Deprecate trivial helpers in style.core.

STYLE_FILE_PATTERN is unused, the usual approach would be to check
whether a path's suffix matches STYLE_EXTENSION.

load_base_library is just a one-liner used in one place; hiding it
behind a function only obscures things.

iter_user_library is a combo of os.path.expanduser (which can be
inlined) and checking whether entries are existing directories (which is
unnecessary in update_user_library) as Path.glob helpfully returns empty
lists when globbing non-existing paths or into non-directories.

(Also note that none of these APIs are exported in `style/__init__.py`,
which is the real "intended public" API.)

Author: anntzer

doc/api/next_api_changes/deprecations/19585-AL.rst

@@ -0,0 +1,4 @@
+matplotlib.style.core deprecations
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+``STYLE_FILE_PATTERN``, ``load_base_library``, and ``iter_user_libraries`` are
+deprecated.

lib/matplotlib/style/core.py

@@ -30,6 +30,8 @@
 # Users may want multiple library paths, so store a list of paths.
 USER_LIBRARY_PATHS = [os.path.join(mpl.get_configdir(), 'stylelib')]
 STYLE_EXTENSION = 'mplstyle'
+
+# Deprecated in Matplotlib 3.5.
 STYLE_FILE_PATTERN = re.compile(r'([\S]+).%s$' % STYLE_EXTENSION)
 
 
@@ -155,12 +157,14 @@ def context(style, after_reset=False):
         yield
 
 
+@_api.deprecated("3.5")
 def load_base_library():
     """Load style library defined in this package."""
     library = read_style_directory(BASE_LIBRARY_PATH)
     return library
 
 
+@_api.deprecated("3.5")
 def iter_user_libraries():
     for stylelib_path in USER_LIBRARY_PATHS:
         stylelib_path = os.path.expanduser(stylelib_path)
@@ -170,7 +174,7 @@ def iter_user_libraries():
 
 def update_user_library(library):
     """Update style library with user-defined rc files."""
-    for stylelib_path in iter_user_libraries():
+    for stylelib_path in map(os.path.expanduser, USER_LIBRARY_PATHS):
         styles = read_style_directory(stylelib_path)
         update_nested_dict(library, styles)
     return library
@@ -204,10 +208,8 @@ def update_nested_dict(main_dict, new_dict):
 
 # Load style library
 # ==================
-_base_library = load_base_library()
-
+_base_library = read_style_directory(BASE_LIBRARY_PATH)
 library = None
-
 available = []