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Improving error message for width and position type mismatch in violinplot #30545

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Improving error message for width and position type mismatch in violi…
hasanrashid Sep 11, 2025
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Integrate tests in test_axes.py
hasanrashid Sep 30, 2025
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anntzer Sep 30, 2025
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Update lib/matplotlib/axes/_axes.py
hasanrashid Oct 13, 2025
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Merge branch 'matplotlib:main' into enh-30417
hasanrashid Oct 13, 2025
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Improving error message for width and position type mismatch in violi…
hasanrashid Sep 11, 2025
c5e3e0d
Integrate tests in test_axes.py
hasanrashid Sep 30, 2025
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Update lib/matplotlib/axes/_axes.py
hasanrashid Oct 13, 2025
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Merge branch 'enh-30417' of github.com:hasanrashid/matplotlib into en…
hasanrashid Oct 13, 2025
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Improving error message for width and position type mismatch in violi… 
…nplot
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@hasanrashid
hasanrashid committed Oct 13, 2025
commit 1e2bcae3e50a0b63950b017d4aea45593eeb1e7d
15 changes: 15 additions & 0 deletions lib/matplotlib/axes/_axes.py
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Original file line number Diff line number Diff line change
@@ -1,3 +1,4 @@
import datetime
import functools
import itertools
import logging
Expand Down Expand Up @@ -9047,6 +9048,20 @@ def violin(self, vpstats, positions=None, vert=None,
elif len(widths) != N:
raise ValueError(datashape_message.format("widths"))

# Proactive validation: if positions are datetime-like
# widths must be timedelta-like.
if any(isinstance(p, (datetime.datetime, datetime.date))
for p in positions):

@timhoffm timhoffm Sep 11, 2025

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Isn't it enough to check the first position? I think mixed positions are not allowed anyway. Also, do we need to handle np.datetime64 arrays?

@timhoffm timhoffm Sep 15, 2025

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@hasanrashid You've marked this as resolved. Please comment how my above questions are addressed.

@hasanrashid hasanrashid Sep 21, 2025

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@timhoffm Yes, it is enough to check the first position. The position argument can't mix datetime with float. width can't mix timedelta and float either.

I was approaching this by converting entries into numeric equivalent until it was suggested that I provide clearer message instead.

Also, yes, datetime64 isn't needed either. I was thinking of all test cases, but I realize datetime64 isn't a supported type in violinplot.

Making the adjustments

@timhoffm timhoffm Sep 21, 2025 •
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Why is datetime64 not supported? Should we rather fix this? (Can be later, but already add the check here?)

@hasanrashid hasanrashid Sep 28, 2025 •
edited by timhoffm
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@timhoffm

If I try a testcase like this:

def test_np_datetime64_positions_with_float_widths_behaves_like_numeric():
    """Test that np.datetime64 positions with float widths do not raise TypeError (treated as numeric)."""
    fig, ax = plt.subplots()
    try:
        vpstats = violin_plot_stats()
        positions = [np.datetime64('2020-01-01'), np.datetime64('2021-01-01')]
        widths = [0.5, 1.0]
        # Should not raise TypeError, but may raise downstream if not supported
        ax.violin(vpstats, positions=positions, widths=widths)
    finally:
        plt.close(fig)

This produces the following error:
numpy._core._exceptions._UFuncBinaryResolutionError: ufunc 'add' cannot use operands with types dtype('float64') and dtype('<M8[D]')

I took this to mean that datetime64 was not an expected type of datetime for biolinplots- thus unsupported.

If we need to investigate adding datetime64 support in violineplot, should it be another issue? We are updating the error message enforcing timedelta with datetimes; datetime64 support- to me- seems like it should be investigated in a separate issue (which I am happy to open and work on)

_widths = widths if not np.isscalar(widths) else [widths] * N
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if any(not isinstance(w, (datetime.timedelta, np.timedelta64))
for w in _widths):
raise TypeError(
"If positions are datetime/date values, pass widths as "
"datetime.timedelta (e.g., datetime.timedelta(days=10))"
"or numpy.timedelta64."
)

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# Validate side
_api.check_in_list(["both", "low", "high"], side=side)

Expand Down
95 changes: 95 additions & 0 deletions lib/matplotlib/tests/test_violinplot_datetime.py
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@@ -0,0 +1,95 @@
"""
Unit tests for proactive validation of datetime
positions and timedelta widths in violinplot.
"""

import datetime
import pytest

import matplotlib.pyplot as plt


def make_vpstats():
"""Create minimal valid stats for a violin plot."""


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def violin_plot_stats():
# Stats for violin plot
datetimes = [
datetime.datetime(2023, 2, 10),
datetime.datetime(2023, 5, 18),
datetime.datetime(2023, 6, 6)
]
return [{
'coords': datetimes,
'vals': [0.1, 0.5, 0.2],
'mean': datetimes[1],
'median': datetimes[1],
'min': datetimes[0],
'max': datetimes[-1],
'quantiles': datetimes
}, {
'coords': datetimes,
'vals': [0.2, 0.3, 0.4],
'mean': datetimes[2],
'median': datetimes[2],
'min': datetimes[0],
'max': datetimes[-1],
'quantiles': datetimes
}]


def test_datetime_positions_with_float_widths_raises():
"""Test that datetime positions with float widths raise TypeError."""
fig, ax = plt.subplots()
try:
vpstats = violin_plot_stats()
positions = [datetime.datetime(2020, 1, 1), datetime.datetime(2021, 1, 1)]
widths = [0.5, 1.0]
with pytest.raises(TypeError,
match="positions are datetime/date.*widths as datetime\\.timedelta"):
ax.violin(vpstats, positions=positions, widths=widths)
finally:
plt.close(fig)
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def test_datetime_positions_with_scalar_float_width_raises():
"""Test that datetime positions with scalar float width raise TypeError."""
fig, ax = plt.subplots()
try:
vpstats = violin_plot_stats()
positions = [datetime.datetime(2020, 1, 1), datetime.datetime(2021, 1, 1)]
widths = 0.75
with pytest.raises(TypeError,
match="positions are datetime/date.*widths as datetime\\.timedelta"):
ax.violin(vpstats, positions=positions, widths=widths)
finally:
plt.close(fig)


def test_numeric_positions_with_float_widths_ok():
"""Test that numeric positions with float widths work."""
fig, ax = plt.subplots()
try:
vpstats = violin_plot_stats()
positions = [1.0, 2.0]
widths = [0.5, 1.0]
ax.violin(vpstats, positions=positions, widths=widths)
finally:
plt.close(fig)


def test_mixed_positions_datetime_and_numeric_behaves():
"""Test that mixed datetime and numeric positions
with float widths raise TypeError.
"""
fig, ax = plt.subplots()
try:
vpstats = violin_plot_stats()
positions = [datetime.datetime(2020, 1, 1), 2.0]
widths = [0.5, 1.0]
with pytest.raises(TypeError,
match="positions are datetime/date.*widths as datetime\\.timedelta"):
ax.violin(vpstats, positions=positions, widths=widths)
finally:
plt.close(fig)