open-mmlab/mmdetection · error · RuntimeError

lap is not installed, please install it by…

Error message

lap is not installed,                 please install it by: pip install lap

What it means

BYTETracker associates detections to tracks with linear assignment provided by the optional 'lap' package. If the lap import failed, constructing ByteTrack raises RuntimeError.

Solutions

  1. pip install lap
  2. If the source build fails on Windows, install a prebuilt wheel: pip install lap -f https://github.com/gatagat/lap/releases or use conda
  3. Verify with python -c 'import lap' in the same interpreter as training

Example fix

# before: RuntimeError: lap is not installed
# after
pip install lap
Defensive patterns

Strategy: validation

Validate before calling

import importlib.util
if importlib.util.find_spec('lap') is None:
    raise SystemExit('pip install lap')

Try / catch

try:
    tracker = ByteTracker(...)
except RuntimeError as e:
    if 'lap' in str(e):
        raise SystemExit('Missing dependency: pip install lap')
    raise

Prevention

When it happens

Trigger: Building BYTETracker (tracker=dict(type='ByteTrack', ...)) in an environment where 'import lap' fails or lap is not installed.

Common situations: Running MOT configs (bytetrack_yolox_x_8xb4-80e_mot17half...) after a base mmdet install; lap needs a C compiler and can fail to build on Windows or older pip.

Related errors

AI-assisted analysis of open-mmlab/mmdetection@cfd5d3a985 (2026-08-27). Data as JSON: /api/errors/fb054e977871fe0b. Report an issue: GitHub.

Appendix: source

Thrown at mmdet/models/trackers/byte_tracker.py:53

            - low (float): Threshold of the second matching. Defaults to 0.5.
            - tentative (float): Threshold of the matching for tentative
                tracklets. Defaults to 0.3.
        num_tentatives (int, optional): Number of continuous frames to confirm
            a track. Defaults to 3.
    """

    def __init__(self,
                 motion: Optional[dict] = None,
                 obj_score_thrs: dict = dict(high=0.6, low=0.1),
                 init_track_thr: float = 0.7,
                 weight_iou_with_det_scores: bool = True,
                 match_iou_thrs: dict = dict(high=0.1, low=0.5, tentative=0.3),
                 num_tentatives: int = 3,
                 **kwargs):
        super().__init__(**kwargs)

        if lap is None:
            raise RuntimeError('lap is not installed,\
                 please install it by: pip install lap')
        if motion is not None:
            self.motion = TASK_UTILS.build(motion)

        self.obj_score_thrs = obj_score_thrs
        self.init_track_thr = init_track_thr

        self.weight_iou_with_det_scores = weight_iou_with_det_scores
        self.match_iou_thrs = match_iou_thrs

        self.num_tentatives = num_tentatives

    @property
    def confirmed_ids(self) -> List:
        """Confirmed ids in the tracker."""
        ids = [id for id, track in self.tracks.items() if not track.tentative]
        return ids

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