bnf.py

import inspect
import ast


def terminal(s):
    return "<%s>" % s


def define_name(o):
    return terminal(o.id) if isinstance(o, ast.Name) else o.s


def define_expr(op):
    if isinstance(op, ast.BinOp) and isinstance(op.op, ast.Add):
        return (*define_expr(op.left), define_name(op.right))
    return (define_name(op),)


def get_alternatives(op, to_expr=lambda o: o.s):
    if isinstance(op, ast.BinOp) and isinstance(op.op, ast.BitOr):
        return get_alternatives(op.left, to_expr) + [to_expr(op.right)]
    return [to_expr(op)]


def funct_parser(tree, to_expr=lambda o: o.s):
    return {
        assign.targets[0].id: get_alternatives(assign.value, to_expr)
        for assign in tree.body[0].body
    }


def define_grammar(fn, to_expr=lambda o: o.s):
    source = inspect.getsource(fn)
    tree = ast.parse(source)
    grammar = funct_parser(tree, to_expr)
    return grammar


def define_ex_grammar(fn):
    return define_grammar(fn, define_expr)


@define_ex_grammar
def expression_grammar():
    start = expr
    expr = term + "+" + expr | term + "-" + expr
    term = factor + "*" + term | factor + "/" + term | factor
    factor = (
        "+" + factor
        | "-" + factor
        | "(" + expr + ")"
        | integer + "." + integer
        | integer
    )
    integer = digit + integer | digit
    digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"


if __name__ == "__main__":
    for symbol in expression_grammar:
        print(symbol, "::=\n\t|", '\n\t| '.join(
            [' '.join(i) for i in expression_grammar[symbol]]))