Don't expose private styles in style.available

They remain in `style.library`, because that's how we look them up, but
this prevents them being exposed as something someone might use.

Also, fix `reload_library`, which was accidentally modifying the
original base library information each time.

Fixes itprojects/MasVisGtk#13

Author: QuLogic

lib/matplotlib/style/__init__.py

@@ -6,7 +6,8 @@
 ``context``
     Context manager to use a style sheet temporarily.
 ``available``
-    List available style sheets.
+    List available style sheets. Underscore-prefixed names are considered private and
+    not listed, though may still be accessed directly from ``library``.
 ``library``
     A dictionary of style names and matplotlib settings.
 """
@@ -245,8 +246,8 @@ def update_nested_dict(main_dict, new_dict):
 def reload_library():
     """Reload the style library."""
     library.clear()
-    library.update(_update_user_library(_base_library))
-    available[:] = sorted(library.keys())
+    library.update(_update_user_library(_base_library.copy()))
+    available[:] = sorted(name for name in library if not name.startswith('_'))
 
 
 reload_library()

lib/matplotlib/tests/test_style.py

@@ -21,6 +21,7 @@ def temp_style(style_name, settings=None):
     if not settings:
         settings = DUMMY_SETTINGS
     temp_file = f'{style_name}.mplstyle'
+    orig_library_paths = style.USER_LIBRARY_PATHS
     try:
         with TemporaryDirectory() as tmpdir:
             # Write style settings to file in the tmpdir.
@@ -32,6 +33,7 @@ def temp_style(style_name, settings=None):
             style.reload_library()
             yield
     finally:
+        style.USER_LIBRARY_PATHS = orig_library_paths
         style.reload_library()
 
 
@@ -46,8 +48,17 @@ def test_invalid_rc_warning_includes_filename(caplog):
 
 
 def test_available():
-    with temp_style('_test_', DUMMY_SETTINGS):
-        assert '_test_' in style.available
+    # Private name should not be listed in available but still usable.
+    assert '_classic_test_patch' not in style.available
+    assert '_classic_test_patch' in style.library
+
+    with temp_style('_test_', DUMMY_SETTINGS), temp_style('dummy', DUMMY_SETTINGS):
+        assert 'dummy' in style.available
+        assert 'dummy' in style.library
+        assert '_test_' not in style.available
+        assert '_test_' in style.library
+    assert 'dummy' not in style.available
+    assert '_test_' not in style.available
 
 
 def test_use():