Implement Path.__deepcopy__ avoiding infinite recursion

To deep copy an object without calling deepcopy on the object itself, create a new object of the correct class and iterate calling deepcopy on its __dict__. Closes #29157 without relying on private CPython methods. Does not fix the other issue with TransformNode.__copy__. Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
tacaswell · Jun 26, 2025 · Jun 21, 2025 · Jun 24, 2025 · Jun 21, 2025 · Jun 25, 2025
commit c7edce3820d918474fadb42e55818d1c40be1979
diff --git a/lib/matplotlib/path.py b/lib/matplotlib/path.py
@@ -275,17 +275,37 @@ def copy(self):
         """
         return copy.copy(self)
 
-    def __deepcopy__(self, memo=None):
+    def __deepcopy__(self, memo):
         """
         Return a deepcopy of the `Path`.  The `Path` will not be
         readonly, even if the source `Path` is.
         """
         # Deepcopying arrays (vertices, codes) strips the writeable=False flag.
-        p = copy.deepcopy(super(), memo)
+        cls = type(self)
+        memo[id(self)] = p = cls.__new__(cls)
+
+        for k, v in self.__dict__.items():
+            setattr(p, k, copy.deepcopy(v, memo))
+
         p._readonly = False
         return p
 
-    deepcopy = __deepcopy__
+    def deepcopy(self, memo=None):
+        """
+        Return a deep copy of the `Path`.  The `Path` will not be readonly,
+        even if the source `Path` is.
+
+        Parameters
+        ----------
+        memo : dict, optional
+            A dictionary to use for memoizing, passed to `copy.deepcopy`.
+
+        Returns
+        -------
+        Path
+            A deep copy of the `Path`, but not readonly.
+        """
+        return copy.deepcopy(self, memo)
 
     @classmethod
     def make_compound_path_from_polys(cls, XY):

diff --git a/lib/matplotlib/path.pyi b/lib/matplotlib/path.pyi
@@ -44,8 +44,8 @@ class Path:
     @property
     def readonly(self) -> bool: ...
     def copy(self) -> Path: ...
-    def __deepcopy__(self, memo: dict[int, Any] | None = ...) -> Path: ...
-    deepcopy = __deepcopy__
+    def __deepcopy__(self, memo: dict[int, Any]) -> Path: ...
+    def deepcopy(self, memo: dict[int, Any] | None = None) -> Path: ...
 
     @classmethod
     def make_compound_path_from_polys(cls, XY: ArrayLike) -> Path: ...

diff --git a/lib/matplotlib/tests/test_path.py b/lib/matplotlib/tests/test_path.py
@@ -355,15 +355,49 @@ def test_path_deepcopy():
     # Should not raise any error
     verts = [[0, 0], [1, 1]]
     codes = [Path.MOVETO, Path.LINETO]
-    path1 = Path(verts)
-    path2 = Path(verts, codes)
+    path1 = Path(verts, readonly=True)
+    path2 = Path(verts, codes, readonly=True)
     path1_copy = path1.deepcopy()
     path2_copy = path2.deepcopy()
     assert path1 is not path1_copy
     assert path1.vertices is not path1_copy.vertices
+    assert np.all(path1.vertices == path1_copy.vertices)
+    assert path1.readonly
+    assert not path1_copy.readonly
     assert path2 is not path2_copy
     assert path2.vertices is not path2_copy.vertices
+    assert np.all(path2.vertices == path2_copy.vertices)
     assert path2.codes is not path2_copy.codes
+    assert all(path2.codes == path2_copy.codes)
+    assert path2.readonly
+    assert not path2_copy.readonly
+
+
+def test_path_deepcopy_cycle():
+    class PathWithCycle(Path):
+        def __init__(self, *args, **kwargs):
+            super().__init__(*args, **kwargs)
+            self.x = self
+
+    p = PathWithCycle([[0, 0], [1, 1]], readonly=True)
+    p_copy = p.deepcopy()
+    assert p_copy is not p
+    assert p.readonly
+    assert not p_copy.readonly
+    assert p_copy.x is p_copy
+
+    class PathWithCycle2(Path):
+        def __init__(self, *args, **kwargs):
+            super().__init__(*args, **kwargs)
+            self.x = [self] * 2
+
+    p2 = PathWithCycle2([[0, 0], [1, 1]], readonly=True)
+    p2_copy = p2.deepcopy()
+    assert p2_copy is not p2
+    assert p2.readonly
+    assert not p2_copy.readonly
+    assert p2_copy.x[0] is p2_copy
+    assert p2_copy.x[1] is p2_copy
 
 
 def test_path_shallowcopy():