[Python-Dev] Modifying Grammar/grammar and other foul acts

[Georg Brandl]

Am 09.03.2010 14:42, schrieb Jeremy Hylton:
> On Sat, Mar 6, 2010 at 11:27 AM, Gregg Lind <gregg.lind at gmail.com> wrote:
>> Python-devs,
>>
>> I'm writing to you for some help in understanding the Python grammar.  As an
>> excuse to deep dive into Python's tokenizer / grammar, I decided (as a
>> hideous, hideous joke) to want to allow braces where colons are allowed (as
>> flow control).
>>
>> Starting from PEP 306 (and branch r311), I hacked on Grammar/Grammer
>>
>> As a first example:
>>
>> funcdef: ('def' NAME parameters ['->' test] ':' suite |
>>           'def' NAME parameters ['->' test] '{' suite '}' )
>>
>> I reran Parser/pgen and the dfa changes, but python (3.1) when recompiled,
>> throws errors on things like:
>>
>> def a() { None }
>>
>> Strangely enough:
>>
>> lambdef: ( 'lambda' [varargslist] ':' test  |
>>                'lambda' [varargslist] '{' test '}' )
>>
>> works fine!  I this simplely some difference between "test" and "suite".
>>
>> I have tried tackling this with gdb, looking at err_input clearly isn't
>> enough.
>>
>>     (gdb) break err_input
>>     (gdb) break PyParser_ASTFromString
>>     import sys
>>     b = compile("def a() {pass}","sys.stdout","single")
>>     # yet a simple grammar fix is enough for this!
>>     c = compile("lambda x {None}","sys.stdout","single")
>>
>> I'm in over my head!
> 
> You don't say what errors occur when you try to compile strings in
> your new language.  You may have changed the Grammar, which allows you
> to tokenize the input.  That isn't enough to get the input to compile.
>  You also need to change the compiler to understand the new tokens.

In particular, many AST creation functions check for specific counts of
children on many nodes.   I haven't checked, but in the case of the
"funcdef" rule, it may check for either 7 or 5 children to determine
whether the optional return annotation ['->' test] is present.

Georg


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