json_parser.py

# ---------------
# User Instructions
#
# In this problem, you will be using many of the tools and techniques
# that you developed in unit 3 to write a grammar that will allow
# us to write a parser for the JSON language. 
#
# You will have to visit json.org to see the JSON grammar. It is not 
# presented in the correct format for our grammar function, so you 
# will need to translate it.

# ---------------
# Provided functions
#
# These are all functions that were built in unit 3. They will help
# you as you write the grammar.  Add your code at line 102.

from functools import update_wrapper
from string import split
import re

def grammar(description, whitespace=r'\s*'):
    """Convert a description to a grammar.  Each line is a rule for a
    non-terminal symbol; it looks like this:
        Symbol =>  A1 A2 ... | B1 B2 ... | C1 C2 ...
    where the right-hand side is one or more alternatives, separated by
    the '|' sign.  Each alternative is a sequence of atoms, separated by
    spaces.  An atom is either a symbol on some left-hand side, or it is
    a regular expression that will be passed to re.match to match a token.
    
    Notation for *, +, or ? not allowed in a rule alternative (but ok
    within a token). Use '\' to continue long lines.  You must include spaces
    or tabs around '=>' and '|'. That's within the grammar description itself.
    The grammar that gets defined allows whitespace between tokens by default;
    specify '' as the second argument to grammar() to disallow this (or supply
    any regular expression to describe allowable whitespace between tokens)."""
    G = {' ': whitespace}
    description = description.replace('\t', ' ') # no tabs!
    for line in split(description, '\n'):
        lhs, rhs = split(line, ' => ', 1)
        alternatives = split(rhs, ' | ')
        G[lhs] = tuple(map(split, alternatives))
    return G

def decorator(d):
    "Make function d a decorator: d wraps a function fn."
    def _d(fn):
        return update_wrapper(d(fn), fn)
    update_wrapper(_d, d)
    return _d

@decorator
def memo(f):
    """Decorator that caches the return value for each call to f(args).
    Then when called again with same args, we can just look it up."""
    cache = {}
    def _f(*args):
        try:
            return cache[args]
        except KeyError:
            cache[args] = result = f(*args)
            return result
        except TypeError:
            # some element of args can't be a dict key
            return f(args)
    return _f

def parse(start_symbol, text, grammar):
    """Example call: parse('Exp', '3*x + b', G).
    Returns a (tree, remainder) pair. If remainder is '', it parsed the whole
    string. Failure iff remainder is None. This is a deterministic PEG parser,
    so rule order (left-to-right) matters. Do 'E => T op E | T', putting the
    longest parse first; don't do 'E => T | T op E'
    Also, no left recursion allowed: don't do 'E => E op T'"""

    tokenizer = grammar[' '] + '(%s)'

    def parse_sequence(sequence, text):
        result = []
        for atom in sequence:
            tree, text = parse_atom(atom, text)
            if text is None: return Fail
            result.append(tree)
        return result, text

    @memo
    def parse_atom(atom, text):
        if atom in grammar:  # Non-Terminal: tuple of alternatives
            for alternative in grammar[atom]:
                tree, rem = parse_sequence(alternative, text)                
                if rem is not None: return [atom]+tree, rem  
            return Fail
        else:  # Terminal: match characters against start of text           
            print "regex = " + atom + " text = " + text
            m = re.match(tokenizer % atom, text)
            return Fail if (not m) else (m.group(1), text[m.end():])
    
    # Body of parse:
    return parse_atom(start_symbol, text)

Fail = (None, None)

JSON = grammar(r"""object => \{ members \} | \{ \}
members => pair , members | pair
pair => string : value
array => \[ elements \] | \[ \]
elements => value , elements | value 
value => string | number | object | array | true | false | null 
string => "[^"\\]*"
number => int frac exp | int frac | int exp | int 
int => -?[1-9]+
frac => [.][0-9]+ 
exp => [Ee][+-]?[1-9]+""", whitespace='\s*')

def json_parse(text):
    result =  parse('value', text, JSON)
    print result
    return result

def test():
    assert json_parse('["testing", 1, 2, 3]') == (                      
                       ['value', ['array', '[', ['elements', ['value', 
                       ['string', '"testing"']], ',', ['elements', ['value', ['number', 
                       ['int', '1']]], ',', ['elements', ['value', ['number', 
                       ['int', '2']]], ',', ['elements', ['value', ['number', 
                       ['int', '3']]]]]]], ']']], '')
    
    assert json_parse('-123.456e+789') == (
                       ['value', ['number', ['int', '-123'], ['frac', '.456'], ['exp', 'e+789']]], '')
    
    assert json_parse('{"age": 21, "state":"CO","occupation":"rides the rodeo"}') == (
                      ['value', ['object', '{', ['members', ['pair', ['string', '"age"'], 
                       ':', ['value', ['number', ['int', '21']]]], ',', ['members', 
                      ['pair', ['string', '"state"'], ':', ['value', ['string', '"CO"']]], 
                      ',', ['members', ['pair', ['string', '"occupation"'], ':', 
                      ['value', ['string', '"rides the rodeo"']]]]]], '}']], '')
    return 'tests pass'

print test()