Include close matches when key not found

dstansby · dstansby · commit 3f98b36395ac · 2025-05-03T18:20:35.000+01:00
diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
@@ -740,12 +740,11 @@ def __setitem__(self, key, val):
                 and val is rcsetup._auto_backend_sentinel
                 and "backend" in self):
             return
+        valid_key = _api.check_getitem(
+            self.validate, rcParam=key, _suggest_close_matches=True, _error_cls=KeyError
+        )
         try:
-            cval = self.validate[key](val)
-        except KeyError as err:
-            raise KeyError(
-                f"{key} is not a valid rc parameter (see rcParams.keys() for "
-                f"a list of valid parameters)") from err
+            cval = valid_key(val)
         except ValueError as ve:
             raise ValueError(f"Key {key}: {ve}") from None
         self._set(key, cval)
diff --git a/lib/matplotlib/_api/__init__.py b/lib/matplotlib/_api/__init__.py
@@ -10,6 +10,7 @@
 
 """
 
+import difflib
 import functools
 import itertools
 import pathlib
@@ -174,12 +175,19 @@ def check_shape(shape, /, **kwargs):
             )
 
 
-def check_getitem(mapping, /, **kwargs):
+def check_getitem(
+        mapping, /, _suggest_close_matches=False, _error_cls=ValueError, **kwargs
+    ):
     """
     *kwargs* must consist of a single *key, value* pair.  If *key* is in
     *mapping*, return ``mapping[value]``; else, raise an appropriate
     ValueError.
 
+    Parameters
+    ----------
+    _suggest_close_matches
+        If True, suggest only close matches instead of all valid values.
+
     Examples
     --------
     >>> _api.check_getitem({"foo": "bar"}, arg=arg)
@@ -190,9 +198,14 @@ def check_getitem(mapping, /, **kwargs):
     try:
         return mapping[v]
     except KeyError:
-        raise ValueError(
-            f"{v!r} is not a valid value for {k}; supported values are "
-            f"{', '.join(map(repr, mapping))}") from None
+        if _suggest_close_matches:
+            if len(best := difflib.get_close_matches(v, mapping.keys(), cutoff=0.5)):
+                suggestion = f"Did you mean one of {best}?"
+            else:
+                suggestion = ""
+        else:
+            suggestion = f"Supported values are {', '.join(map(repr, mapping))}"
+        raise _error_cls(f"{v!r} is not a valid value for {k}. {suggestion}") from None
 
 
 def caching_module_getattr(cls):
diff --git a/lib/matplotlib/cm.py b/lib/matplotlib/cm.py
@@ -92,10 +92,10 @@ def __init__(self, cmaps):
         self._builtin_cmaps = tuple(cmaps)
 
     def __getitem__(self, item):
-        try:
-            return self._cmaps[item].copy()
-        except KeyError:
-            raise KeyError(f"{item!r} is not a known colormap name") from None
+        cmap = _api.check_getitem(
+            self._cmaps, colormap=item, _suggest_close_matches=True, _error_cls=KeyError
+        )
+        return cmap.copy()
 
     def __iter__(self):
         return iter(self._cmaps)
diff --git a/lib/matplotlib/tests/test_colors.py b/lib/matplotlib/tests/test_colors.py
@@ -1828,3 +1828,13 @@ def test_LinearSegmentedColormap_from_list_value_color_tuple():
         cmap([value for value, _ in value_color_tuples]),
         to_rgba_array([color for _, color in value_color_tuples]),
     )
+
+
+def test_close_error_name():
+    with pytest.raises(
+        KeyError,
+        match=(
+            "'grays' is not a valid value for colormap. "
+            "Did you mean one of ['gray', 'Grays', 'gray_r']?"
+        )):
+        matplotlib.colormaps["grays"]
diff --git a/lib/matplotlib/tests/test_style.py b/lib/matplotlib/tests/test_style.py
@@ -140,7 +140,9 @@ def test_context_with_badparam():
     with style.context({PARAM: other_value}):
         assert mpl.rcParams[PARAM] == other_value
         x = style.context({PARAM: original_value, 'badparam': None})
-        with pytest.raises(KeyError):
+        with pytest.raises(
+            KeyError, match="\'badparam\' is not a valid value for rcParam. "
+        ):
             with x:
                 pass
         assert mpl.rcParams[PARAM] == other_value
