idl_parser: Always use PLY from third_party.

tools/idl_parser/idl_lexer.py

 #!/usr/bin/env python
 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """ Lexer for PPAPI IDL
 
 The lexer uses the PLY library to build a tokenizer which understands both
 WebIDL and Pepper tokens.
 
 WebIDL, and WebIDL regular expressions can be found at:
    http://www.w3.org/TR/2012/CR-WebIDL-20120419/
 PLY can be found at:
    http://www.dabeaz.com/ply/
 """
 
 import os.path
 import sys
 
-#
-# Try to load the ply module, if not, then assume it is in the third_party
-# directory.
-#
-try:
-  # Disable lint check which fails to find the ply module.
-  # pylint: disable=F0401
-  from ply import lex
-except ImportError:
-  module_path, module_name = os.path.split(__file__)
-  third_party = os.path.join(module_path, '..', '..', 'third_party')
-  sys.path.append(third_party)
-  # pylint: disable=F0401
-  from ply import lex
+SRC_DIR = os.path.join(os.path.dirname(__file__), os.pardir, os.pardir)
+sys.path.insert(0, os.path.join(SRC_DIR, 'third_party'))
+from ply import lex
+
 
 #
 # IDL Lexer
 #
 class IDLLexer(object):
   # 'literals' is a value expected by lex which specifies a list of valid
   # literal tokens, meaning the token type and token value are identical.
   literals = r'"*.(){}[],;:=+-/~|&^?<>'
 
   # 't_ignore' contains ignored characters (spaces and tabs)
@@ skipping 242 matching lines @@
     self.tokens = []
     self._AddTokens(IDLLexer.tokens)
     self._AddKeywords(IDLLexer.keywords)
     self._lexobj = None
     self.last = None
     self.lines = None
 
 # If run by itself, attempt to build the lexer
 if __name__ == '__main__':
   lexer_object = IDLLexer()