[Fix] Support trace-specific color sequences in Plotly Express via templates

plotly/express/_core.py

@@ -1003,7 +1003,7 @@ def one_group(x):
     return ""
 
 
-def apply_default_cascade(args, constructor=None):
+def apply_default_cascade(args, constructor):
     # first we apply px.defaults to unspecified args
 
     for param in defaults.__slots__:
@@ -1047,16 +1047,15 @@ def apply_default_cascade(args, constructor=None):
             else:
                 trace_type = constructor().type
             if trace_data_list := getattr(args["template"].data, trace_type, None):
-                args["color_discrete_sequence"] = [
+                trace_specific_colors = [
                     trace_data.marker.color
                     for trace_data in trace_data_list
                     if hasattr(trace_data, "marker")
                     and hasattr(trace_data.marker, "color")
                 ]
-                if not args["color_discrete_sequence"] or not any(
-                    args["color_discrete_sequence"]
-                ):
-                    args["color_discrete_sequence"] = None
+                # If template contains at least one color for this trace type, assign to color_discrete_sequence
+                if any(trace_specific_colors):
+                    args["color_discrete_sequence"] = trace_specific_colors
         # fallback to layout.colorway if trace-specific colors not available
         if args["color_discrete_sequence"] is None and args["template"].layout.colorway:
             args["color_discrete_sequence"] = args["template"].layout.colorway

tests/test_optional/test_px/test_px.py

@@ -1,6 +1,7 @@
 from itertools import permutations
 import warnings
 
+import pandas as pd
 import plotly.express as px
 import plotly.io as pio
 import narwhals.stable.v1 as nw
@@ -227,21 +228,23 @@ def test_px_templates(backend):
 
 
 def test_px_templates_trace_specific_colors(backend):
-    import pandas as pd
-
     tips = px.data.tips(return_type=backend)
 
     # trace-specific colors: each trace type uses its own template colors
     template = {
-        "data_histogram": [
-            {"marker": {"color": "orange"}},
-            {"marker": {"color": "purple"}},
-        ],
-        "data_bar": [
-            {"marker": {"color": "red"}},
-            {"marker": {"color": "blue"}},
-        ],
-        "layout_colorway": ["yellow", "green"],
+        "data": {
+            "histogram": [
+                {"marker": {"color": "orange"}},
+                {"marker": {"color": "purple"}},
+            ],
+            "bar": [
+                {"marker": {"color": "red"}},
+                {"marker": {"color": "blue"}},
+            ],
+        },
+        "layout": {
+            "colorway": ["yellow", "green"],
+        },
     }
     # histogram uses histogram colors
     fig = px.histogram(tips, x="total_bill", color="sex", template=template)