Add minimal dependencies to Python path

Only add pythonfutures on Python 2. The concurrent.futures module is
part of the standard library on Python 3.

.ycm_extra_conf.py

@@ -29,6 +29,7 @@
 # For more information, please refer to <http://unlicense.org/>
 
 import os
+import subprocess
 
 DIR_OF_THIS_SCRIPT = os.path.abspath( os.path.dirname( __file__ ) )
 DIR_OF_THIRD_PARTY = os.path.join( DIR_OF_THIS_SCRIPT, 'third_party' )
@@ -46,19 +47,23 @@ def GetStandardLibraryIndexInSysPath( sys_path ):
 def PythonSysPath( **kwargs ):
   sys_path = kwargs[ 'sys_path' ]
 
-  for folder in os.listdir( DIR_OF_THIRD_PARTY ):
-    sys_path.insert( 0, os.path.realpath( os.path.join( DIR_OF_THIRD_PARTY,
-                                                        folder ) ) )
+  dependencies = [ os.path.join( DIR_OF_THIS_SCRIPT, 'python' ),
+                   os.path.join( DIR_OF_THIRD_PARTY, 'requests-futures' ),
+                   os.path.join( DIR_OF_THIRD_PARTY, 'ycmd' ),
+                   os.path.join( DIR_OF_YCMD_THIRD_PARTY, 'frozendict' ),
+                   os.path.join( DIR_OF_YCMD_THIRD_PARTY, 'requests' ) ]
 
-  for folder in os.listdir( DIR_OF_YCMD_THIRD_PARTY ):
-    if folder == 'python-future':
-      folder = os.path.join( folder, 'src' )
-      sys_path.insert( GetStandardLibraryIndexInSysPath( sys_path ) + 1,
-                       os.path.realpath( os.path.join( DIR_OF_YCMD_THIRD_PARTY,
-                                                       folder ) ) )
-      continue
+  # The concurrent.futures module is part of the standard library on Python 3.
+  interpreter_path = kwargs[ 'interpreter_path' ]
+  major_version = int( subprocess.check_output( [
+    interpreter_path, '-c', 'import sys; print( sys.version_info[ 0 ] )' ]
+  ).rstrip().decode( 'utf8' ) )
+  if major_version == 2:
+    dependencies.append( os.path.join( DIR_OF_THIRD_PARTY, 'pythonfutures' ) )
 
-    sys_path.insert( 0, os.path.realpath( os.path.join( DIR_OF_YCMD_THIRD_PARTY,
-                                                        folder ) ) )
+  sys_path[ 0:0 ] = dependencies
+  sys_path.insert( GetStandardLibraryIndexInSysPath( sys_path ) + 1,
+                   os.path.join( DIR_OF_YCMD_THIRD_PARTY, 'python-future',
+                                 'src' ) )
 
   return sys_path

autoload/youcompleteme.vim

@@ -186,23 +186,34 @@ from __future__ import print_function
 from __future__ import division
 from __future__ import absolute_import
 
-import os
+import os.path as p
 import sys
 import traceback
 import vim
 
-# Add python sources folder to the system path.
-script_folder = vim.eval( 's:script_folder_path' )
-sys.path.insert( 0, os.path.join( script_folder, '..', 'python' ) )
-sys.path.insert( 0, os.path.join( script_folder, '..', 'third_party', 'ycmd' ) )
+root_folder = p.normpath( p.join( vim.eval( 's:script_folder_path' ), '..' ) )
+third_party_folder = p.join( root_folder, 'third_party' )
+ycmd_third_party_folder = p.join( third_party_folder, 'ycmd', 'third_party' )
+
+# Add dependencies to Python path.
+dependencies = [ p.join( root_folder, 'python' ),
+                 p.join( third_party_folder, 'requests-futures' ),
+                 p.join( third_party_folder, 'ycmd' ),
+                 p.join( ycmd_third_party_folder, 'frozendict' ),
+                 p.join( ycmd_third_party_folder, 'requests' ) ]
+
+# The concurrent.futures module is part of the standard library on Python 3.
+if sys.version_info[ 0 ] == 2:
+  dependencies.append( p.join( third_party_folder, 'pythonfutures' ) )
+
+sys.path[ 0:0 ] = dependencies
 
 # We enclose this code in a try/except block to avoid backtraces in Vim.
 try:
-  from ycmd import server_utils as su
-  su.AddNearestThirdPartyFoldersToSysPath( script_folder )
-  # We need to import ycmd's third_party folders as well since we import and
-  # use ycmd code in the client.
-  su.AddNearestThirdPartyFoldersToSysPath( su.__file__ )
+  # The python-future module must be inserted after the standard library path.
+  from ycmd.server_utils import GetStandardLibraryIndexInSysPath
+  sys.path.insert( GetStandardLibraryIndexInSysPath() + 1,
+                   p.join( ycmd_third_party_folder, 'python-future', 'src' ) )
 
   # Import the modules used in this file.
   from ycm import base, vimsupport, youcompleteme