matplotlib/matplotlib · error · ValueError

triangles are indices into the points and must be in the ran

Error message

triangles are indices into the points and must be in the range 0 <= i < {len(self.x)} but found value {self.triangles.max()}

What it means

Error "triangles are indices into the points and must be in the range 0 <= i < {len(self.x)} but found value {self.triangles.max()}" thrown in matplotlib/matplotlib.

Source

Thrown at lib/matplotlib/tri/_triangulation.py:75

        if triangles is None:
            # No triangulation specified, so use matplotlib._qhull to obtain
            # Delaunay triangulation.
            self.triangles, self._neighbors = _qhull.delaunay(x, y, sys.flags.verbose)
            self.is_delaunay = True
        else:
            # Triangulation specified. Copy, since we may correct triangle
            # orientation.
            try:
                self.triangles = np.array(triangles, dtype=np.int32, order='C')
            except ValueError as e:
                raise ValueError('triangles must be a (N, 3) int array, not '
                                 f'{triangles!r}') from e
            if self.triangles.ndim != 2 or self.triangles.shape[1] != 3:
                raise ValueError(
                    'triangles must be a (N, 3) int array, but found shape '
                    f'{self.triangles.shape!r}')
            if self.triangles.max() >= len(self.x):
                raise ValueError(
                    'triangles are indices into the points and must be in the '
                    f'range 0 <= i < {len(self.x)} but found value '
                    f'{self.triangles.max()}')
            if self.triangles.min() < 0:
                raise ValueError(
                    'triangles are indices into the points and must be in the '
                    f'range 0 <= i < {len(self.x)} but found value '
                    f'{self.triangles.min()}')

        # Underlying C++ object is not created until first needed.
        self._cpp_triangulation = None

        # Default TriFinder not created until needed.
        self._trifinder = None

        self.set_mask(mask)

    def calculate_plane_coefficients(self, z):

View on GitHub (pinned to b379c1b69e)

When it happens

Trigger: Thrown at lib/matplotlib/tri/_triangulation.py:75 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of matplotlib/matplotlib@b379c1b69e (2026-08-21). Data as JSON: /api/errors/bc3c89b0ef529df5. Report an issue: GitHub.