fix: handle empty px.histogram() by skipping None label in hover template#5535
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emilykl merged 1 commit intoplotly:mainfrom Mar 10, 2026
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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 plotly#5534
emilykl
approved these changes
Mar 10, 2026
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Thanks for the fix @tysoncung !
That whole massive if statement inside make_trace_kwargs() is a bit messy. This is a perfectly reasonable fix and addresses the issue.
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Summary
px.histogram()crashes withTypeError: unsupported operand type(s) for +: 'NoneType' and 'str'when called with no data arguments, while other chart types likepx.scatter(),px.bar(), andpx.pie()handle empty invocations correctly.Root Cause
The histogram-specific hover label path in
_core.pyunconditionally adds an entry tomapping_labelseven when no column is specified. When no data is provided,attr_labelisNone(fromget_decorated_label(args, None, 'x')), resulting in a{None: '%{x}'}entry. The hover template list comprehension then fails:Fix
Skip adding to
mapping_labelswhenattr_labelisNone, consistent with how other chart types handle empty invocations (they never reach the mapping_labels code path since their attrs checkattr_value is not Nonefirst).Test
Added
test_empty_histogramto verifypx.histogram()returns a valid figure without raising.Fixes #5534