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

CHANGELOG.md

@@ -4,6 +4,9 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
 
+### Fixed
+- Fix issue where Plotly Express ignored trace-specific color sequences defined in templates via `template.data.<trace_type>` [[#5437](https://github.com/plotly/plotly.py/pull/5437)]
+
 ### Updated
 - Speed up `validate_gantt` function [[#5386](https://github.com/plotly/plotly.py/pull/5386)], with thanks to @misrasaurabh1 for the contribution!
 

plotly/express/_core.py

@@ -1003,7 +1003,7 @@ def one_group(x):
     return ""
 
 
-def apply_default_cascade(args):
+def apply_default_cascade(args, constructor=None):
     # first we apply px.defaults to unspecified args
 
     for param in defaults.__slots__:
@@ -1037,9 +1037,30 @@ def apply_default_cascade(args):
         if args["color_continuous_scale"] is None:
             args["color_continuous_scale"] = sequential.Viridis
 
+    # if color_discrete_sequence not set explicitly or in px.defaults,
+    # see if we can defer to template. Try trace-specific colors first,
+    # then layout.colorway, then set reasonable defaults
     if "color_discrete_sequence" in args:
+        if args["color_discrete_sequence"] is None and constructor is not None:
+            if constructor == "timeline":
+                trace_type = "bar"
+            else:
+                trace_type = constructor().type
+            if trace_data_list := getattr(args["template"].data, trace_type, None):
+                args["color_discrete_sequence"] = [
+                    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
+        # 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
+        # final fallback to default qualitative palette
         if args["color_discrete_sequence"] is None:
             args["color_discrete_sequence"] = qualitative.D3
 
@@ -2486,7 +2507,7 @@ def get_groups_and_orders(args, grouper):
 def make_figure(args, constructor, trace_patch=None, layout_patch=None):
     trace_patch = trace_patch or {}
     layout_patch = layout_patch or {}
-    apply_default_cascade(args)
+    apply_default_cascade(args, constructor=constructor)
 
     args = build_dataframe(args, constructor)
     if constructor in [go.Treemap, go.Sunburst, go.Icicle] and args["path"] is not None:

tests/test_optional/test_px/test_px.py

@@ -226,6 +226,52 @@ def test_px_templates(backend):
         pio.templates.default = "plotly"
 
 
+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"],
+    }
+    # histogram uses histogram colors
+    fig = px.histogram(tips, x="total_bill", color="sex", template=template)
+    assert fig.data[0].marker.color == "orange"
+    assert fig.data[1].marker.color == "purple"
+    # fallback to layout.colorway when trace-specific colors don't exist
+    fig = px.box(tips, x="day", y="total_bill", color="sex", template=template)
+    assert fig.data[0].marker.color == "yellow"
+    assert fig.data[1].marker.color == "green"
+    # timeline special case (maps to bar)
+    df_timeline = pd.DataFrame(
+        {
+            "Task": ["Job A", "Job B"],
+            "Start": ["2009-01-01", "2009-03-05"],
+            "Finish": ["2009-02-28", "2009-04-15"],
+            "Resource": ["Alex", "Max"],
+        }
+    )
+    fig = px.timeline(
+        df_timeline,
+        x_start="Start",
+        x_end="Finish",
+        y="Task",
+        color="Resource",
+        template=template,
+    )
+    assert fig.data[0].marker.color == "red"
+    assert fig.data[1].marker.color == "blue"
+
+
 def test_px_defaults():
     px.defaults.labels = dict(x="hey x")
     px.defaults.category_orders = dict(color=["b", "a"])