conftest.py

import threading
def pytest_runtest_setup(item):
    item.activeCount = threading.activeCount()
    item.session._setupstate.prepare(item)

def pytest_runtest_teardown(item, nextitem):
    item.session._setupstate.teardown_exact(item, nextitem)
    after = threading.activeCount()
    before = item.activeCount
    assert after == before, "number of threads changed from %s to %s" % (before, after)

# def pytest_runtest_call(item):
#     before = threading.activeCount()
#     item.runtest()
#     after = threading.activeCount()
#     assert after == before, "activeCount changed"