locustio/locust · error · ValueError

Multiple fixtures found for parameter {name!r} in {function.

Error message

Multiple fixtures found for parameter {name!r} in {function.name}: {defs}

What it means

In locust's pytest integration, each test function's parameters must resolve to exactly one pytest fixture via fixturemanager.getfixturedefs. If a parameter matches more than one fixture definition (e.g. a fixture overridden in multiple scopes or duplicated via conftest/plugin chains), load_locustfile cannot pick one and raises this ValueError while building the PytestUser class.

Source

Thrown at locust/util/load_locustfile.py:128

        ],
    )
    config._do_configure()
    session = pytest.Session.from_config(config)
    config.hook.pytest_sessionstart(session=session)
    session.perform_collect()
    config.hook.pytest_collection_modifyitems(session=session, config=config, items=session.items)
    fm = session._fixturemanager

    for function in session.items:
        if isinstance(function, pytest.Function):
            sig = inspect.signature(function.obj)
            function.kwargs = {}  # type: ignore[attr-defined]
            for name in sig.parameters:
                defs = fm.getfixturedefs(name, function)
                if not defs:
                    raise ValueError(f"Could not find fixture for parameter {name!r} in {function.name}")
                if len(defs) > 1:
                    raise ValueError(f"Multiple fixtures found for parameter {name!r} in {function.name}: {defs}")
                function.fixturedef = defs[0]  # type: ignore[attr-defined]
            if not function.name in user_classes:
                user_classes[function.name] = type(function.name, (PytestUser,), {})
                user_classes[function.name].functions = []
            user_classes[function.name].functions.append(function)
        else:
            pass  # Skipping non-function item
    return user_classes  # type: ignore

View on GitHub (pinned to f391a716e1)

Solutions

Rename one of the conflicting fixtures so each parameter maps to exactly one fixture
Remove the duplicated fixture definition (keep the conftest.py one and delete the module-level copy, or vice versa)
Use @pytest.fixture(autouse=False) scoping or parametrization restructuring to avoid duplicate names
Run `pytest --fixtures <file>` to list which fixture definitions match the parameter name and delete/shadow the extra one

Example fix

# before
tests/conftest.py
@pytest.fixture
def client(): ...

locustfile.py
@pytest.fixture
def client(): ...

# after
locustfile.py
@pytest.fixture
def http_client(): ...  # renamed so 'client' resolves to a single fixture

Defensive patterns

Strategy: validation

Validate before calling

import pytest
def ensure_unique_fixtures(path):
    from _pytest.config import get_config, get_plugin_manager
    cfg = get_config(); cfg.parse([str(path)]); cfg._preparse([], addopts=False)
    fm = cfg.pluginmanager.get_plugin('funcmanage')
    for name in {n for n in collect_param_names(path)}:
        assert len(fm.getfixturedefs(name, None) or []) <= 1, f"duplicate fixture: {name}"

Try / catch

try:
    user_classes = load_locustfile(path)
except ValueError as e:
    if 'Multiple fixtures found' in str(e):
        log.error('Duplicate fixture definitions; run `pytest --fixtures` to find them: %s', e)
        sys.exit(2)
    raise

Prevention

Define each fixture exactly once, ideally in conftest.py
Avoid generic fixture names like 'user' or 'client' that plugins may also register
Run `pytest --fixtures` when adding plugins to spot name collisions
Keep pytest fixtures out of the locustfile itself

When it happens

Trigger: Running `locust -f locustfile.py` where the locustfile imports/loads pytest-style test functions and one function parameter has >1 matching fixture defs, e.g. the same fixture name defined in both conftest.py and the module, or via a plugin autouse fixture with the same name.

Common situations: Projects mixing pytest suites with locust load tests where conftest.py fixtures shadow module fixtures; installing plugins that register fixtures with common names (like 'client' or 'user'); copy-pasting fixture definitions into both conftest and test files.

Related errors

Could not find fixture for parameter {name!r} in {function.n

AI-assisted analysis of locustio/locust@f391a716e1 (2026-08-29). Data as JSON: /api/errors/20529329d4946228. Report an issue: GitHub.