Preserve legend settings when regenerating in Qt figure options

Sanat Gupta · Sanat Gupta · commit 3a7f060d176a · 2026-04-23T03:26:58.000-04:00
When using the Qt figure options dialog to regenerate a legend, previously only ncols and draggable state were preserved. This caused all other legend customizations (loc, fontsize, frameon, shadow, title, mode, spacing, etc.) to be lost. This change extracts and reapplies these properties when regenerating the legend. Closes #17775
diff --git a/lib/matplotlib/backends/qt_editor/figureoptions.py b/lib/matplotlib/backends/qt_editor/figureoptions.py
@@ -248,11 +248,34 @@ def apply_callback(data):
         if generate_legend:
             draggable = None
             ncols = 1
+            legend_kwargs = {}
             if axes.legend_ is not None:
                 old_legend = axes.get_legend()
                 draggable = old_legend._draggable is not None
                 ncols = old_legend._ncols
-            new_legend = axes.legend(ncols=ncols)
+                # Preserve location and positioning
+                legend_kwargs['loc'] = old_legend._loc
+                if old_legend._bbox_to_anchor is not None:
+                    legend_kwargs['bbox_to_anchor'] = (
+                        old_legend._bbox_to_anchor.bounds)
+                # Preserve styling
+                legend_kwargs['fontsize'] = old_legend._fontsize
+                legend_kwargs['frameon'] = old_legend.get_frame_on()
+                legend_kwargs['shadow'] = old_legend.shadow
+                legend_kwargs['framealpha'] = (
+                    old_legend.get_frame().get_alpha())
+                legend_kwargs['title'] = (
+                    old_legend.get_title().get_text())
+                # Preserve layout settings
+                if old_legend._mode is not None:
+                    legend_kwargs['mode'] = old_legend._mode
+                legend_kwargs['columnspacing'] = (
+                    old_legend.columnspacing)
+                legend_kwargs['labelspacing'] = old_legend.labelspacing
+                legend_kwargs['handlelength'] = old_legend.handlelength
+                legend_kwargs['handletextpad'] = (
+                    old_legend.handletextpad)
+            new_legend = axes.legend(ncols=ncols, **legend_kwargs)
             if new_legend:
                 new_legend.set_draggable(draggable)
 
diff --git a/lib/matplotlib/tests/test_figureoptions_legend.py b/lib/matplotlib/tests/test_figureoptions_legend.py
@@ -0,0 +1,128 @@
+"""Tests for legend preservation in Qt figure options dialog."""
+import matplotlib
+matplotlib.use('Agg')
+import matplotlib.pyplot as plt
+
+
+def test_legend_properties_preserved():
+    """
+    Test that legend properties are preserved when the legend is
+    regenerated via the Qt figure options dialog.
+
+    Regression test for https://github.com/matplotlib/matplotlib/issues/17775
+    """
+    fig, ax = plt.subplots()
+    ax.plot(range(5), label='a')
+    ax.plot(range(3)[::-1], label='b')
+
+    ax.legend(
+        loc='upper right',
+        fontsize=14,
+        frameon=False,
+        shadow=True,
+        title='My Legend',
+        ncols=2,
+        columnspacing=3.0,
+        labelspacing=1.5,
+        handlelength=4.0,
+        handletextpad=1.2,
+    )
+
+    old_legend = ax.get_legend()
+    old_loc = old_legend._loc
+    old_fontsize = old_legend._fontsize
+    old_frameon = old_legend.get_frame_on()
+    old_shadow = old_legend.shadow
+    old_title = old_legend.get_title().get_text()
+    old_ncols = old_legend._ncols
+    old_columnspacing = old_legend.columnspacing
+    old_labelspacing = old_legend.labelspacing
+    old_handlelength = old_legend.handlelength
+    old_handletextpad = old_legend.handletextpad
+
+    # Simulate what the patched figureoptions code does
+    draggable = old_legend._draggable is not None
+    ncols = old_legend._ncols
+    legend_kwargs = {}
+    legend_kwargs['loc'] = old_legend._loc
+    if old_legend._bbox_to_anchor is not None:
+        legend_kwargs['bbox_to_anchor'] = (
+            old_legend._bbox_to_anchor.bounds)
+    legend_kwargs['fontsize'] = old_legend._fontsize
+    legend_kwargs['frameon'] = old_legend.get_frame_on()
+    legend_kwargs['shadow'] = old_legend.shadow
+    legend_kwargs['framealpha'] = old_legend.get_frame().get_alpha()
+    legend_kwargs['title'] = old_legend.get_title().get_text()
+    if old_legend._mode is not None:
+        legend_kwargs['mode'] = old_legend._mode
+    legend_kwargs['columnspacing'] = old_legend.columnspacing
+    legend_kwargs['labelspacing'] = old_legend.labelspacing
+    legend_kwargs['handlelength'] = old_legend.handlelength
+    legend_kwargs['handletextpad'] = old_legend.handletextpad
+
+    new_legend = ax.legend(ncols=ncols, **legend_kwargs)
+    if new_legend:
+        new_legend.set_draggable(draggable)
+
+    assert new_legend._loc == old_loc
+    assert new_legend._fontsize == old_fontsize
+    assert new_legend.get_frame_on() == old_frameon
+    assert new_legend.shadow == old_shadow
+    assert new_legend.get_title().get_text() == old_title
+    assert new_legend._ncols == old_ncols
+    assert new_legend.columnspacing == old_columnspacing
+    assert new_legend.labelspacing == old_labelspacing
+    assert new_legend.handlelength == old_handlelength
+    assert new_legend.handletextpad == old_handletextpad
+
+    plt.close(fig)
+
+
+def test_legend_regeneration_no_existing_legend():
+    """
+    Test that regenerating a legend when none exists still works.
+    """
+    fig, ax = plt.subplots()
+    ax.plot(range(5), label='a')
+
+    assert ax.get_legend() is None
+
+    new_legend = ax.legend(ncols=1)
+    assert new_legend is not None
+    assert len(new_legend.get_texts()) == 1
+    assert new_legend.get_texts()[0].get_text() == 'a'
+
+    plt.close(fig)
+
+
+def test_legend_mode_preserved():
+    """
+    Test that mode='expand' is preserved on legend regeneration.
+    """
+    fig, ax = plt.subplots()
+    ax.plot(range(5), label='a')
+    ax.plot(range(3)[::-1], label='b')
+
+    ax.legend(
+        loc='lower left',
+        mode='expand',
+        ncols=2,
+    )
+
+    old_legend = ax.get_legend()
+    old_mode = old_legend._mode
+    old_loc = old_legend._loc
+
+    legend_kwargs = {}
+    legend_kwargs['loc'] = old_legend._loc
+    legend_kwargs['fontsize'] = old_legend._fontsize
+    legend_kwargs['frameon'] = old_legend.get_frame_on()
+    if old_legend._mode is not None:
+        legend_kwargs['mode'] = old_legend._mode
+
+    new_legend = ax.legend(ncols=old_legend._ncols, **legend_kwargs)
+
+    assert new_legend._mode == old_mode
+    assert new_legend._loc == old_loc
+
+    plt.close(fig)
