Hello all, I'm working on a parser for part of the Python language (expressions but not statements basically). I'm using PLY to generate the parser and it's mostly done.
I've hit on what looks like a fundamental ambiguity in the Python grammar which is difficult to get round with PLY; and I'm wondering *why* the grammar is defined in this way. It's possible there is a reason that I've missed, which means I need to rethink my workaround. List displays (list comprehensions) are defined as (from http://docs.python.org/ref/lists.html ) : test ::= and_test ( "or" and_test )* | lambda_form testlist ::= test ( "," test )* [ "," ] list_display ::= "[" [listmaker] "]" listmaker ::= expression ( list_for | ( "," expression )* [","] ) list_iter ::= list_for | list_if list_for ::= "for" expression_list "in" testlist [list_iter] list_if ::= "if" test [list_iter] The problem is that list_for is defined as : "for" expression_list "in" testlist This allows arbitrary expressions in the 'assignment' part of a list comprehension. As a result, the following is valid syntax according to the grammar : [x for x + 1 in y] Obviously it isn't valid ! This parses to an ast, but the syntax error is thrown when you compile the resulting ast. The problem is that for the basic case of a list comprehension ( ``[x for x in y]``), ``x in y`` is a valid expression. That makes it extremely hard to disambiguate the grammar so that the ``in`` is treated correctly, and not part of an expression. My question is, why are arbitrary expressions allowed here in the grammar ? As far as I can tell, only identifiers (nested in parentheses or brackets) are valid here. I've got round the problem by creating a new node 'identifier_list' and just having that return the expected syntax tree (actually an expression list). This gets round the ambiguity [#]_. It worries me that there might be a valid expression allowed here that I haven't thought of. My current rules allow anything that looks like ``(a, [b, c, (d, e)], f)`` - any nested identifier list. Would anything else be allowed ? If not, why not modify the grammar so that the compiler has less possible invalid syntax trees to work with ? (Also the grammar definition of string conversion is wrong as it states that a trailing comma is valid, which isn't the case. As far as I can tell that is necessary to allow nesting string conversions.) Fuzzyman http://www.voidspace.org.uk/python/index.shtml .. [#] If I could make precedence work in PLY I could also solve it I guess. However I can't. :-) _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
