Re: [Pyparsing] How to distinguish a variable from a integer

[Gustavo N. <me...@gu...>]

Paul said:
> > Hello, everybody.
> >
> > First of all, I wanted to thank you for this awesome package. I'm having
> > fun with it. :)
>
> Well, well, my friend, so we meet again!  I'm pleased to see you have been
> bitten by the pyparsing bug. :)

Hello, Paul!

Good to see you here :)

> > How can I fix this?
>
> In general, I think this is why variable names in most computing languages
> I know do *not* permit the name to begin with a number.  But you are the
> language designer, so I will show you how to do this in pyparsing.
>
> Two suggestions, not sure if I have a preference:
> 1. use "operand = number ^ variable" instead of "operand = number |
> variable".  '|' returns MatchFirst expressions, which return, well, the
> first matching expression.  '^' returns Or expressions, which return
> *longest* match of all the alternative expressions.  Think of the '^' as a
> little set of dividers, measuring the returned values of all the
> expressions, and picking the longest.  '^' is not a cure-all, though, and
> can cause infinite run-time recursion in self-referencing grammars (those
> that include operatorPrecedence or Forward expressions).

I use both operatorPrecedence and Forward :/


> 2. As you say, invert operand to "operand = variable | number", and then
> attach a parse action to variable that first tries to evaluate the result
> as a number.  In your current parser, you may eventually attach a parse
> action to number, something like this:
> number.setParseAction(lambda tokens: float(tokens[0]))
> so that at post-parse time, the returned string has already been converted
> to a float.  So instead, attaching something like this to variable
> (untested):
>
> def numOrVar(tokens):
>     try:
>         return float(tokens[0])
>     except ValueError:
>         pass
> variable.setParseAction(numOrVar)
>
> Now you don't even need the alternation, since as you observed, variable
> will also match "22", so just define "operand = variable".
> You could also try this for defining variable:
>
> variable = Word(unicode(alphanums+'_'))
>
> or
>
> variable = Word(unicode(alphanums+alphas8bit+'_'))
>
> or to absolutely cover all bases (for 2-byte Unicode, anyway):
>
> allUnicodeAlphas = u''.join(c for c in map(unichr,range(65536)) if
> c.isalpha())
> allUnicodeNums = = u''.join(c for c in map(unichr,range(65536)) if
> c.isdigit())
> variable = Word(allUnicodeAlphas + allUnicodeNums + u'_')
>
> (It's surprising how many Unicode digits there are besides '0'-'9'.)

I said that an operand could be a variable or a number to simplify things 
given that the problem was between numbers and variables. But it's actually 
more complex than that: It could be a quoted string or a set (in the form 
"{element1, element2, ...}" where each element can be a number, variable, 
quoted string or even another set) too:

operand = number | string | variable | set

Therefore setting a parse action for the whole operand wouldn't be desirable, 
I'd rather set it in the types individually -- specially to be able to test 
them separately too.

Sorry for not pointing this out.


>
>
> BTW, this definition of decimals:
>
> decimals = Optional(decimal_sep + OneOrMore(Word(nums)))
>
> includes some unnecessary repetition.  It should be sufficient to write:
>
> decimals = Optional(decimal_sep + Word(nums))
>
> Unless I misunderstood your intent here.

Thank you so much! I thought I had to set the quantifier explicitly.


> So, will we see some pyparsing sneak into a repoze package one of these
> days, perhaps some sort of authorization rights syntax, hmmm?

You guessed right! :)

I'm working on a package called PyACL, which as the name implies, implements 
Access Control Lists in Python (and repoze.what 2 will use it a lot). But one 
of the things that I was missing was the way to allow system administrators to 
filter the access rules easily, so I started working on this generic 
Pyparsing-based library which I'll announce here as soon as it's usable:
https://launchpad.net/booleano


> Buena suerte, y mucho gusto!

¡Lo mismo digo! ;-)

Thank you! =)
-- 
Gustavo Narea <xri://=Gustavo>.
| Tech blog: =Gustavo/(+blog)/tech  ~  About me: =Gustavo/about |