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

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.

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.py


def 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.py

pytest_plugins = [
    'plugins.example_plugin',
]


# content of plugins/example_plugin.py
def pytest_configure(config):
    pass


def pytest_unconfigure(config):
    pass