What causes "Empty parameter set in '{func.__name__}' at line {lineno + 1}" in pytest-dev/pytest?

Supply at least one parameter row, or guard the parametrize call so it is only applied when data exists.

pytest-dev/pytest · error · CollectError

Empty parameter set in

Error message

Empty parameter set in '{func.__name__}' at line {lineno + 1}

What it means

A Collector.CollectError raised when a parametrize call yields an empty parameter set and the ini option empty_parameter_set_mark is set to 'fail_at_collect'. By default pytest skips such tests; only this explicit configuration turns the empty set into a hard collection failure. The function name and 1-based line number are included so the user can locate the offending parametrize.

Solutions

Supply at least one parameter row, or guard the parametrize call so it is only applied when data exists.
Change the ini option to 'skip' (default) or 'xfail' if an empty set is legitimate.
If the emptiness is a bug, fix the upstream data source / generator so it yields the expected rows.

Example fix

# before
@pytest.mark.parametrize('a', [])
def test_x(a): ...
# after
@pytest.mark.parametrize('a', [1])
def test_x(a): ...

Defensive patterns

Strategy: validation

Validate before calling

def has_params(param_values) -> bool:
    return len(list(param_values)) > 0

Try / catch

import pytest
try:
    # collection happens here
    ...
except pytest.Collector.CollectError as e:
    if 'Empty parameter set' in str(e):
        # provide default data or skip
        ...

Prevention

Guard parametrize so it is only applied when data exists.
Prefer 'skip' (default) over 'fail_at_collect' unless you want strict CI.
Log when data sources are empty so you notice early.

When it happens

Trigger: A `@pytest.mark.parametrize('a', [])` decorator (or parametrize that evaluates to zero parameter rows) when pytest.ini / pyproject.toml sets `empty_parameter_set_mark = fail_at_collect`. Also triggered when a parametrize generator function yields nothing.

Common situations: Data-driven tests where the data source is empty at runtime (e.g. a filtered list, a weekend schedule, a disabled feature flag), combined with strict collection policy. CI configs that set fail_at_collect to surface empty suites.

Related errors

AI-assisted analysis of pytest-dev/pytest@0d6fbdeffa (2026-08-11). Data as JSON: /api/errors/5de5e6b87c226eb8. Report an issue: GitHub.

Appendix: source

Thrown at src/_pytest/mark/structures.py:76


def get_empty_parameterset_mark(
    config: Config, argnames: Sequence[str], func
) -> MarkDecorator:
    from ..nodes import Collector

    argslisting = ", ".join(argnames)

    _fs, lineno = getfslineno(func)
    reason = f"got empty parameter set for ({argslisting})"
    requested_mark: _EmptyParameterSetMark = config.getini(EMPTY_PARAMETERSET_OPTION)
    match requested_mark:
        case "skip":
            return MARK_GEN.skip(reason=reason)
        case "xfail":
            return MARK_GEN.xfail(reason=reason, run=False)
        case "fail_at_collect":
            raise Collector.CollectError(
                f"Empty parameter set in '{func.__name__}' at line {lineno + 1}"
            )
        case unreachable:
            assert_never(unreachable)


class ParameterSet(NamedTuple):
    """A set of values for a set of parameters along with associated marks and
    an optional ID for the set.

    Examples::

        pytest.param(1, 2, 3)
        # ParameterSet(values=(1, 2, 3), marks=(), id=None)

        pytest.param("hello", id="greeting")
        # ParameterSet(values=("hello",), marks=(), id="greeting")

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