[Python-Dev] Modifying Grammar/grammar and other foul acts (original) (raw)

Guido van Rossum guido at python.org
Sat Mar 6 22:41:01 CET 2010

On Sat, Mar 6, 2010 at 10:26 AM, Gregg Lind <gregg.lind at gmail.com> wrote:

Sorry, re: question one, forgive the ill-formed question.  I meant more, are the parser rules applied "first matching".   Essentially trying to confirm that the parser is "top down" or "bottom up" or whether or not it even matters.

That's not how it works at all. I can't explain it in a few words -- but any text on LL(1) parsing should clarify this. The parser uses no backtracking and a 1-token lookahead. The only unusual thing is that individual rules use a regex-like notation, but that is all converted to a DFA. If one token is not enough to know which path to take through the DFA (this may invoke another rule -- but you always know which one) you're hosed.

I suspect you've introduced ambiguities, though I don't immediately see where (they could be in the combination of different rules).

Another possibility is that you may be running into problems where the parser expects a newline at the end of a suite.

(FWIW since you're not proposing a language change, this is technically off-topic for python-dev. :-)

--Guido

Thanks for the tip -- it seems to be exactly what I want.  To make it explicit, this seems to be fuller (unix) recipe for how to make this style of debugging happen.

$ ./configure --with-pydebug $ make $ set PYTHONDEBUG=1 $ ./python -d   # then this shows the parsing info

On Sat, Mar 6, 2010 at 10:56 AM, "Martin v. Löwis" <martin at v.loewis.de> wrote:

> 1.)  I assume the Grammar/grammar is read top to bottom.  Confirm? Confirm - but this is not surprising: any source file is typically read from top to bottom. Randoma access reading is typically done for binary files, only. So you must be asking something else, but I can't guess what that might be. > 2.)  More help figuring out how to debug what python thinks it's > seeing when it see "def a() {pass}".  It's not getting to the ast > construction stage, as near as I can tell.  What additional breakpoints > can I set to see where it's failing. Enable the D() debugging in parser.c (configure with --with-pydebug, and set PYTHONDEBUG). Regards, Martin

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-- --Guido van Rossum (python.org/~guido)

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