add legend title to px.pie

packages/python/plotly/plotly/express/_core.py

@@ -2146,6 +2146,9 @@ def make_figure(args, constructor, trace_patch=None, layout_patch=None):
     layout_patch["legend"] = dict(tracegroupgap=0)
     if trace_name_labels:
         layout_patch["legend"]["title_text"] = ", ".join(trace_name_labels)
+    elif "showlegend" in trace_patch.keys():
+        if trace_patch["showlegend"] and constructor == go.Pie:
+            layout_patch["legend"]["title_text"] = args["names"]
     if args["title"]:
         layout_patch["title_text"] = args["title"]
     elif args["template"].layout.margin.t is None:

packages/python/plotly/plotly/tests/test_optional/test_px/test_pie.py

@@ -0,0 +1,25 @@
+import plotly.express as px
+import plotly.graph_objects as go
+from numpy.testing import assert_equal
+import numpy as np
+import pandas as pd
+import pytest
+
+
+def _compare_legend_to_column_name(name_column, name_from_fig):
+    """Compare legend title name to the name given to it
+    in "names" value to see if is equal to it and not None.
+    """
+    assert_equal(name_from_fig, name_column)
+    assert name_from_fig
+
+
+def test_pie_like_px():
+    # Pie
+    data_name = "col1"
+    names_name = "col2"
+    df = pd.DataFrame(data={data_name: [3, 2, 1], names_name: [1, 2, 4]})
+    fig = px.pie(
+        df, values=data_name, names=names_name, title="Population of European continent"
+    )
+    _compare_legend_to_column_name(names_name, fig.layout.legend.title.text)