Improving error message for width and position type mismatch in violinplot

hasanrashid · hasanrashid · commit 9ac50435387b · 2025-09-10T21:23:30.000-04:00
diff --git a/lib/matplotlib/axes/_axes.py b/lib/matplotlib/axes/_axes.py
@@ -1,3 +1,4 @@
+import datetime
 import functools
 import itertools
 import logging
@@ -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):
+            _widths = widths if not np.isscalar(widths) else [widths] * N
+            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."
+                )
+
+
         # Validate side
         _api.check_in_list(["both", "low", "high"], side=side)
 
diff --git a/lib/matplotlib/tests/test_violinplot_datetime.py b/lib/matplotlib/tests/test_violinplot_datetime.py
@@ -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."""
+
+
+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)
+
+
+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)
