How to run a method before all tests in all classes?

Using session fixture as suggested by hpk42 is great solution for many cases,but fixture will run only after all tests are collected.

Here are two more solutions:

conftest hooks

Write a pytest_configure or pytest_sessionstart hook in your conftest.py file:

# content of conftest.pydef pytest_configure(config):    """    Allows plugins and conftest files to perform initial configuration.    This hook is called for every plugin and initial conftest    file after command line options have been parsed.    """def pytest_sessionstart(session):    """    Called after the Session object has been created and    before performing collection and entering the run test loop.    """def pytest_sessionfinish(session, exitstatus):    """    Called after whole test run finished, right before    returning the exit status to the system.    """def pytest_unconfigure(config):    """    called before test process is exited.    """

pytest plugin

Create a pytest plugin with pytest_configure and pytest_unconfigure hooks.
Enable your plugin in conftest.py:

# content of conftest.pypytest_plugins = [    'plugins.example_plugin',]# content of plugins/example_plugin.pydef pytest_configure(config):    passdef pytest_unconfigure(config):    pass

You might want to use a session-scoped "autouse" fixture:

# content of conftest.py or a tests file (e.g. in your tests or root directory)@pytest.fixture(scope="session", autouse=True)def do_something(request):    # prepare something ahead of all tests    request.addfinalizer(finalizer_function)

This will run ahead of all tests. The finalizer will be called after the last test finished.

Starting from version 2.10 there is a cleaner way to tear down the fixture as well as defining its scope. So you may use this syntax:

@pytest.fixture(scope="module", autouse=True)def my_fixture():    print ('INITIALIZATION')    yield param    print ('TEAR DOWN')

The autouse parameter:From documentation:

Here is how autouse fixtures work in other scopes:

• autouse fixtures obey the scope= keyword-argument: if an autouse fixture has scope='session' it will only be run once, no matter where it is defined. scope='class' means it will be run once per class, etc.

• if an autouse fixture is defined in a test module, all its test functions automatically use it.

• if an autouse fixture is defined in a conftest.py file then all tests in all test modules below its directory will invoke the fixture.

  ...

The "request" parameter:Note that the "request" parameter is not necessary for your purpose although you might want to use it for other purposes. From documentation:

"Fixture function can accept the request object to introspect the “requesting” test function, class or module context.."