Adding new g:ycm_path_to_python_interpreter option

Fixes #663

README.md

@@ -887,6 +887,20 @@ Default: `[]`
 
     let g:ycm_extra_conf_vim_data = []
 
+### The `g:ycm_path_to_python_interpreter` option
+
+YCM will by default search for an appropriate Python interpreter on your system.
+You can use this option to override that behavior and force the use of a
+specific interpreter of your choosing.
+
+NOTE: This interpreter is only used for the `ycmd` server. The YCM client
+running inside Vim always uses the Python interpreter that's embedded inside
+Vim.
+
+Default: `''`
+
+    let g:ycm_path_to_python_interpreter = ''
+
 ### The `g:ycm_server_use_vim_stdout` option
 
 By default, the `ycmd` completion server writes logs to logfiles. When this

plugin/youcompleteme.vim

@@ -116,6 +116,9 @@ let g:ycm_server_idle_suicide_seconds =
 let g:ycm_extra_conf_vim_data =
       \ get( g:, 'ycm_extra_conf_vim_data', [] )
 
+let g:ycm_path_to_python_interpreter =
+      \ get( g:, 'path_to_python_interpreter', '' )
+
 
 " On-demand loading. Let's use the autoload folder and not slow down vim's
 " startup procedure.

python/ycm/utils.py

@@ -79,7 +79,28 @@ def GetUnusedLocalhostPort():
   return port
 
 
+def Memoize( obj ):
+  cache = obj.cache = {}
+
+  @functools.wraps( obj )
+  def memoizer( *args, **kwargs ):
+    key = str( args ) + str( kwargs )
+    if key not in cache:
+      cache[ key ] = obj( *args, **kwargs )
+    return cache[ key ]
+  return memoizer
+
+
+@Memoize
 def PathToPythonInterpreter():
+  if not RunningInsideVim():
+    return sys.executable
+
+  import vim  # NOQA
+  user_path_to_python = vim.eval( 'g:ycm_path_to_python_interpreter' )
+  if user_path_to_python:
+    return user_path_to_python
+
   # We check for 'python2' before 'python' because some OS's (I'm looking at you
   # Arch Linux) have made the... interesting decision to point /usr/bin/python
   # to python3.
@@ -138,18 +159,6 @@ def AddThirdPartyFoldersToSysPath():
     sys.path.insert( 0, os.path.realpath( os.path.join( path_to_third_party,
                                                         folder ) ) )
 
-def Memoize( obj ):
-  cache = obj.cache = {}
-
-  @functools.wraps( obj )
-  def memoizer( *args, **kwargs ):
-    key = str( args ) + str( kwargs )
-    if key not in cache:
-      cache[ key ] = obj( *args, **kwargs )
-    return cache[ key ]
-  return memoizer
-
-
 def ForceSemanticCompletion( request_data ):
   return ( 'force_semantic' in request_data and
            bool( request_data[ 'force_semantic' ] ) )