Replace various __import__ calls with one Powerline.get_module_attr

Also makes some more errors non-fatal:
- Failure to import renderer class in case there is an existing renderer object
- Failure to import segment function
- Failure to import matcher function

One of the purposes: create a function that is able to collect all imported
modules to reload them (really purge out from `sys.modules` and let the python
do its job when reimporting powerline and recreating Powerline and other
objects).

powerline/__init__.py

@@ -407,6 +407,9 @@ class Powerline(object):
 						'common_config': self.common_config,
 						'run_once': self.run_once,
 						'shutdown_event': self.shutdown_event,
+						# Note: creates implicit reference to self meaning 
+						# reference cycle.
+						'get_module_attr': self.get_module_attr,
 					},
 				)
 
@@ -468,11 +471,12 @@ class Powerline(object):
 			self.renderer_options['theme_config'] = self.load_theme_config(self.ext_config.get('theme', 'default'))
 
 		if create_renderer:
-			try:
-				Renderer = __import__(self.renderer_module, fromlist=['renderer']).renderer
-			except Exception as e:
-				self.exception('Failed to import renderer module: {0}', str(e))
-				sys.exit(1)
+			Renderer = self.get_module_attr(self.renderer_module, 'renderer')
+			if not Renderer:
+				if hasattr(self, 'renderer'):
+					return
+				else:
+					raise ImportError('Failed to obtain renderer')
 
 			# Renderer updates configuration file via segments’ .startup thus it 
 			# should be locked to prevent state when configuration was updated, 
@@ -652,6 +656,34 @@ class Powerline(object):
 				with self.cr_kwargs_lock:
 					self.cr_kwargs.clear()
 
+	def get_module_attr(self, module, attr, prefix='powerline'):
+		'''Import module and get its attribute.
+
+		Replaces ``from {module} import {attr}``.
+
+		:param str module:
+			Module name, will be passed as first argument to ``__import__``.
+		:param str attr:
+			Module attribute, will be passed to ``__import__`` as the only value 
+			in ``fromlist`` tuple.
+
+		:return:
+			Attribute value or ``None``. Note: there is no way to distinguish 
+			between successfull import of attribute equal to ``None`` and 
+			unsuccessfull import.
+		'''
+		oldpath = sys.path
+		sys.path = self.import_paths + sys.path
+		module = str(module)
+		attr = str(attr)
+		try:
+			return getattr(__import__(module, fromlist=(attr,)), attr)
+		except Exception as e:
+			self.pl.exception('Failed to import attr {0} from module {1}: {2}', attr, module, str(e), prefix=prefix)
+			return None
+		finally:
+			sys.path = oldpath
+
 	def render(self, *args, **kwargs):
 		'''Update/create renderer if needed and pass all arguments further to 
 		``self.renderer.render()``.

powerline/matcher.py

@@ -1,19 +0,0 @@
-# vim:fileencoding=utf-8:noet
-
-from __future__ import absolute_import
-import sys
-
-
-def gen_matcher_getter(ext, import_paths):
-	def get(match_name):
-		match_module, separator, match_function = match_name.rpartition('.')
-		if not separator:
-			match_module = 'powerline.matchers.{0}'.format(ext)
-			match_function = match_name
-		oldpath = sys.path
-		sys.path = import_paths + sys.path
-		try:
-			return getattr(__import__(str(match_module), fromlist=[str(match_function)]), match_function)
-		finally:
-			sys.path = oldpath
-	return get

powerline/segment.py

@@ -55,14 +55,11 @@ def get_segment_key(merge, *args, **kwargs):
 
 
 def get_function(data, segment):
-	oldpath = sys.path
-	sys.path = data['path'] + sys.path
 	segment_module = str(segment.get('module', data['default_module']))
-	name = str(segment['name'])
-	try:
-		return None, getattr(__import__(segment_module, fromlist=[name]), name), segment_module
-	finally:
-		sys.path = oldpath
+	function = data['get_module_attr'](segment_module, segment['name'], prefix='segment_generator')
+	if not function:
+		raise ImportError('Failed to obtain segment function')
+	return None, function, segment_module
 
 
 def get_string(data, segment):
@@ -162,10 +159,10 @@ def process_segment(pl, side, segment_info, parsed_segments, segment):
 		parsed_segments.append(segment)
 
 
-def gen_segment_getter(pl, ext, common_config, theme_configs, default_module=None):
+def gen_segment_getter(pl, ext, common_config, theme_configs, default_module, get_module_attr):
 	data = {
 		'default_module': default_module or 'powerline.segments.' + ext,
-		'path': common_config['paths'],
+		'get_module_attr': get_module_attr,
 	}
 
 	def get_key(merge, segment, module, key, default=None):

powerline/theme.py

@@ -29,6 +29,7 @@ class Theme(object):
 	             theme_config,
 	             common_config,
 	             pl,
+	             get_module_attr,
 	             main_theme_config=None,
 	             run_once=False,
 	             shutdown_event=None):
@@ -53,7 +54,7 @@ class Theme(object):
 		theme_configs = [theme_config]
 		if main_theme_config:
 			theme_configs.append(main_theme_config)
-		get_segment = gen_segment_getter(pl, ext, common_config, theme_configs, theme_config.get('default_module'))
+		get_segment = gen_segment_getter(pl, ext, common_config, theme_configs, theme_config.get('default_module'), get_module_attr)
 		for segdict in itertools.chain((theme_config['segments'],),
 		                               theme_config['segments'].get('above', ())):
 			self.segments.append(new_empty_segment_line())

powerline/vim.py

@@ -6,7 +6,6 @@ import sys
 from powerline.bindings.vim import vim_get_func, vim_getvar
 from powerline import Powerline
 from powerline.lib import mergedicts
-from powerline.matcher import gen_matcher_getter
 import vim
 from itertools import count
 
@@ -80,18 +79,30 @@ class VimPowerline(Powerline):
 		)
 
 	def get_local_themes(self, local_themes):
-		self.get_matcher = gen_matcher_getter(self.ext, self.import_paths)
-
 		if not local_themes:
 			return {}
 
 		return dict((
-			(
-				(None if key == '__tabline__' else self.get_matcher(key)),
-				{'config': self.load_theme_config(val)}
+			(matcher, {'config': self.load_theme_config(val)})
+			for matcher, key, val in (
+				(
+					(None if k == '__tabline__' else self.get_matcher(k)),
+					k,
+					v
+				)
+				for k, v in local_themes.items()
+			) if (
+				matcher or
+				key == '__tabline__'
 			)
-			for key, val in local_themes.items())
-		)
+		))
+
+	def get_matcher(self, match_name):
+		match_module, separator, match_function = match_name.rpartition('.')
+		if not separator:
+			match_module = 'powerline.matchers.{0}'.format(self.ext)
+			match_function = match_name
+		return self.get_module_attr(match_module, match_function, prefix='matcher_generator')
 
 	def get_config_paths(self):
 		try: