Use g:powerline_theme_overrides dictionary in place of g:…overrides__

Ref #1126

docs/source/configuration/local.rst

@@ -16,11 +16,11 @@ Vim configuration can be overridden using the following options:
     Dictionary, recursively merged with contents of 
     :file:`powerline/config.json`.
 
-``g:powerline_theme_overrides__{theme_name}``
-    Dictionary, recursively merged with contents of 
-    :file:`powerline/themes/vim/{theme_name}.json`. Note that this way you can’t 
-    redefine some value (e.g. segment) in list, only the whole list itself: only 
-    dictionaries are merged recursively.
+``g:powerline_theme_overrides``
+    Dictionary mapping theme names to theme overrides, recursively merged with 
+    contents of :file:`powerline/themes/vim/{key}.json`. Note that this way you 
+    can’t redefine some value (e.g. segment) in list, only the whole list 
+    itself: only dictionaries are merged recursively.
 
 ``g:powerline_config_paths``
     Paths list (each path must be expanded, ``~`` shortcut is not supported). 

powerline/vim.py

@@ -15,11 +15,16 @@ from powerline.lib.dict import mergedicts
 from powerline.lib.unicode import u
 
 
-def _override_from(config, override_varname):
+def _override_from(config, override_varname, key=None):
 	try:
 		overrides = vim_getvar(override_varname)
 	except KeyError:
 		return config
+	if key is not None:
+		try:
+			overrides = overrides[key]
+		except KeyError:
+			return config
 	mergedicts(config, overrides)
 	return config
 
@@ -111,12 +116,10 @@ class VimPowerline(Powerline):
 		return _override_from(super(VimPowerline, self).load_main_config(), 'powerline_config_overrides')
 
 	def load_theme_config(self, name):
-		# Note: themes with non-[a-zA-Z0-9_] names are impossible to override 
-		# (though as far as I know exists() won’t throw). Won’t fix, use proper 
-		# theme names.
 		return _override_from(
 			super(VimPowerline, self).load_theme_config(name),
-			'powerline_theme_overrides__' + name
+			'powerline_theme_overrides',
+			name
 		)
 
 	def get_local_themes(self, local_themes):

tests/test_local_overrides.vim

@@ -2,7 +2,7 @@
 set encoding=utf-8
 let g:powerline_config_paths = [expand('<sfile>:p:h:h') . '/powerline/config_files']
 let g:powerline_config_overrides = {'common': {'default_top_theme': 'ascii'}}
-let g:powerline_theme_overrides__default = {'segment_data': {'line_current_symbol': {'contents': 'LN '}, 'branch': {'before': 'B '}}}
+let g:powerline_theme_overrides = {'default': {'segment_data': {'line_current_symbol': {'contents': 'LN '}, 'branch': {'before': 'B '}}}}
 
 redir => g:messages