Add default_log_stream Powerline attribute, make it sys.stdout in Vim

Reason for changes in VimPowerline:

    try
        python import sys; sys.stderr.write('abc')
    catch
        echomsg v:exception
    endtry

reports an exception. This should not happen because stack trace that ought to 
be printed by the logger will not be shown to the user, making it hard to 
determine the error and turning some recoverable errors to non-recoverable when 
using powerline.vim.

powerline/__init__.py

@@ -103,7 +103,7 @@ class PowerlineLogger(object):
 _fallback_logger = None
 
 
-def get_fallback_logger():
+def get_fallback_logger(stream=None):
 	global _fallback_logger
 	if _fallback_logger:
 		return _fallback_logger
@@ -112,7 +112,7 @@ def get_fallback_logger():
 	formatter = logging.Formatter(log_format)
 
 	level = logging.WARNING
-	handler = logging.StreamHandler()
+	handler = logging.StreamHandler(stream)
 	handler.setLevel(level)
 	handler.setFormatter(formatter)
 
@@ -195,7 +195,7 @@ def load_config(cfg_path, find_config_files, config_loader, loader_callback=None
 	return ret
 
 
-def _get_log_handler(common_config):
+def _get_log_handler(common_config, stream=None):
 	'''Get log handler.
 
 	:param dict common_config:
@@ -211,17 +211,17 @@ def _get_log_handler(common_config):
 			os.mkdir(log_dir)
 		return logging.FileHandler(log_file)
 	else:
-		return logging.StreamHandler()
+		return logging.StreamHandler(stream)
 
 
-def create_logger(common_config):
+def create_logger(common_config, stream=None):
 	'''Create logger according to provided configuration
 	'''
 	log_format = common_config['log_format']
 	formatter = logging.Formatter(log_format)
 
 	level = getattr(logging, common_config['log_level'])
-	handler = _get_log_handler(common_config)
+	handler = _get_log_handler(common_config, stream)
 	handler.setLevel(level)
 	handler.setFormatter(formatter)
 
@@ -456,7 +456,7 @@ class Powerline(object):
 				self.import_paths = self.common_config['paths']
 
 				if not self.logger:
-					self.logger = create_logger(self.common_config)
+					self.logger = create_logger(self.common_config, self.default_log_stream)
 
 				if not self.pl:
 					self.pl = PowerlineLogger(self.use_daemon_threads, self.logger, self.ext)
@@ -562,6 +562,14 @@ class Powerline(object):
 			else:
 				self.renderer = renderer
 
+	default_log_stream = sys.stdout
+	'''Default stream for default log handler
+
+	Usually it is ``sys.stderr``, but there is sometimes a reason to prefer 
+	``sys.stdout`` or a custom file-like object. It is not supposed to be used 
+	to write to some file.
+	'''
+
 	def setup_components(self, components):
 		'''Run component-specific setup
 
@@ -847,7 +855,7 @@ class Powerline(object):
 		if 'prefix' not in kwargs:
 			kwargs['prefix'] = 'powerline'
 		exception = kwargs.pop('exception', None)
-		pl = getattr(self, 'pl', None) or get_fallback_logger()
+		pl = getattr(self, 'pl', None) or get_fallback_logger(self.default_log_stream)
 		if exception:
 			try:
 				reraise(exception)

powerline/vim.py

@@ -31,6 +31,8 @@ class VimPowerline(Powerline):
 		self.pyeval = pyeval
 		self.window_statusline = '%!' + pyeval + '(\'powerline.statusline({0})\')'
 
+	default_log_stream = sys.stdout
+
 	def add_local_theme(self, key, config):
 		'''Add local themes at runtime (during vim session).