Include close matches when key not found

dstansby · dstansby · commit 43bdfcec7100 · 2025-05-02T21:05:41.000+01:00
diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
@@ -740,12 +740,16 @@ def __setitem__(self, key, val):
                 and val is rcsetup._auto_backend_sentinel
                 and "backend" in self):
             return
+
+        _api.check_in_keys(
+            key,
+            self.validate,
+            error_string=("{value} is not a valid rc parameter. "
+                          "See rcParams.keys() for a list of valid parameters. "
+                          "{matches_str}")
+        )
         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
         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
@@ -139,6 +140,35 @@ def check_in_list(values, /, *, _print_supported_values=True, **kwargs):
             raise ValueError(msg)
 
 
+def check_in_keys(value, mappable, *,
+                  error_string="{value!r} not found. {matches_str}"):
+    """
+    Check if *value* is in the keys of *mappable*.
+
+    Parameters
+    ----------
+    value :
+        Value to check for in keys of *mappable*.
+    mappable :
+        A mappable object with keys.
+    error_string:
+        String to use to format error.
+        Must contain fields for *value* and *matches_str*.
+
+    Raises
+    ------
+    KeyError
+        If *value* is not in the keys of *mappable*.
+    """
+    valid = list(mappable)
+    if value not in valid:
+        matches_str = ""
+        if len(best := difflib.get_close_matches(value, valid, cutoff=.1)):
+            matches_str = f"Did you mean one of {best}?"
+        raise KeyError(error_string.format(value=value, matches_str=matches_str))
+
+
+
 def check_shape(shape, /, **kwargs):
     """
     For each *key, value* pair in *kwargs*, check that *value* has the shape *shape*;
diff --git a/lib/matplotlib/cm.py b/lib/matplotlib/cm.py
@@ -92,10 +92,12 @@ 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
+        _api.check_in_keys(
+            item,
+            self._cmaps,
+            error_string="{value!r} is not a known colormap name. {matches_str}"
+        )
+        return self._cmaps[item].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 +1828,14 @@ 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) as exinfo:
+        matplotlib.colormaps["grays"]
+
+    msg = exinfo.value.args[0]
+    assert msg == (
+        "'grays' is not a known colormap name. "
+        "Did you mean one of ['gray', 'Grays', 'gray_r']?"
+    )
