MNT: improve the error message in Path init

tacaswell · tacaswell · commit b8ffb8c10c5c · 2020-05-13T20:23:09.000-04:00
diff --git a/lib/matplotlib/path.py b/lib/matplotlib/path.py
@@ -131,16 +131,20 @@ def __init__(self, vertices, codes=None, _interpolation_steps=1,
         vertices = _to_unmasked_float_array(vertices)
         if vertices.ndim != 2 or vertices.shape[1] != 2:
             raise ValueError(
-                "'vertices' must be a 2D list or array with shape Nx2")
+                "'vertices' must be a 2D list or array with shape (N, 2). "
+                f"Your input has shape {vertices.shape}.")
 
         if codes is not None:
             codes = np.asarray(codes, self.code_type)
             if codes.ndim != 1 or len(codes) != len(vertices):
                 raise ValueError("'codes' must be a 1D list or array with the "
-                                 "same length of 'vertices'")
+                                 "same length of 'vertices'. "
+                                 f"Your vertices have shape {vertices.shape} "
+                                 f"but your codes have shape {codes.shape}")
             if len(codes) and codes[0] != self.MOVETO:
                 raise ValueError("The first element of 'code' must be equal "
-                                 "to 'MOVETO' ({})".format(self.MOVETO))
+                                 f"to 'MOVETO' ({self.MOVETO}).  "
+                                 f"Your first code if {codes[0]}")
         elif closed and len(vertices):
             codes = np.empty(len(vertices), dtype=self.code_type)
             codes[0] = self.MOVETO
diff --git a/lib/matplotlib/tests/test_path.py b/lib/matplotlib/tests/test_path.py
@@ -1,5 +1,5 @@
 import copy
-
+import re
 import numpy as np
 
 from numpy.testing import assert_array_equal
@@ -30,6 +30,25 @@ def modify_vertices():
         modify_vertices()
 
 
+def test_path_exceptions():
+    bad_verts1 = np.arange(12).reshape(4, 3)
+    with pytest.raises(ValueError,
+                       match=re.escape(f'has shape {bad_verts1.shape}')):
+        Path(bad_verts1)
+
+    bad_verts2 = np.arange(12).reshape(2, 3, 2)
+    with pytest.raises(ValueError,
+                       match=re.escape(f'has shape {bad_verts2.shape}')):
+        Path(bad_verts2)
+
+    good_verts = np.arange(12).reshape(6, 2)
+    bad_codes = np.arange(2)
+    msg = re.escape(f"Your vertices have shape {good_verts.shape} "
+                    f"but your codes have shape {bad_codes.shape}")
+    with pytest.raises(ValueError, match=msg):
+        Path(good_verts, bad_codes)
+
+
 def test_point_in_path():
     # Test #1787
     verts2 = [(0, 0), (0, 1), (1, 1), (1, 0), (0, 0)]
