reorder pytest tests based on dependencies

conftest.py

def pytest_collection_modifyitems(session, config, items):
    """Reorder the execution of tests based on the dependency marks on tests.
    
    To setup a test dependency, decorate the test function with the "depends" 
    mark and give a list of dependencies via the "on" keyword arg to the 
    decorator. Dependencies can be specified by name only when in the same
    file as the test being decorated, or by pytest node path for tests in 
    other files/classes.
    
        @pytest.mark.depends(on=["test_in_same_file", 
                                 "test_other.py::test_in_other_file", 
                                 "test_other.py::TestClass::test_method"])
        def test_with_dependencies():
            pass
            
    Tests without dependencies will run in arbitrary order.            
    """
    def _nodeid_cleaner(nodeid):
        """Clean node ids for class-based tests so they're easier to refer to."""
        return nodeid.replace('::()::', '::')

    dag = nx.DiGraph()

    nodeid_to_item_map = {_nodeid_cleaner(item.nodeid): item for item in items}

    list(map(lambda x: dag.add_node(_nodeid_cleaner(x.nodeid)), items))

    for item in items:
        marker = item.get_marker("depends")
        if marker is not None:
            depends = marker.kwargs.get('on', [])
            for dependency in depends:
                if '::' not in dependency:
                    base_item_nodeid = _nodeid_cleaner(item.nodeid).rsplit('::', 1)[0]
                    dependency = f"{base_item_nodeid}::{dependency}"
                dag.add_edge(dependency, _nodeid_cleaner(item.nodeid))

    reordered = [nodeid_to_item_map[node] for node in nx.topological_sort(dag)]

    items[:] = reordered

requirements.pip

pytest
networkx

Could use pytestmark = [pytest.mark.depends(on=["other_file.py::last_test"]) at the top of a test file to make sure every test there runs after the last test in other_file.py

Put conftest.py in your top-level test directory.

@jwineinger thanks for sharing your code.
If you are interested, I have forked your gist (https://gist.github.com/ftesser/4919ac83ad9e912e72e8a532d7ef6504) and make a little change to achieve the compatibility with pytest > 4.0:
to use get_closest_marker() instead of get_marker() as stated in https://stackoverflow.com/questions/54254337/pytest-attributeerror-function-object-has-no-attribute-get-marker.

thanks!