fix: fall back to altair.Chart when specific chart type can't parse the spec

Specs with properties Altair can't deserialize (e.g., boxplot layers
with extra y2 encoding attributes) now gracefully degrade to
altair.Chart instead of raising TypeError.

Author: cpsievert

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 - Upgraded to ggsql Rust crate v0.3.2.
 - `render_altair()` and `VegaLiteWriter.render_chart()` now default to `validate=False` when creating Altair chart objects. This avoids `ValidationError`s for valid specs (e.g., boxplots) that use Vega-Lite features not yet reflected in Altair's schema. Pass `validate=True` to re-enable.
+- When Altair can't deserialize a spec into the expected chart subclass (e.g., `LayerChart`), the converter now falls back to `altair.Chart` instead of raising. The chart still displays correctly; only Altair-level round-tripping (`.to_dict()`) is lost.
 
 ## 0.3.1
 

python/ggsql/__init__.py

@@ -52,20 +52,27 @@ def _json_to_altair_chart(vegalite_json: str, **kwargs: Any) -> AltairChart:
     kwargs.setdefault("validate", False)
     spec = json.loads(vegalite_json)
 
+    chart_class: type[AltairChart] | None = None
     if "layer" in spec:
-        return altair.LayerChart.from_json(vegalite_json, **kwargs)
+        chart_class = altair.LayerChart
     elif "facet" in spec or "spec" in spec:
-        return altair.FacetChart.from_json(vegalite_json, **kwargs)
+        chart_class = altair.FacetChart
     elif "concat" in spec:
-        return altair.ConcatChart.from_json(vegalite_json, **kwargs)
+        chart_class = altair.ConcatChart
     elif "hconcat" in spec:
-        return altair.HConcatChart.from_json(vegalite_json, **kwargs)
+        chart_class = altair.HConcatChart
     elif "vconcat" in spec:
-        return altair.VConcatChart.from_json(vegalite_json, **kwargs)
+        chart_class = altair.VConcatChart
     elif "repeat" in spec:
-        return altair.RepeatChart.from_json(vegalite_json, **kwargs)
-    else:
-        return altair.Chart.from_json(vegalite_json, **kwargs)
+        chart_class = altair.RepeatChart
+
+    if chart_class is not None:
+        try:
+            return chart_class.from_json(vegalite_json, **kwargs)
+        except Exception:
+            pass
+
+    return altair.Chart.from_json(vegalite_json, **kwargs)
 
 
 class VegaLiteWriter:

tests/test_ggsql.py

@@ -351,6 +351,49 @@ def test_chart_with_color_encoding(self):
         assert isinstance(chart, altair.LayerChart)
 
 
+class TestAltairChartFallback:
+    """Tests for fallback to Chart when a specific Altair type can't parse the spec."""
+
+    def test_layer_fallback_to_chart(self):
+        """Specs with 'layer' fall back to Chart when LayerChart.from_json fails."""
+        from ggsql import _json_to_altair_chart
+
+        spec = json.dumps(
+            {
+                "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
+                "data": {"values": [{"x": 1, "y": 2}]},
+                "layer": [
+                    {
+                        "mark": "point",
+                        "encoding": {"x": {"field": "x", "type": "quantitative"}},
+                        "not_a_real_property": True,
+                    }
+                ],
+            }
+        )
+        chart = _json_to_altair_chart(spec)
+        assert isinstance(chart, altair.Chart)
+
+    def test_layer_uses_specific_type_when_possible(self):
+        """Specs with 'layer' use LayerChart when parsing succeeds."""
+        from ggsql import _json_to_altair_chart
+
+        spec = json.dumps(
+            {
+                "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
+                "data": {"values": [{"x": 1, "y": 2}]},
+                "layer": [
+                    {
+                        "mark": "point",
+                        "encoding": {"x": {"field": "x", "type": "quantitative"}},
+                    }
+                ],
+            }
+        )
+        chart = _json_to_altair_chart(spec)
+        assert isinstance(chart, altair.LayerChart)
+
+
 class TestRenderAltairErrorHandling:
     """Tests for error handling in render_altair()."""