locustio/locust · error · ValueError

Could not find fixture for parameter

Error message

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

What it means

When loading a locustfile via pytest (PytestUser), each test function's parameters must be resolvable to exactly one pytest fixture. If fixturemanager.getfixturedefs returns none for a parameter, ValueError is raised.

Solutions

1. Define the missing fixture (in the locustfile or an imported conftest.py)
2. Fix the parameter name typo to match an existing fixture
3. Install/import the plugin providing the fixture
4. Ensure exactly one fixture matches the parameter name

Example fix

// before
def test_login(user_client):
    ...  # no fixture named user_client
// after
@pytest.fixture
def user_client():
    return ...

def test_login(user_client):
    ...

import pytest
for p in inspect.signature(test_fn).parameters:
    assert fm.getfixturedefs(p, test_fn), f"Missing fixture {p}"

try:
    load_locustfile_pytest(path)
except ValueError as e:
    logging.error("Fixture problem in locustfile: %s", e)

When it happens

Trigger: A pytest-style user test function declares a parameter that matches no fixture (not defined, autouse-only mismatch, or defined in a non-imported plugin/conftest).

Common situations: Typos in fixture names; fixtures defined in a conftest.py outside the collection scope; missing plugin; parametrized fixtures yielding multiple defs (separate error).

Related errors

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AI-assisted analysis of locustio/locust@f391a716e1 (2026-08-29). Data as JSON: /api/errors/71c92364f424e5db. Report an issue: GitHub.

Appendix: source

Thrown at locust/util/load_locustfile.py:126

            "-s",  # dont capture stdin (locust uses it for opening a browser and other input events)
            path,
        ],
    )
    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

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