Rewrite SetUpPython function

Do not create YouCompleteMe object if setup failed. Use a try/except
block to prevent backtraces in Vim.

autoload/youcompleteme.vim

@@ -131,38 +131,38 @@ endfunction
 
 function! s:SetUpPython() abort
 python << EOF
+import os
 import sys
+import traceback
 import vim
-import os
-import subprocess
 
+# 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' ) )
-from ycmd import server_utils
-server_utils.AddNearestThirdPartyFoldersToSysPath( script_folder )
-
-# We need to import ycmd's third_party folders as well since we import and
-# use ycmd code in the client.
-server_utils.AddNearestThirdPartyFoldersToSysPath( server_utils.__file__ )
-from ycmd import utils
-from ycm import base
-base.LoadJsonDefaultsIntoVim()
-from ycmd import user_options_store
-user_options_store.SetAll( base.BuildServerConf() )
-from ycm import paths, vimsupport
-
-popen_args = [ paths.PathToPythonInterpreter(),
-               paths.PathToCheckCoreVersion() ]
-
-if utils.SafePopen( popen_args ).wait() == 2:
-  vimsupport.PostVimMessage(
-    'YouCompleteMe unavailable: YCM support libs too old, PLEASE RECOMPILE' )
-  vim.command( 'return 0' )
+sys.path.insert( 0, os.path.join( script_folder, '..', 'python' ) )
+
+from ycm.setup import SetUpSystemPaths, SetUpYCM
 
-from ycm.youcompleteme import YouCompleteMe
-ycm_state = YouCompleteMe( user_options_store.GetAll() )
-vim.command( 'return 1' )
+# We enclose this code in a try/except block to avoid backtraces in Vim.
+try:
+  SetUpSystemPaths()
+
+  # Import the modules used in this file.
+  from ycm import base, vimsupport
+
+  ycm_state = SetUpYCM()
+except Exception as error:
+  # We don't use PostVimMessage or EchoText from the vimsupport module because
+  # importing this module may fail.
+  vim.command( 'redraw | echohl WarningMsg' )
+  for line in traceback.format_exc().splitlines():
+    vim.command( "echom '{0}'".format( line.replace( "'", "''" ) ) )
+
+  vim.command( "echo 'YouCompleteMe unavailable: {0}'"
+               .format( str( error ).replace( "'", "''" ) ) )
+  vim.command( 'echohl None' )
+  vim.command( 'return 0' )
+else:
+  vim.command( 'return 1' )
 EOF
 endfunction
 

python/ycm/paths.py

@@ -20,9 +20,10 @@ import sys
 import vim
 import functools
 import re
-from ycmd import utils
 
 DIR_OF_CURRENT_SCRIPT = os.path.dirname( os.path.abspath( __file__ ) )
+DIR_OF_YCMD = os.path.join( DIR_OF_CURRENT_SCRIPT, '..', '..', 'third_party',
+                            'ycmd' )
 
 WIN_PYTHON_PATH = os.path.join( sys.exec_prefix, 'python.exe' )
 PYTHON_BINARY_REGEX = re.compile( r'python(2(\.[67])?)?(.exe)?$' )
@@ -42,6 +43,8 @@ def Memoize( obj ):
 
 @Memoize
 def PathToPythonInterpreter():
+  from ycmd import utils
+
   python_interpreter = vim.eval( 'g:ycm_path_to_python_interpreter' )
 
   if python_interpreter:
@@ -82,6 +85,8 @@ def EndsWithPython( path ):
 
 def IsPythonVersionCorrect( path ):
   """Check if given path is the Python interpreter version 2.6 or 2.7."""
+  from ycmd import utils
+
   if not EndsWithPython( path ):
     return False
 
@@ -95,10 +100,8 @@ def IsPythonVersionCorrect( path ):
 
 
 def PathToServerScript():
-  return os.path.join( DIR_OF_CURRENT_SCRIPT, '..', '..', 'third_party',
-                       'ycmd', 'ycmd' )
+  return os.path.join( DIR_OF_YCMD, 'ycmd' )
 
 
 def PathToCheckCoreVersion():
-  return os.path.join( DIR_OF_CURRENT_SCRIPT, '..', '..', 'third_party',
-                       'ycmd', 'check_core_version.py' )
+  return os.path.join( DIR_OF_YCMD, 'check_core_version.py' )

python/ycm/setup.py

@@ -0,0 +1,48 @@
+# Copyright (C) 2016 YouCompleteMe contributors
+#
+# This file is part of YouCompleteMe.
+#
+# YouCompleteMe is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# YouCompleteMe is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with YouCompleteMe.  If not, see <http://www.gnu.org/licenses/>.
+
+import sys
+import os
+import paths
+
+
+def SetUpSystemPaths():
+  sys.path.insert( 0, os.path.join( paths.DIR_OF_YCMD ) )
+
+  from ycmd import server_utils as su
+  su.AddNearestThirdPartyFoldersToSysPath( paths.DIR_OF_CURRENT_SCRIPT )
+  # 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__ )
+
+
+def SetUpYCM():
+  import base
+  from ycmd import user_options_store, utils
+  from youcompleteme import YouCompleteMe
+
+  base.LoadJsonDefaultsIntoVim()
+
+  user_options_store.SetAll( base.BuildServerConf() )
+
+  popen_args = [ paths.PathToPythonInterpreter(),
+                 paths.PathToCheckCoreVersion() ]
+
+  if utils.SafePopen( popen_args ).wait() == 2:
+    raise RuntimeError( 'YCM support libs too old, PLEASE RECOMPILE.' )
+
+  return YouCompleteMe( user_options_store.GetAll() )