Parameters:
names (str) – The module names to register.
This is especially important when you write a pytest plugin which is
created using a package. The import hook only treats conftest.py
files and any modules which are listed in the pytest11
entrypoint
as plugins. As an example consider the following package:
pytest_foo/__init__.py
pytest_foo/plugin.py
pytest_foo/helper.py
With the following typical setup.py
extract:
setup(..., entry_points={"pytest11": ["foo = pytest_foo.plugin"]}, ...)
In this case only pytest_foo/plugin.py
will be rewritten. If the
helper module also contains assert statements which need to be
rewritten it needs to be marked as such, before it gets imported.
This is easiest by marking it for rewriting inside the
__init__.py
module, which will always be imported first when a
module inside a package is imported. This way plugin.py
can still
import helper.py
normally. The contents of
pytest_foo/__init__.py
will then need to look like this:
import pytest
pytest.register_assert_rewrite("pytest_foo.helper")
Requiring/Loading plugins in a test module or conftest file¶
You can require plugins in a test module or a conftest.py
file using pytest_plugins
:
pytest_plugins = ["name1", "name2"]
When the test module or conftest plugin is loaded the specified plugins will be loaded as well. Any module can be blessed as a plugin, including internal application modules:
pytest_plugins = "myapp.testsupport.myplugin"
pytest_plugins
are processed recursively, so note that in the example above
if myapp.testsupport.myplugin
also declares pytest_plugins
, the contents
of the variable will also be loaded as plugins, and so on.
Note
Requiring plugins using pytest_plugins
variable in non-root
conftest.py
files is deprecated.
This is important because conftest.py
files implement per-directory
hook implementations, but once a plugin is imported, it will affect the
entire directory tree. In order to avoid confusion, defining
pytest_plugins
in any conftest.py
file which is not located in the
tests root directory is deprecated, and will raise a warning.
This mechanism makes it easy to share fixtures within applications or even external applications without the need to create external plugins using the entry point packaging metadata technique.
Plugins imported by pytest_plugins
will also automatically be marked
for assertion rewriting (see pytest.register_assert_rewrite()
).
However for this to have any effect the module must not be
imported already; if it was already imported at the time the
pytest_plugins
statement is processed, a warning will result and
assertions inside the plugin will not be rewritten. To fix this you
can either call pytest.register_assert_rewrite()
yourself before
the module is imported, or you can arrange the code to delay the
importing until after the plugin is registered.
Accessing another plugin by name¶
If a plugin wants to collaborate with code from another plugin it can obtain a reference through the plugin manager like this:
plugin = config.pluginmanager.get_plugin("name_of_plugin")
If you want to look at the names of existing plugins, use
the --trace-config
option.
Registering custom markers¶
If your plugin uses any markers, you should register them so that they appear in
pytest’s help text and do not cause spurious warnings.
For example, the following plugin would register cool_marker
and
mark_with
for all users:
def pytest_configure(config):
config.addinivalue_line("markers", "cool_marker: this one is for cool tests.")
config.addinivalue_line(
"markers", "mark_with(arg, arg2): this marker takes arguments."
)
Testing plugins¶
pytest comes with a plugin named pytester
that helps you write tests for
your plugin code. The plugin is disabled by default, so you will have to enable
it before you can use it.
You can do so by adding the following line to a conftest.py
file in your
testing directory:
# content of conftest.py
pytest_plugins = ["pytester"]
Alternatively you can invoke pytest with the -p pytester
command line
option.
This will allow you to use the pytester
fixture for testing your plugin code.
Let’s demonstrate what you can do with the plugin with an example. Imagine we
developed a plugin that provides a fixture hello
which yields a function
and we can invoke this function with one optional parameter. It will return a
string value of Hello World!
if we do not supply a value or Hello
{value}!
if we do supply a string value.
import pytest
def pytest_addoption(parser):
group = parser.getgroup("helloworld")
group.addoption(
"--name",
action="store",
dest="name",
default="World",
help='Default "name" for hello().',
)
@pytest.fixture
def hello(request):
name = request.config.getoption("name")
def _hello(name=None):
if not name:
name = request.config.getoption("name")
return f"Hello {name}!"
return _hello
Now the pytester
fixture provides a convenient API for creating temporary
conftest.py
files and test files. It also allows us to run the tests and
return a result object, with which we can assert the tests’ outcomes.
def test_hello(pytester):
"""Make sure that our plugin works."""
# create a temporary conftest.py file
pytester.makeconftest(
"""
import pytest
@pytest.fixture(params=[
"Brianna",
"Andreas",
"Floris",
])
def name(request):
return request.param
"""
)
# create a temporary pytest test file
pytester.makepyfile(
"""
def test_hello_default(hello):
assert hello() == "Hello World!"
def test_hello_name(hello, name):
assert hello(name) == "Hello {0}!".format(name)
"""
)
# run all tests with pytest
result = pytester.runpytest()
# check that all 4 tests passed
result.assert_outcomes(passed=4)
Additionally it is possible to copy examples to the pytester
’s isolated environment
before running pytest on it. This way we can abstract the tested logic to separate files,
which is especially useful for longer tests and/or longer conftest.py
files.
Note that for pytester.copy_example
to work we need to set pytester_example_dir
in our pytest.ini
to tell pytest where to look for example files.
# content of pytest.ini
[pytest]
pytester_example_dir = .
# content of test_example.py
def test_plugin(pytester):
pytester.copy_example("test_example.py")
pytester.runpytest("-k", "test_example")
def test_example():
pass
$ pytest
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-8.x.y, pluggy-1.x.y
rootdir: /home/sweet/project
configfile: pytest.ini
collected 2 items
test_example.py .. [100%]
============================ 2 passed in 0.12s =============================
For more information about the result object that runpytest()
returns, and
the methods that it provides please check out the RunResult
documentation.