Bugfix and move plugin to python specific ftplugin

Move to python specific plugin (ftplugin),
Fixed bug regarding empty buffer.

ftplugin/python/pythonhelper.py

@@ -151,7 +151,11 @@ class PythonHelper(object):
         line_number = vim.current.window.cursor[0] - 1
 
         while True:
-            line = vim.current.buffer[line_number]
+            try:
+                line = vim.current.buffer[line_number]
+            except IndexError:
+                return None
+
             line_indent = len(RE_INDENT.match(line).group(1))
             if line.strip():
                 break

ftplugin/python/test_pythonhelper.py

@@ -17,8 +17,7 @@ class Window(object):
 
 class Current(object):
     def __init__(self):
-        with open('test_py_example.py') as fobj:
-            self.buffer = fobj.read().split('\n')
+        self.buffer = []
         self.window = Window()
 
 
@@ -26,6 +25,9 @@ class MockVim(object):
     current = Current()
     command = Command()
 
+with open('test_py_example.py') as fobj:
+    BUFFER = fobj.read().split('\n')
+
 
 sys.modules['vim'] = vim = MockVim
 
@@ -33,8 +35,22 @@ sys.modules['vim'] = vim = MockVim
 import pythonhelper
 
 
+class TestTagsHelperWithEmptyBuffer(unittest.TestCase):
+
+    def setUp(self):
+        vim.current.buffer = []
+
+    def test_get_tag(self):
+        vim.current.window.cursor = (1, 1)
+        tag = pythonhelper.PythonHelper._get_tag(1, 2)
+        self.assertIsNone(tag)
+
+
 class TestTagsHelper(unittest.TestCase):
 
+    def setUp(self):
+        vim.current.buffer = BUFFER
+
     def test_import(self):
         vim.current.window.cursor = (1, 1)
         tag = pythonhelper.PythonHelper._get_tag(1, 2)