[Pyparsing] RE: python indentation grammar

[Paul M. <pa...@al...>]

Michel -

Not so much global data, as it is parsing state preserved inside the
pyparsing class instances (namely the cacheing of exception instances).  I
am fairly certain that calling parseString is not thread-safe, and you
should interlock calls to it if you have multiple threads calling it.

I have made a few attempts at indentation-based parsing in the past, but I
looked at them last night, and they are really not so good.  I think the key
will be in a) using a parse action with col() to detect the indentation
level of the current line, and b) keeping a global stack of indentations
levels seen thus far, so that you can tell if your current line is part of
the current indent level, a deeper level or a higher level. 

When creating your test cases, be sure to add unfriendly tests, such as
nested levels that unwind to a higher nesting than just the immediate
parent.  That is:

A
  A1
  A2
     A2a
         A2aa
         A2ab
     A2b
         A2ba
  A3

Since there is no A2c entry (to be a peer of A2a and A2b), your parser will
end up doing a double pop from the indentation stack.

Also, what would this data signify?

A
  A1
  A2
     A2a
         A2aa
         A2ab
     A2b
         A2ba
   A2.5
  A3

Note that A2.5 is more indented than A2 and A3, but less indented than A2a
and A2b.  I'm guessing this case should probably be an error (and if you
detect it in a parse action, you should raise ParseFatalException instead of
simple ParseException, to halt parsing immediately).

-- Paul