How can I patch a plugin registered through a setuptools entry point

[dgard1981]

I'm developing a small plugin for use internally that will be registered through a setuptools entry point.

In my attempts to write tests for the plugin, I wish to patch a method that is part of an object included in one of the modules. However, patching directly doesn't have any affect. This isn't really surprising, but I'm unsure of exactly how the plugin is loaded and what I would need to patch. I suspect there is something in the pytestconfig object that I can patch.

monkeypatch.setattr(MyPluginObject, '_get_node_desc', lambda: 'mocked node description')

Is it possible to patch a plugin that is installed via a setup.py file?

[RonnyPfannschmidt]

the plugin-manager provides mechanisms to access plugin objects

without a clear idea of how your plugin self-enables is hard to give further suggestions

if you self-register a plugin object, i'd recommend to put a reference into the stash using a stash-key to ease finding it

[dgard1981]

@RonnyPfannschmidt, hopefully this example plugin that you can create in a docker container will demonstrate what I wish to do.

The plugin simply adds a pytest_terminal_summary hook that calls out to the method _some_info. What I wish to do is patch the return value of _some_info. I've added a short explanation of why I want to do this below, if it helps.

My attempts to do so thus far have failed, including in this example where I import MyPlugin and use monkeypatch.setattr(MyPlugin, '_some_info', lambda: 'foobar').

# Run in a docker container.
docker run --rm -it python:3.10-bullseye /bin/bash

# Make the require directories and change to the plugin project root.
mkdir -p /tmp/example_project/src/test
mkdir -p /tmp/example_project/src/pytest_example
cd tmp/example_project

# Make the plugin file.
cat <<EOF > /tmp/example_project/src/pytest_example/plugin.py
import pytest

class MyPlugin:

    def _some_info(self):
        return 'Some info.'

    @pytest.hookimpl
    def pytest_terminal_summary(self, *_) -> None:
        '''Print details of passed assertions in the terminal summary.'''
        print(self._some_info())

@pytest.hookimpl
def pytest_configure(config: pytest.Config) -> None:
    '''Register the pytest-expect plugin.'''

    config.pluginmanager.register(MyPlugin(), '_example')
EOF

# Make the test file.
cat <<EOF > /tmp/example_project/src/test/test_something.py
from pytest_example.plugin import MyPlugin

def test_something(monkeypatch):

    monkeypatch.setattr(MyPlugin, '_some_info', lambda: 'foobar')
    assert 1 == 1
EOF

# Make the project TOML file.
cat <<EOF > /tmp/example_project/pyproject.toml
[build-system]
requires = ["setuptools"]
build-backend = "setuptools.build_meta"

[project]
name = "pytest_example"
version = "0.0.1"

[project.entry-points.pytest11]
pytest_example = "pytest_example.plugin"
EOF

# Install pytest and the plugin, then run pytest.
pip install pytest /tmp/example_project/
pytest

Why I'm doing this

In short, my plugin adds output to the terminal summary, and I need to test that the terminal summary includes the expected output. Most of it is predictable, but one line depends on the environment in which the test run occurs, hence the need to mock that part.

The only way I can think to do this is run pytest.main() from within a test, and then use the capsys fixture to check for the expected output in the terminal summary.

The tests are configured in such a way that running pytest only triggers the main test, and the that test passes a specific module to pytest.main(), ensuring the tests aren't caught in a never ending loop.

[RonnyPfannschmidt]

As far as I can tell,you need the pytester Plugin to run your testing

If you monkeypatch inside a test,it gets undone After the test function

[dgard1981]

OK, I see what you mean - the mock is undone before the pytest_terminal_summary is fired, so that's why it seems that monkeypatch isn't working.

I will take a look at pytester (not come across it before) and see if I can figure that out. But from the description, sounds hopeful that it's the right plugin for the job.

[The-Compiler]

Yep, pytester is definitely the right way to test pytest plugins. Also see the relevant docs: Writing plugins — pytest documentation

[dgard1981]

@The-Compiler, @RonnyPfannschmidt, you are correct, pyteter is the best tool for the job. However, I'm still stuck on my initial point... How do I mock a method in a plugin class?

Take the example below. It will print Some info. in the terminal summary (the pytest_terminal_summary hook calls out to another method in the same class to get that text). In reality, the value returned by MyPlugin._some_info() is unknown (it's to do with the where the tests are run), and that's why I want to mock it to make the output.

So how could I amend this example to mock the return value of MyPlugin._some_info() in tests run by pytester?

# Run in a docker container.
docker run --rm -it python:3.10-bullseye /bin/bash


# Make the require directories and change to the plugin project root.
mkdir -p /tmp/example_project/test
mkdir -p /tmp/example_project/src/pytest_example
cd tmp/example_project/test


# Make the plugin file.
cat <<EOF > /tmp/example_project/src/pytest_example/plugin.py
import pytest

class MyPlugin:

    def _some_info(self):
        return 'Some info.'

    @pytest.hookimpl
    def pytest_terminal_summary(self, config, *_) -> None:
        print(self._some_info())

@pytest.hookimpl
def pytest_configure(config: pytest.Config) -> None:
    config.pluginmanager.register(MyPlugin(), '_example')
EOF


# Make the test file.
cat <<EOF > /tmp/example_project/test/test_something.py
import pytest

def test_something(pytester: pytest.Pytester):

    result = pytester.runpytest('-q')
    assert result.stdout.fnmatch_lines('Some info.') is None
EOF


# Make the test INI.
cat <<EOF > /tmp/example_project/test/pytest.ini
[pytest]
addopts = -p no:cacheprovider -p no:example -p pytester
EOF


# Make the project TOML file.
cat <<EOF > /tmp/example_project/pyproject.toml
[build-system]
requires = ["setuptools"]
build-backend = "setuptools.build_meta"

[project]
name = "pytest_example"
version = "0.0.1"

[project.entry-points.pytest11]
pytest_example = "pytest_example.plugin"
EOF


# Install pytest and the plugin, then run pytest.
pip install pytest /tmp/example_project/
pytest