IDL parser rewrite in Python

Source/bindings/scripts/blink_idl_lexer.py

Index: Source/bindings/scripts/blink_idl_lexer.py
diff --git a/Source/bindings/scripts/blink_idl_lexer.py b/Source/bindings/scripts/blink_idl_lexer.py
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@@ -0,0 +1,92 @@
+# Copyright (C) 2013 Google Inc. All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+#
+#     * Redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer.
+#     * Redistributions in binary form must reproduce the above
+# copyright notice, this list of conditions and the following disclaimer
+# in the documentation and/or other materials provided with the
+# distribution.
+#     * Neither the name of Google Inc. nor the names of its
+# contributors may be used to endorse or promote products derived from
+# this software without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+"""Lexer for Blink IDL.
+
+The lexer uses the PLY (Python Lex-Yacc) library to build a tokenizer which
+understands the Blink dialect of Web IDL and produces a token stream suitable
+for the Blink IDL parser.
+
+Blink IDL is identical to Web IDL at the token level, but the base lexer
+does not discard comments. We need to override (and not include comments in
+the token stream), as otherwise comments must be explicitly included in the
+phrase grammar of the parser.
+
+FIXME: Change base lexer to discard comments, and simply used the base
+lexer, eliminating this separate lexer.
+
+Web IDL:
+    http://www.w3.org/TR/WebIDL/
+Web IDL Grammar:
+    http://www.w3.org/TR/WebIDL/#idl-grammar
+PLY:
+    http://www.dabeaz.com/ply/
+"""
+
+# Disable attribute validation, as lint can't import parent class to check
+# pylint: disable=E1101
+
+import os.path
+import sys
+
+# Base lexer is in Chromium src/tools/idl_parser
+module_path, module_name = os.path.split(__file__)
+tools_dir = os.path.join(module_path, os.pardir, os.pardir, os.pardir, os.pardir, os.pardir, 'tools')
+sys.path.append(tools_dir)
+
+from idl_parser.idl_lexer import IDLLexer
+
+REMOVE_TOKENS = ['COMMENT']
+
+
+class BlinkIDLLexer(IDLLexer):
+    # ignore comments
+    def t_COMMENT(self, t):
+        r'(/\*(.|\n)*?\*/)|(//.*(\n[ \t]*//.*)*)'
+        self.AddLines(t.value.count('\n'))

[haraken, 2013/07/22 01:50:23]

Don't you need to write 'return t' at the end? idl_ppapi_lexer.py has 'return
t'.

Actually I still don't understand how t_XXX works (Probably I just don't
understand Python syntax).

def t_XXX(self, t):
  r'yyy'  # This just defines a raw string, that's it. How can it affect
behavior of the lexer?
  self.AddLines(...)

[Nils Barth (inactive), 2013/07/22 06:32:01]

We don’t want to “return t” because we’re discarding comments.
There are two issues: Python syntax, and PLY syntax.
1. Python docstrings
In general in Python, you document by having a docstring, a string literal as
the first statement of a definition (function etc.):
http://www.python.org/dev/peps/pep-0257/
“A docstring is a string literal that occurs as the first statement in a module,
function, class, or method definition. Such a docstring becomes the __doc__
special attribute of that object.”

2. PLY syntax
PLY uses the docstring as the pattern to match: a regex in the lexer, a
production rule (in BNF) in the parser. That is, the PLY lexer framework goes
through the attributes of the lexer object, and extracts the pattern by checking
their __doc__ attribute; likewise for the parser.

There’s a good comment in the base lexer:
https://code.google.com/p/chromium/codesearch#chromium/src/tools/idl_parser/i...
  # Lex assumes any value or function in the form of 't_<TYPE>' represents a
  # regular expression where a match will emit a token of type <TYPE>.  In the
  # case of a function, the function is called when a match is made. These
  # definitions come from WebIDL.

...so e.g., in:
  def t_string(self, t):
    r'"[^"]*"'
...this defines a token type “string”, matching the pattern r'"[^"]*"';
sequences that match this token are then passed to this function, which
processes it (in this case the function strips the quotes and returns the
stripped string).

I’ve added a more detailed comment in the parser, which should help, and I’ll
hopefully eliminate the Blink-specific lexer soon.

+
+    # Analogs to _AddToken/_AddTokens in base lexer
+    # Needed to remove COMMENT token, since comments ignored
+    def _RemoveToken(self, token):
+        try:
+            self.tokens.remove(token)
+        except ValueError:
+            raise RuntimeError('Token "%s" not present, so cannot be removed ' % token)
+
+    def _RemoveTokens(self, tokens):
+        for token in tokens:
+            self._RemoveToken(token)
+
+    def __init__(self):
+        IDLLexer.__init__(self)
+        self._RemoveTokens(REMOVE_TOKENS)
+
+
+# If run by itself, attempt to build the lexer
+if __name__ == '__main__':
+    lexer = BlinkIDLLexer()