Pytest Fixtures and Conftest.py: Overriding Session-Scoped State Across Directory Hierarchies

Direct Answer

Overriding a session-scoped fixture in a sub-directory does not create a new instance per test. The overridden fixture remains session-scoped, so pytest creates exactly one instance of that specific fixture definition per test session—the first time any test in that sub-directory requests it. Tests outside the sub-directory continue to receive the root fixture, potentially resulting in two separate session-scoped instances if both fixture definitions are exercised during the same run.

Teardown occurs after the entire test session finishes, in reverse order of first creation. Each fixture definition (root and overridden) is torn down exactly once, regardless of how many tests use it. Overriding does not alter this global teardown sequence.

How Fixture Resolution Works

Pytest resolves fixtures by walking the directory hierarchy from the test file upward. When a sub-directory conftest.py defines a fixture with the same name as a parent, that definition shadows the parent for all tests collected under that sub-tree. The two fixtures are completely independent—different objects, different setup/teardown calls, different lifecycles.

• Root tests → receive root fixture instance
• Sub-directory tests → receive overridden fixture instance
• Both used in one session → two distinct session-scoped instances exist simultaneously

Teardown Order

Session-scoped fixtures are torn down in reverse order of their first setup. If the root fixture sets up first (because a root test runs first), its teardown runs last. If a sub-directory test runs first and triggers the overridden fixture setup, that fixture tears down last. The directory hierarchy does not dictate teardown order—execution order does.

Verification Steps

1. Create conftest.py at project root with a session fixture printing SETUP_ROOT/TEARDOWN_ROOT.
2. Create subdir/conftest.py overriding the same fixture name, printing SETUP_SUB/TEARDOWN_SUB.
3. Add tests in both root and subdir/ that request the fixture.
4. Run pytest -s and observe:
   • Each SETUP_* message appears exactly once
   • Sub-directory tests show only SETUP_SUB/TEARDOWN_SUB
   • Root tests show only SETUP_ROOT/TEARDOWN_ROOT
   • Teardown messages appear in reverse order of first setup after all tests complete

Practical Implication

If you need one shared resource across the entire suite with localized configuration, avoid overriding. Instead, make the root fixture configurable (e.g., via a session-scoped config fixture or environment variables) so sub-directories influence behavior without creating a second instance. Override only when you genuinely need independent resources (e.g., separate database connections, distinct temp directories).

One Diagnostic Detail

Are both the root and overridden fixtures actually used in the same test session? If your test suite only runs tests from the sub-directory, the root fixture is never instantiated and the question of dual instances/teardown order is moot.