Naive JSON parser

json_parser.py

#!/usr/bin/env python

import re
import sys

HEX_DIGITS = '0123456789abcdefABCDEF'
NUMBER_RE = re.compile(r'-?(?:0|[1-9]\d*)(?:\.\d+)?(?:[eE][+-]?\d+)?')

class ParseError(Exception):
    pass

def w(s):
    """Remove leading whitespace characters of a string."""

    return s.lstrip()

def n(s):
    """Return the first non-whitespace character of a string,
    or ``None`` if it is empty."""

    s = w(s)
    if len(s) == 0:
        return None
    else:
        return s[0]

def chars(c, s):
    s = w(s)
    if s.startswith(c):
        return s[len(c):]
    else:
        raise ParseError(s)

def hex4(s):
    s = w(s)
    if len(s) < 4:
        raise ParseError(s)
    elif all(c in HEX_DIGITS for c in s[:4]):
        return s[4:]
    else:
        raise ParseError(s)

def number(s):
    s = w(s)
    match = NUMBER_RE.match(s)
    if match:
        return chars(match.group(), s)
    else:
        raise ParseError(s)

def in_array(s):
    s = w(s)
    s = value(s)
    if n(s) == ']':
        return s
    elif n(s) == ',':
        s = chars(',', s)
        return in_array(s)
    else:
        raise ParseError(s)

def array(s):
    s = w(s)
    s = chars('[', s)
    if n(s) != ']': s = in_array(s)
    return chars(']', s)

def in_string(s):
    s = w(s)
    if n(s) == '"':
        return s
    elif n(s) == '\\':
        s = chars('\\', s)
        if n(s) in '"\\/bfnrt':
            s = chars(n(s), s)
        elif n(s) == 'u':
            s = chars('u', s)
            s = hex4(s)
        else:
            raise ParseError(s)
    elif n(s) is None:
        raise ParseError(s)
    else:
        s = chars(n(s), s)
    return in_string(s)

def string(s):
    s = w(s)
    s = chars('"', s)
    s = in_string(s)
    return chars('"', s)

def value(s):
    s = w(s)
    if n(s) == '"':
        return string(s)
    elif n(s) in '-0123456789':
        return number(s)
    elif n(s) == '{':
        return obj(s)
    elif n(s) == '[':
        return array(s)
    elif n(s) == 't':
        return chars('true', s)
    elif n(s) == 'f':
        return chars('false', s)
    elif n(s) == 'n':
        return chars('null', s)
    else:
        raise ParseError(s)

def binding(s):
    s = w(s)
    s = string(s)
    s = chars(':', s)
    return value(s)

def in_obj(s):
    s = w(s)
    s = binding(s)
    if n(s) == '}':
        return s
    elif n(s) == ',':
        s = chars(',', s)
        return in_obj(s)
    else:
        raise ParseError(s)

def obj(s):
    s = w(s)
    s = chars('{', w(s))
    if n(s) != '}': s = in_obj(s)
    s = chars('}', s)
    return s

def empty(s):
    s = w(s)
    if s != '': raise ParseError(s)
    return s

def parse(s):
    s = w(s)
    s = obj(s)
    return empty(s)

if __name__ == '__main__':
    s = ''.join((line for line in sys.stdin))
    parse(s)