From c10fb9c6862ca45bae05c6ba66782ea0d030df99 Mon Sep 17 00:00:00 2001
From: Tyson Cung <tyson.cung@gmail.com>
Date: Tue, 10 Mar 2026 17:20:22 +0000
Subject: [PATCH] fix: handle empty histogram() by skipping None label in hover
 template

When px.histogram() is called with no data arguments, the histogram-
specific hover label path inserts a None key into mapping_labels
(because attr_label is None when no column is specified). The hover
template list comprehension then fails with:

    TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'

Skip adding to mapping_labels when attr_label is None, consistent
with how other chart types handle empty invocations.

Fixes #5534
---
 plotly/express/_core.py                          |  3 ++-
 tests/test_optional/test_px/test_px_functions.py | 10 ++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/plotly/express/_core.py b/plotly/express/_core.py
index 9fbf526cdeb..2756346c55e 100644
--- a/plotly/express/_core.py
+++ b/plotly/express/_core.py
@@ -588,7 +588,8 @@ def make_trace_kwargs(args, trace_spec, trace_data, mapping_labels, sizeref):
             and attr_name == "z"
         ):
             # ensure that stuff like "count" gets into the hoverlabel
-            mapping_labels[attr_label] = "%%{%s}" % attr_name
+            if attr_label is not None:
+                mapping_labels[attr_label] = "%%{%s}" % attr_name
     if trace_spec.constructor not in [go.Parcoords, go.Parcats]:
         # Modify mapping_labels according to hover_data keys
         # if hover_data is a dict
diff --git a/tests/test_optional/test_px/test_px_functions.py b/tests/test_optional/test_px/test_px_functions.py
index 0814898f89d..2822a931fd7 100644
--- a/tests/test_optional/test_px/test_px_functions.py
+++ b/tests/test_optional/test_px/test_px_functions.py
@@ -619,3 +619,13 @@ def test_timeline_cols_already_temporal(constructor, datetime_columns):
     assert len(fig.data) == 3
     assert fig.layout.xaxis.type == "date"
     assert fig.layout.xaxis.title.text is None
+
+
+def test_empty_histogram():
+    """Empty px.histogram() should not raise, matching scatter/bar/pie behavior.
+
+    Regression test for https://github.com/plotly/plotly.py/issues/5534
+    """
+    fig = px.histogram()
+    assert len(fig.data) == 1
+    assert fig.data[0].type == "histogram"