Pyparsing error when evaluating WFF logic expressions?

Question

I'm new to Python and pyparsing, and I'm making a logic expression evaluator.

The formula must be a WFF. The BNF of WFF is:

<alpha set> ::= p | q | r | s | t | u | ... 
(the arbitrary finite set of propositional variables)   

<form> ::= <alpha set> | ¬<form> | (<form>V<form>) | (<form>^<form>) 
           | (<form> -> <form>) | (<form> <-> <form>)

My code is:

'''
Created on 17/02/2012

@author: Juanjo

'''

from pyparsing import *
from string import lowercase

def fbf():
    atom  = Word(lowercase, max=1) #aphabet
    op = oneOf('^ V => <=>') #Operators
    identOp = oneOf('( [ {')
    identCl = oneOf(') ] }')
    form = Forward() #Iniciar de manera recursiva
    #Grammar:
    form << ( (Group(Literal('~') + form)) | ( Group(identOp + form + op + form + identCl) ) | ( Group(identOp + form + identCl) ) | (atom) )

    return form

entrada = raw_input("Input please: ") #userinput
print fbf().parseString(entrada)

The problem is when I use these expressions: a^b and aVb.

The parser should return an error, but there's no error; instead it returns a. Actually, any symbol after a will be ignored.

The WFF version of those forms are: (a^b) and (aVb)

Both work correctly. I think the problem is in the atom definition.

What am I doing wrong?

Answer 1

By default parseString will just parse the beginning of the string.

You can force it to parse the entire string by changing the code to:

print fbf().parseString(entrada, parseAll=True)

Alternatively, you can end the grammar with the StringEnd() token - see the documentation under parseString in http://packages.python.org/pyparsing/ for more details.