matplotlib · BrunoWolf03 · Jun 4, 2025 · Jun 4, 2025 · Jun 4, 2025 · rcomer
diff --git a/lib/matplotlib/collections.py b/lib/matplotlib/collections.py
@@ -1,3 +1,3 @@
 """
 Classes for the efficient drawing of large collections of objects that
 share most properties, e.g., a large number of line segments or
@@ -1132,6 +1132,45 @@
     def get_paths(self):
         return self._paths
 
+    def check_consistency(self):
+        """
+        Emit warnings if the lengths of certain properties do not match
+        the number of paths.
+
+        This method checks whether the lengths of `sizes` and `edgecolors`
+        are either 1 or equal to the number of paths in the collection.
+        If not, a warning is issued.
+
+        This is useful for identifying potential bugs or silent mismatches
+        in collection properties when plotting, especially when properties
+        are expected to be applied per-path.
+
+        Notes
+        -----
+        - This method does not raise an error, only a warning.
+        - It is recommended to call this at draw-time to ensure properties
+        have been fully set.
+
+        Warnings
+        --------
+        UserWarning
+            If the number of sizes or edgecolors does not match the number
+            of paths and is not a singleton (length 1).
+
+        Examples
+        --------
+        >>> paths = [Path(...), Path(...), Path(...)]
+        >>> pc = PathCollection(paths, sizes=[10, 20])  # 3 paths, 2 sizes
+        >>> pc.check_consistency()
+        UserWarning: Number of sizes does not match number of paths.
+        """
+        n_paths = len(self.get_paths())
+        if self._sizes is not None and len(self._sizes) not in (1, n_paths):
-        if self._sizes is not None and len(self._sizes) not in (1, n_paths):
+        if self._sizes.size not in (0, 1, n_paths):
-        if self._sizes is not None and len(self._sizes) not in (1, n_paths):
+        if self._sizes.size not in (0, 1, n_paths):
+            warnings.warn("Number of sizes does not match number of paths.")
+        if self._edgecolors is not None and len(self._edgecolors) not in (1, n_paths):
+            warnings.warn("Number of edgecolors does not match number of paths.")
+
+
     def legend_elements(self, prop="colors", num="auto",
                         fmt=None, func=lambda x: x, **kwargs):
         """
@@ -1272,6 +1311,21 @@
 
         return handles, labels
 
+    def draw(self, renderer):
+        """
+        Draw the collection using the given renderer.
+
+        This override adds consistency checks before delegating
+        to the base implementation.
+
+        Parameters
+        ----------
+        renderer : RendererBase instance
+        The renderer to use for drawing.
+        """
+        self.check_consistency()
+        return super().draw(renderer)
+
 
 class PolyCollection(_CollectionWithSizes):
 

diff --git a/lib/matplotlib/collections.pyi b/lib/matplotlib/collections.pyi
@@ -86,6 +86,7 @@ class PathCollection(_CollectionWithSizes):
     def __init__(
         self, paths: Sequence[Path], sizes: ArrayLike | None = ..., **kwargs
     ) -> None: ...
+    def check_consistency(self) -> None: ...
     def set_paths(self, paths: Sequence[Path]) -> None: ...
     def get_paths(self) -> Sequence[Path]: ...
     def legend_elements(

diff --git a/lib/matplotlib/tests/test_collections.py b/lib/matplotlib/tests/test_collections.py
@@ -16,9 +16,10 @@
 import matplotlib.path as mpath
 import matplotlib.transforms as mtransforms
 from matplotlib.collections import (Collection, LineCollection,
-                                    EventCollection, PolyCollection)
+                                    EventCollection, PolyCollection, PathCollection)
 from matplotlib.collections import FillBetweenPolyCollection
 from matplotlib.testing.decorators import check_figures_equal, image_comparison
+from matplotlib.path import Path
 
 
 @pytest.fixture(params=["pcolormesh", "pcolor"])
@@ -1533,3 +1534,13 @@ def mock_draw_path_collection(self, gc, master_transform, paths, all_transforms,
     ax.add_collection(coll)
 
     plt.draw()
+
+
+def test_pathcollection_size_mismatch_warning():
+
+    paths = [Path([(0, 0), (1, 0), (0.5, 1)]) for _ in range(5)]
+    fig, ax = plt.subplots()
+    with pytest.warns(UserWarning, match="Number of sizes does not match"):
+        pc = PathCollection(paths, sizes=[10, 20])
+        ax.add_collection(pc)
+        fig.canvas.draw()