ebnf-demo.py

from rpython.rlib.parsing.ebnfparse import parse_ebnf, make_parse_function
from rpython.rlib.parsing.parsing import ParseError
from rpython.rlib.parsing.deterministic import LexerError

EBNF = """
IGNORE: " |\n";
NUMBER: "\-?(0|[1-9][0-9]*)(\.[0-9]+)?";
expr : NUMBER >(arith_op NUMBER)+< | <NUMBER>;
arith_op: <"+"> | <"-">;
main: <expr> [EOF];
"""

regexes, rules, _to_ast = parse_ebnf(EBNF)
parse_ebnf = make_parse_function(regexes, rules, eof=True)
to_ast = _to_ast()

def eval(ast):
    if ast.symbol == 'main':
        expr = ast.children[0]
        return eval(expr);
    elif ast.symbol == 'expr':
        term = eval(ast.children[0])
        for i in range(1, len(ast.children[1:]), 2):
            op, term_i = ast.children[i], eval(ast.children[i+1])
            if op.additional_info == '+':
                term = term + term_i
            elif op.additional_info == '-':
                term = term - term_i
        return term
    elif ast.symbol == 'NUMBER':
        if ast.additional_info.startswith('-'):
            return -1 * float(ast.additional_info[1:])
        else:
            return float(ast.additional_info)
    else:
        print(ast)
        raise ValueError('unknown ast.')

def run(source):
    tree = parse_ebnf(source)
    ast = to_ast.transform(tree)
    print(eval(ast))

if __name__ == '__main__':
    while True:
        try:
            print '>',
            expr = run(raw_input())
        except Exception as e:
            print(e)

$ virtualenv venv
$ source venv/bin/activate
$ pip install rpython
$ python ebnf-demo.py
> 1 + 1 + 2
4.0
> -1 + -1 + -2
-4.0
> -1+-1+-1
-3.0
> ^C