Update lib/matplotlib/legend.py to simplify the logic as per suggested by jklymak

Remove title_prop from public API and update test for robustness
Change title_prop to title_fontproperties

Author: smartlixx

doc/users/next_whats_new/legend_title_fontproperties_kwarg.rst

@@ -0,0 +1,15 @@
+Legend now has a title_fontproperties kwarg
+-------------------------------------------
+
+The title for a `.Figure.legend` and `.Axes.legend` can now have its
+title's font properties set via the ``title_fontproperties`` kwarg, defaults 
+to ``None``, which means the legend's title will have the font properties
+as set by the :rc:`legend.title_fontsize`. Through this new kwarg, one
+can set the legend's title font properties through either FontProperties
+or dict, for example:
+
+.. plot::
+
+    fig, ax = plt.subplots(figsize=(4, 3))
+    ax.plot(range(10))
+    ax.legend(title='Points', title_fontproperties={'family': 'serif'}, title_fontsize=22) 

doc/users/next_whats_new/legend_title_prop_kwarg.rst

@@ -237,7 +237,7 @@ def _update_bbox_to_anchor(self, loc_in_canvas):
 title : str or None
     The legend's title. Default is no title (``None``).
 
-title_prop : None or `matplotlib.font_manager.FontProperties` or dict
+title_fontproperties : None or `matplotlib.font_manager.FontProperties` or dict
     The font properties of the legend's title. If None (default), the current
     :data:`matplotlib.rcParams` will be used.
 
@@ -327,7 +327,6 @@ def __init__(self, parent, handles, labels,
                                  # box, none use rc
                  shadow=None,
                  title=None,  # set a title for the legend
-                 title_prop=None,  # properties for the legend title
                  title_fontsize=None,  # the font size for the title
                  framealpha=None,  # set frame alpha
                  edgecolor=None,  # frame patch edgecolor
@@ -337,6 +336,7 @@ def __init__(self, parent, handles, labels,
                  bbox_transform=None,  # transform for the bbox
                  frameon=None,  # draw frame
                  handler_map=None,
+                 title_fontproperties=None,  # properties for the legend title
                  ):
         """
         Parameters
@@ -511,23 +511,18 @@ def __init__(self, parent, handles, labels,
         self._set_loc(loc)
         self._loc_used_default = tmp  # ignore changes done by _set_loc
 
-        # figure out title fontsize:
-        if title_prop is None:
+        # figure out title font properties:
+        title_prop_fp = FontProperties._from_any(title_fontproperties)
+        if isinstance(title_fontproperties, dict) and \
+           "size" not in title_fontproperties:
             if title_fontsize is None:
                 title_fontsize = mpl.rcParams["legend.title_fontsize"]
-            self.title_prop = FontProperties(
-                size=title_fontsize)
+            title_prop_fp.set_size(title_fontsize)
         else:
-            self.title_prop = FontProperties._from_any(title_prop)
-            if isinstance(title_prop, dict) and "size" not in title_prop:
-                if title_fontsize is None:
-                    title_fontsize = mpl.rcParams["legend.title_fontsize"]
-                self.title_prop.set_size(title_fontsize)
-            else:
-                if title_fontsize is not None:
-                    self.title_prop.set_size(title_fontsize)
+            if title_fontsize is not None:
+                title_prop_fp.set_size(title_fontsize)
 
-        self.set_title(title, prop=self.title_prop)
+        self.set_title(title, prop=title_prop_fp)
         self._draggable = None
 
         # set the text color

lib/matplotlib/legend.py

@@ -544,27 +544,37 @@ def test_window_extent_cached_renderer():
     leg2.get_window_extent()
 
 
-def test_legend_title_fontsize():
+def test_legend_title_fontprop_fontsize():
     # test the title_fontsize kwarg
-    fig, axes = plt.subplots(2, 2, figsize=(8, 6))
+    fig, axes = plt.subplots(2, 3, figsize=(10, 6))
     axes = axes.flat
     axes[0].plot(range(10))
     leg = axes[0].legend(title='Aardvark', title_fontsize=22)
     assert leg.get_title().get_fontsize() == 22
     axes[1].plot(range(10))
     leg2 = axes[1].legend(title='Aardvark',
-                          title_prop={'family': 'serif', 'size': 22})
+                          title_fontproperties={'family': 'serif', 'size': 22})
     assert leg2.get_title().get_fontsize() == 22
     axes[2].plot(range(10))
     leg3 = axes[2].legend(title='Aardvark',
-                          title_prop={'family': 'serif'},
+                          title_fontproperties={'family': 'serif'},
                           title_fontsize=22)
     assert leg3.get_title().get_fontsize() == 22
     axes[3].plot(range(10))
     leg4 = axes[3].legend(title='Aardvark',
-                          title_prop={'family': 'serif', 'size': 10},
+                          title_fontproperties={'family': 'serif', 'size': 10},
                           title_fontsize=22)
     assert leg4.get_title().get_fontsize() == 22
+    mpl.rcParams['legend.title_fontsize'] = None
+    axes[4].plot(range(10))
+    leg5 = axes[4].legend(title='Aardvark',
+                          title_fontproperties={'family': 'serif'})
+    assert leg5.get_title().get_fontsize() == mpl.rcParams['font.size']
+    mpl.rcParams['legend.title_fontsize'] = 20
+    axes[5].plot(range(10))
+    leg6 = axes[5].legend(title='Aardvark',
+                          title_fontproperties={'family': 'serif'})
+    assert leg6.get_title().get_fontsize() == 20
 
 
 def test_legend_labelcolor_single():