[Python-Dev] Python Grammar Ambiguity

Mon Apr 24 16:12:40 CEST 2006

Well, yes, the syntax is supposed to be something like "for varlist in
testlist". Could you report this as a doc bug (if you found this
information in the docs)?
On 4/24/06, Michael Foord <fuzzyman at voidspace.org.uk> wrote:
> 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. :-)
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--
--Guido van Rossum (home page: http://www.python.org/~guido/)