locustio/locust · error · ValueError

There are no users with weight > 0.

Error message

There are no users with weight > 0.

What it means

Environment filters out user classes whose weight is 0 (and fixed_count 0). If filtering removes every user class, no load can be generated, so it raises ValueError (noted in source as needing a better exception type).

Source

Thrown at locust/env.py:267

        else:
            exclude_tags = None

        for user_class in self.user_classes:
            filter_tasks_by_tags(user_class, tags, exclude_tags)

    def _remove_user_classes_with_weight_zero(self) -> None:
        """
        Remove user classes having a weight of zero.
        """
        if len(self.user_classes) == 0:
            # Preserve previous behaviour that allowed no user classes to be specified.
            return
        filtered_user_classes = [
            user_class for user_class in self.user_classes if user_class.weight > 0 or user_class.fixed_count > 0
        ]
        if len(filtered_user_classes) == 0:
            # TODO: Better exception than `ValueError`?
            raise ValueError("There are no users with weight > 0.")
        self.user_classes[:] = filtered_user_classes

    def assign_equal_weights(self) -> None:
        """
        Update the user classes such that each user runs their specified tasks with equal
        probability.
        """
        for u in self.user_classes:
            u.weight = 1
            user_tasks: list[TaskSet | Callable] = []
            tasks_frontier = u.tasks
            while len(tasks_frontier) != 0:
                t = tasks_frontier.pop()
                if isinstance(t, TaskHolder):
                    tasks_frontier.extend(t.tasks)
                elif callable(t):
                    if t not in user_tasks:
                        user_tasks.append(t)

View on GitHub (pinned to f391a716e1)

Solutions

  1. Give at least one user class a weight > 0 (or a fixed_count > 0)
  2. If a class should be excluded, keep another active class in the list
  3. Validate weights before constructing Environment

Example fix

// before
env = Environment(user_classes=[UserA])  # UserA.weight = 0
// after
UserA.weight = 1
env = Environment(user_classes=[UserA])
Defensive patterns

Strategy: validation

Validate before calling

classes = [UserA, UserB]
assert any(getattr(c, "weight", 1) > 0 or getattr(c, "fixed_count", 0) > 0 for c in classes), "need at least one user with weight > 0"
env = Environment(user_classes=classes)

Try / catch

try:
    env = Environment(user_classes=classes)
except ValueError as e:
    if "no users with weight" in str(e):
        raise RuntimeError("Enable at least one user class (weight > 0)") from e
    raise

Prevention

When it happens

Trigger: Setting `weight = 0` on all user classes passed to Environment(user_classes=[...]); passing an empty list of user classes where all get filtered; dynamically zeroing weights via config to 'disable' the only user class.

Common situations: Driving user weights from a config file where all weights resolve to 0; disabling a single-class test by setting weight 0; typos in config keys so weight defaults are lost.

Related errors

AI-assisted analysis of locustio/locust@f391a716e1 (2026-08-29). Data as JSON: /api/errors/19a13f3238b93ece. Report an issue: GitHub.