Display more helpful error message when two fields strings are used i…

…n condition (#2979) * Raise helpful error if a field is used in both branches of a condition * Add test * Add changelog entry * Blacken * Blacken more * Improve error message wording * Apply suggestions from code review Co-authored-by: Mattijn van Hoek <[email protected]> --------- Co-authored-by: Mattijn van Hoek <[email protected]>
vega · Mar 26, 2023 · b27b829 · b27b829
1 parent 1317f6d
commit b27b829
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Showing 3 changed files with 22 additions and 2 deletions.
diff --git a/altair/vegalite/v5/api.py b/altair/vegalite/v5/api.py
@@ -773,8 +773,13 @@ def condition(predicate, if_true, if_false, **kwargs):
         # dict in the appropriate schema
         if_true = if_true.to_dict()
     elif isinstance(if_true, str):
-        if_true = utils.parse_shorthand(if_true)
-        if_true.update(kwargs)
+        if isinstance(if_false, str):
+            raise ValueError(
+                "A field cannot be used for both the `if_true` and `if_false` values of a condition. One of them has to specify a `value` or `datum` definition."
+            )
+        else:
+            if_true = utils.parse_shorthand(if_true)
+            if_true.update(kwargs)
     condition.update(if_true)
 
     if isinstance(if_false, core.SchemaBase):

diff --git a/doc/releases/changes.rst b/doc/releases/changes.rst
@@ -24,6 +24,7 @@ Enhancements
 - Substantially improved error handling. Both in terms of finding the more relevant error (#2842), and in terms of improving the formatting and clarity of the error messages (#2824, #2568).
 - Include experimental support for the DataFrame Interchange Protocol (through ``__dataframe__`` attribute). This requires ``pyarrow>=11.0.0`` (#2888).
 - Support data type inference for columns with special characters (#2905).
+- Changed error message to be more informative when two field strings are using in a condition (#2979).
 
 Grammar Changes
 ~~~~~~~~~~~~~~~

diff --git a/tests/utils/test_schemapi.py b/tests/utils/test_schemapi.py
@@ -610,6 +610,20 @@ def test_chart_validation_errors(chart_func, expected_error_message):
         chart.to_dict()
 
 
+def test_multiple_field_strings_in_condition():
+    selection = alt.selection_point()
+    expected_error_message = "A field cannot be used for both the `if_true` and `if_false` values of a condition. One of them has to specify a `value` or `datum` definition."
+    with pytest.raises(ValueError, match=expected_error_message):
+        (
+            alt.Chart(data.cars())
+            .mark_circle()
+            .add_params(selection)
+            .encode(
+                color=alt.condition(selection, "Origin", "Origin"),
+            )
+        ).to_dict()
+
+
 def test_serialize_numpy_types():
     m = MySchema(
         a={"date": np.datetime64("2019-01-01")},