fix: show informative error message if a non-existent column name is …

…passed

altair/vegalite/v5/schema/channels.py

@@ -14,7 +14,7 @@
 from typing import TYPE_CHECKING, Any, Literal, Sequence, TypedDict, Union, overload
 from typing_extensions import TypeAlias
 
-from narwhals.dependencies import is_pandas_dataframe as _is_pandas_dataframe
+import narwhals.stable.v1 as nw
 
 from altair.utils import infer_encoding_types as _infer_encoding_types
 from altair.utils import parse_shorthand
@@ -179,7 +179,7 @@ def to_dict(
                 # We still parse it out of the shorthand, but drop it here.
                 parsed.pop("type", None)
             elif not (type_in_shorthand or type_defined_explicitly):
-                if _is_pandas_dataframe(context.get("data", None)):
+                if isinstance(context.get("data", None), nw.DataFrame):
                     msg = (
                         f'Unable to determine data type for the field "{shorthand}";'
                         " verify that the field name is not misspelled."

tests/utils/test_schemapi.py

@@ -881,6 +881,17 @@ def test_multiple_field_strings_in_condition():
         )
 
 
+def test_non_existent_column_name():
+    df = pd.DataFrame({"a": [1, 2], "b": [4, 5]})
+    msg = (
+        'Unable to determine data type for the field "c"; verify that the field name '
+        "is not misspelled. If you are referencing a field from a transform, also "
+        "confirm that the data type is specified correctly."
+    )
+    with pytest.raises(ValueError, match=msg):
+        alt.Chart(df).mark_line().encode(x="a", y="c").to_json()
+
+
 def test_serialize_numpy_types():
     m = MySchema(
         a={"date": np.datetime64("2019-01-01")},