Fall back to Python search in PATH

python/ycm/paths.py

@@ -47,12 +47,12 @@ def PathToPythonInterpreter():
   python_interpreter = vim.eval( 'g:ycm_path_to_python_interpreter' )
 
   if python_interpreter:
-    if not IsPythonVersionCorrect( python_interpreter ):
+    if IsPythonVersionCorrect( python_interpreter ):
+      return python_interpreter
+
     raise RuntimeError( "Path in 'g:ycm_path_to_python_interpreter' option "
                         "does not point to a valid Python 2.6 or 2.7." )
 
-    return python_interpreter
-
   # On UNIX platforms, we use sys.executable as the Python interpreter path.
   # We cannot use sys.executable on Windows because for unknown reasons, it
   # returns the Vim executable. Instead, we use sys.exec_prefix to deduce the
@@ -60,13 +60,22 @@ def PathToPythonInterpreter():
   python_interpreter = ( WIN_PYTHON_PATH if utils.OnWindows() else
                          sys.executable )
 
-  if not IsPythonVersionCorrect( python_interpreter ):
+  if IsPythonVersionCorrect( python_interpreter ):
+    return python_interpreter
+
+  # As a last resort, we search python in the PATH. We check 'python2' before
+  # 'python' because on some distributions (Arch Linux for example), python
+  # refers to python3.
+  python_interpreter = utils.PathToFirstExistingExecutable( [ 'python2',
+                                                              'python' ] )
+
+  if IsPythonVersionCorrect( python_interpreter ):
+    return python_interpreter
+
   raise RuntimeError( "Cannot find Python 2.6 or 2.7. You can set its path "
                       "using the 'g:ycm_path_to_python_interpreter' "
                       "option." )
 
-  return python_interpreter
-
 
 def EndsWithPython( path ):
   """Check if given path ends with a python 2.6 or 2.7 name."""