diff --git a/python/ycm/diagnostic_filter.py b/python/ycm/diagnostic_filter.py
index 24936dd2..67ac8c38 100644
--- a/python/ycm/diagnostic_filter.py
+++ b/python/ycm/diagnostic_filter.py
@@ -1,4 +1,4 @@
-# Copyright (C) 2013  Google Inc.
+# Copyright (C) 2016  YouCompleteMe contributors
 #
 # This file is part of YouCompleteMe.
 #
@@ -23,11 +23,7 @@ from future import standard_library
 standard_library.install_aliases()
 from builtins import *  # noqa
 
-from future.utils import itervalues, iteritems
-from collections import defaultdict, namedtuple
-from ycm import vimsupport
 import re
-import vim
 
 
 class DiagnosticFilter( object ):
@@ -35,14 +31,14 @@ class DiagnosticFilter( object ):
     self._filters = []
 
     for filter_type in config.iterkeys():
-      wrapper = lambda x: x
+      wrapper = _AsIs
       actual_filter_type = filter_type
 
       if filter_type[0] == '!':
         wrapper = _Not
         filter_type = filter_type[1:]
       compiler = FILTER_COMPILERS.get( filter_type )
-      
+
       if compiler is not None:
         for filter_config in _ListOf( config[ actual_filter_type ] ):
           fn = wrapper( compiler( filter_config ) )
@@ -56,7 +52,7 @@ class DiagnosticFilter( object ):
       if f( diagnostic ):
         return False
 
-    return True 
+    return True
 
 
   @staticmethod
@@ -64,18 +60,22 @@ class DiagnosticFilter( object ):
     base = dict( user_options.get( 'quiet_messages', {} ) )
 
     for filetype in filetypes:
-      type_specific = user_options.get( filetype + '_quiet_messages', {} ) 
+      type_specific = user_options.get( filetype + '_quiet_messages', {} )
       base.update( type_specific )
     return DiagnosticFilter( base )
 
 
 def _ListOf( config_entry ):
-  if type( config_entry ) == type( [] ):
+  if isinstance( config_entry, list ):
     return config_entry
 
   return [ config_entry ]
 
 
+def _AsIs( fn ):
+  return fn
+
+
 def _Not( fn ):
   def Inverted( diagnostic ):
     return not fn( diagnostic )
@@ -93,7 +93,7 @@ def _CompileRegex( raw_regex ):
 
 
 def _CompileLevel( level ):
-  # valid kinds are WARNING and ERROR; 
+  # valid kinds are WARNING and ERROR;
   #  expected input levels are `warnings` and `errors`
   # NB: we don't validate the input...
   expected_kind = level.upper()[:-1]
diff --git a/python/ycm/diagnostic_interface.py b/python/ycm/diagnostic_interface.py
index 089eac2d..e0e349b3 100644
--- a/python/ycm/diagnostic_interface.py
+++ b/python/ycm/diagnostic_interface.py
@@ -66,8 +66,11 @@ class DiagnosticInterface( object ):
 
 
   def UpdateWithNewDiagnostics( self, diags ):
-    diag_filter = DiagnosticFilter.from_filetype( self._user_options, vimsupport.CurrentFiletypes() )
-    normalized_diags = [ _NormalizeDiagnostic( x ) for x in diags if diag_filter.Accept(x) ]
+    diag_filter = DiagnosticFilter.from_filetype(
+            self._user_options,
+            vimsupport.CurrentFiletypes() )
+    normalized_diags = [ _NormalizeDiagnostic( x ) for x in diags
+            if diag_filter.Accept(x) ]
     self._buffer_number_to_line_to_diags = _ConvertDiagListToDict(
         normalized_diags )
 
diff --git a/python/ycm/tests/diagnostic_filter_tests.py b/python/ycm/tests/diagnostic_filter_tests.py
index dc84bcf9..c3126940 100644
--- a/python/ycm/tests/diagnostic_filter_tests.py
+++ b/python/ycm/tests/diagnostic_filter_tests.py
@@ -1,4 +1,4 @@
-# Copyright (C) 2013  Google Inc.
+# Copyright (C) 2016  YouCompleteMe contributors
 #
 # This file is part of YouCompleteMe.
 #
@@ -26,20 +26,21 @@ from builtins import *  # noqa
 from ycm.test_utils import MockVimModule
 MockVimModule()
 
-import os
 from hamcrest import assert_that, equal_to
 from ycm.diagnostic_filter import DiagnosticFilter
 
 
 def _assert_accept_equals( filter, text_or_obj, expected ):
-  if type( text_or_obj ) is not type( {} ):
+  if not isinstance( text_or_obj, dict ):
     text_or_obj = { 'text': text_or_obj }
 
   assert_that( filter.Accept( text_or_obj ), equal_to( expected ) )
 
+
 def _assert_accepts( filter, text ):
   _assert_accept_equals( filter, text, True )
 
+
 def _assert_rejects( filter, text ):
   _assert_accept_equals( filter, text, False )
 
@@ -111,9 +112,9 @@ class Level_test():
     opts = { 'quiet_messages' : { 'level': 'warnings' } }
     f = DiagnosticFilter.from_filetype( opts, [ 'java' ] )
 
-    _assert_rejects( f, { 'text': 'This is an unimportant taco', 
+    _assert_rejects( f, { 'text': 'This is an unimportant taco',
                           'kind': 'WARNING' } )
-    _assert_accepts( f, { 'text': 'This taco will be shown', 
+    _assert_accepts( f, { 'text': 'This taco will be shown',
                           'kind': 'ERROR' } )
 
 
@@ -121,7 +122,7 @@ class Level_test():
     opts = { 'quiet_messages' : { 'level': 'errors' } }
     f = DiagnosticFilter.from_filetype( opts, [ 'java' ] )
 
-    _assert_accepts( f, { 'text': 'This is an IMPORTANT taco', 
+    _assert_accepts( f, { 'text': 'This is an IMPORTANT taco',
                           'kind': 'WARNING' } )
-    _assert_rejects( f, { 'text': 'This taco will NOT be shown', 
+    _assert_rejects( f, { 'text': 'This taco will NOT be shown',
                           'kind': 'ERROR' } )