On Feb 19, 2008 1:38 PM, Andrew Dalke <[EMAIL PROTECTED]> wrote: > I'm finishing up a PLY lexer and parser for the current CVS version of > the Python grammar. As part of it I've been testing a lot of dark > corners in the grammar definition and implementation. Python 2.5 has > some small and rare problems which I'm pleased to note have been > pretty much fixed in Python 2.6. > > I have two questions about the grammar definition. Here's a > difference between 2.5 and 2.6. > > % cat x.py > c = "This is 'c'" > def spam((a) = c): > print a > spam() > > % python2.5 x.py > This is 'c' > % python2.6 x.py > File "x.py", line 2 > def spam((a) = c): > SyntaxError: non-default argument follows default argument > > > I don't understand why that's changed. This shouldn't be a > SyntaxError and there is no non-default argument following the default > argument. >
The error might be odd, but I don't see why that should be allowed syntax. Having a parameter surrounded by a parentheses like that makes no sense in a context of a place where arbitrary expressions are not allowed. > Note that this is still valid > > >>> def spam((a,) = c): > ... pass > ... > > I think 2.6 is incorrect. According to the documentation at > http://docs.python.org/ref/function.html > > defparameter ::= parameter ["=" expression] > sublist ::= parameter ("," parameter)* [","] > parameter ::= identifier | "(" sublist ")" > funcname ::= identifier Looking at Grammar/Grammar, the relevant rules are:: parameters: '(' [varargslist] ')' varargslist: ((fpdef ['=' test] ',')* ('*' NAME [',' '**' NAME] | '**' NAME) | fpdef ['=' test] (',' fpdef ['=' test])* [',']) fpdef: NAME | '(' fplist ')' fplist: fpdef (',' fpdef)* [','] >From what I can tell the grammar does not prevent it. But it is possible that during AST creation or bytecode compilation a specific check is made that is throwing the exception... And checking Python.ast.c:672 seems to suggest it is something the AST is doing. I don't have time to dig deeper, but if you tried ``svn blame`` and found out when that line was added the log message give some clues as to why this is occurring. > > > Second question is about trailing commas in list comprehension. I > don't understand why the commented out line is not allowed. > > [x for x in 1] > #[x for x in 1,] # This isn't legal > [x for x in 1,2] > [x for x in 1,2,] > [x for x in 1,2,3] > > The Grammar file says > > # Backward compatibility cruft to support: > # [ x for x in lambda: True, lambda: False if x() ] > # even while also allowing: > # lambda x: 5 if x else 2 > # (But not a mix of the two) > testlist_safe: old_test [(',' old_test)+ [',']] > > but if I change it to also allow > > testlist_safe : old_test ',' > > then PLY still doesn't report any ambiguities in the grammar and I > can't find an expression that exhibits a problem. > > Could someone here enlighten me? > Are you asking why the decision was made to make the expression illegal, or why the grammar is flagging it is wrong? -Brett _______________________________________________ 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
