Parser "if then else" expression

ifthenelse.py

# coding: utf-8

from funcparserlib.lexer import make_tokenizer, Spec
from funcparserlib.parser import (maybe, many, eof, skip, oneplus, fwd,
        name_parser_vars, memoize, SyntaxError)
from funcparserlib.util import pretty_tree
from funcparserlib.contrib.common import unarg, flatten, n, op, op_, sometoks
from funcparserlib.contrib.lexer import (make_comment, make_multiline_comment,
        newline, space)


class Template(object):
    def __init__(self, *args):
        print self.__class__.__name__, args
        self.args = args


class Expr(object):
    def __init__(self, *args):
        print self.__class__.__name__, args
        self.args = args


class IF(Template):
    pass


class THEN(Template):
    pass


class ELSE(Template):
    pass


def tokenize(str):
    specs = [
        space,
        Spec('name',    r'[A-Za-z\200-\377_][A-Za-z\200-\377_0-9]*'),
        Spec('op',      r'[;,\\\[\]:=\.\'"\(\)]'),
        Spec('number',  r'-?(\.[0-9]+)|([0-9]+(\.[0-9]*)?)')
    ]
    useless = ['space']
    t = make_tokenizer(specs)

    return [x for x in t(str) if x.type not in useless]


def get_template():
    """
        stmt -> (matched_stmt | unmathced_stmt)
        matched_stmt -> ((if expr then matched_stmt else matched_stmt) | other)
        unmathced_stmt -> ((if expr then stmt) | (if expr then matched_stmt else unmathced_stmt))
    """
    id = sometoks(['name', 'number'])
    expr = oneplus(id)
    unarg2 = lambda f: lambda args: f(*args) if isinstance(args, list) else f(*[args])

    matched_stmt = fwd()
    unmatched_stmt = fwd()

    _if = memoize(n('if'))
    _then = memoize(n('then'))
    _else = memoize(n('else'))

    stmt = (matched_stmt | unmatched_stmt) >> unarg2(Template)

    exp = expr >> unarg2(Expr)

    if_m = _if + exp >> unarg2(IF)
    then_m = _then + matched_stmt >> unarg2(THEN)
    else_m = _else + matched_stmt >> unarg2(ELSE)

    ifthenelse_m = if_m + then_m + else_m >> unarg2(Template)

    matched_stmt.define((ifthenelse_m | exp) >> unarg2(Template))

    then_unm = _then + stmt >> unarg2(THEN)
    else_unm = _else + unmatched_stmt >> unarg2(ELSE)

    ifthen_unm = if_m + then_unm >> unarg2(Template)
    ifthenelse_unm = if_m + then_m + else_unm >> unarg2(Template)

    unmatched_stmt.define((ifthen_unm | ifthenelse_unm) >> unarg2(Template))

    return stmt


template = get_template()


def parse(seq):
    return template.parse(seq)


def main():
    sql = 'if A then if B then C else if D then E else 42'

    parse(tokenize(sql))


if __name__ == '__main__':
    main()