matplotlib · dstansby · Jul 17, 2020 · Jul 15, 2020 · Jul 15, 2020
diff --git a/doc/api/next_api_changes/deprecations/17926-ES.rst b/doc/api/next_api_changes/deprecations/17926-ES.rst
@@ -0,0 +1,5 @@
+Invalid hatch pattern characters are no longer ignored
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When specifying hatching patterns, characters that are not recognized will
+raise a DeprecationWarning. In the future, this will become a hard error.
diff --git a/lib/matplotlib/collections.py b/lib/matplotlib/collections.py
@@ -15,7 +15,7 @@
 
 import matplotlib as mpl
 from . import (_path, artist, cbook, cm, colors as mcolors, docstring,
-               lines as mlines, path as mpath, transforms)
+               hatch as mhatch, lines as mlines, path as mpath, transforms)
 import warnings
 
 
@@ -141,8 +141,8 @@ def __init__(self,
         hatch : str, optional
             Hatching pattern to use in filled paths, if any. Valid strings are
             ['/', '\\', '|', '-', '+', 'x', 'o', 'O', '.', '*']. See
-            :doc:`/gallery/shapes_and_collections/hatch_demo` for the meaning
-            of each hatch type.
+            :doc:`/gallery/shapes_and_collections/hatch_style_reference` for
+            the meaning of each hatch type.
         pickradius : float, default: 5.0
             If ``pickradius <= 0``, then `.Collection.contains` will return
             ``True`` whenever the test point is inside of one of the polygons
@@ -522,6 +522,8 @@ def set_hatch(self, hatch):
         ----------
         hatch : {'/', '\\', '|', '-', '+', 'x', 'o', 'O', '.', '*'}
         """
+        # Use validate_hatch(list) after deprecation.
+        mhatch._validate_hatch_pattern(hatch)
         self._hatch = hatch
         self.stale = True
 

diff --git a/lib/matplotlib/hatch.py b/lib/matplotlib/hatch.py
@@ -1,6 +1,8 @@
 """Contains classes for generating hatch patterns."""
 
 import numpy as np
+
+from matplotlib import cbook
 from matplotlib.path import Path
 
 
@@ -181,6 +183,22 @@ def __init__(self, hatch, density):
     ]
 
 
+def _validate_hatch_pattern(hatch):
+    valid_hatch_patterns = set(r'-+|/\xXoO.*')
+    if hatch is not None:
+        invalids = set(hatch).difference(valid_hatch_patterns)
+        if invalids:
+            valid = ''.join(sorted(valid_hatch_patterns))
+            invalids = ''.join(sorted(invalids))
+            cbook.warn_deprecated(
+                '3.4',
+                message=f'hatch must consist of a string of "{valid}" or '
+                        'None, but found the following invalid values '
+                        f'"{invalids}". Passing invalid values is deprecated '
+                        'since %(since)s and will become an error %(removal)s.'
+            )
+
+
 def get_path(hatchpattern, density=6):
     """
     Given a hatch specifier, *hatchpattern*, generates Path to render

diff --git a/lib/matplotlib/patches.py b/lib/matplotlib/patches.py
@@ -8,7 +8,8 @@
 import numpy as np
 
 import matplotlib as mpl
-from . import artist, cbook, colors, docstring, lines as mlines, transforms
+from . import (artist, cbook, colors, docstring, hatch as mhatch,
+               lines as mlines, transforms)
 from .bezier import (
     NonIntersectingPathException, get_cos_sin, get_intersection,
     get_parallels, inside_circle, make_wedged_bezier2,
@@ -513,6 +514,8 @@ def set_hatch(self, hatch):
         ----------
         hatch : {'/', '\\', '|', '-', '+', 'x', 'o', 'O', '.', '*'}
         """
+        # Use validate_hatch(list) after deprecation.
+        mhatch._validate_hatch_pattern(hatch)
         self._hatch = hatch
         self.stale = True