Make nerdtree segment use bufvar_exists

powerline/bindings/vim/__init__.py

@@ -66,6 +66,18 @@ else:
 		else:
 			raise KeyError(varname)
 
+if hasattr(vim, 'vars') and vim.vvars['version'] > 703:
+	def bufvar_exists(buffer, varname):
+		buffer = buffer or vim.current.buffer
+		return varname in buffer.vars
+else:
+	def bufvar_exists(buffer, varname):  # NOQA
+		if not buffer or buffer.number == vim.current.buffer.number:
+			return vim.eval('exists("b:{0}")'.format(varname))
+		else:
+			return vim.eval('has_key(getbufvar({0}, ""), {1})'
+							.format(buffer.number, varname))
+
 if hasattr(vim, 'options'):
 	def vim_getbufoption(info, option):
 		return info['buffer'].options[option]

powerline/segments/plugin/nerdtree.py

@@ -5,7 +5,7 @@ try:
 except ImportError:
 	vim = object()  # NOQA
 
-from powerline.bindings.vim import getbufvar
+from powerline.bindings.vim import bufvar_exists
 from powerline.segments.vim import window_cached
 
 
@@ -15,9 +15,8 @@ def nerdtree(pl):
 
 	Highlight groups used: ``nerdtree.path`` or ``file_name``.
 	'''
-	ntr = getbufvar('%', 'NERDTreeRoot')
-	if not ntr:
-		return
+	if not bufvar_exists(None, 'NERDTreeRoot'):
+		return None
 	path_str = vim.eval('getbufvar("%", "NERDTreeRoot").path.str()')
 	return [{
 		'contents': path_str,