From d80c24ab6f65d0ca60b789d10d40cb6fc6739f84 Mon Sep 17 00:00:00 2001
From: Marco Gorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Mon, 12 Aug 2024 10:16:36 +0100
Subject: [PATCH] fix: show informative error message if a non-existent column
 name is passed

---
 altair/vegalite/v5/schema/channels.py |  4 ++--
 tests/utils/test_schemapi.py          | 11 +++++++++++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/altair/vegalite/v5/schema/channels.py b/altair/vegalite/v5/schema/channels.py
index 34daf00ab..bf0008e9b 100644
--- a/altair/vegalite/v5/schema/channels.py
+++ b/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."
diff --git a/tests/utils/test_schemapi.py b/tests/utils/test_schemapi.py
index acea9dcb6..4d741ca60 100644
--- a/tests/utils/test_schemapi.py
+++ b/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")},