Python Lisp S-Expression interpreter

pylisp.py

#!/usr/bin/env python3

# Python Lisp S-Expression interpreter
#
# Example:
# ./pylisp.py "(+ (* 2 3) 1)" executes (2 * 3) + 1 and yields 7
#
# currently only + - / + and = are supported operations

import sys

class Lisp:

    def __init__(self):
        setattr(self, "+", lambda args: sum([int(a) for a in args]))
        setattr(self, "-", self.sub)
        setattr(self, "*", self.mul)
        setattr(self, "/", self.div)
        setattr(self, "=", lambda args: "nil" if False in [str(args[0]) == str(i) for i in args] else "T")

    def sub(self, args):
        if len(args) > 1:
            return int(args[0]) - self.add(args[1:])
        elif len(args) == 1:
            return -int(args[0])
        else:
            return 0

    def mul(self, args):
        x = int(bool(len(args)))
        for a in args:
            x *= int(a)
        return x

    def div(self, args):
        if len(args) == 1:
            return 1 / int(args[0])
        elif len(args) > 1:
            return int(args[0]) / self.mul(args[1:])
        else:
            raise RuntimeError("div requires at least one argument")

    def eval(self, expr):
        l = [self.eval(l) if isinstance(l, list) else l for l in expr]
        return getattr(self, l[0])(l[1:])

    def parse(self, expr):
        return self.__parse(expr[1:-1])

    def __parse(self, expr):
        e = expr.strip(" ")
        if len(e) < 1:
            return "nil"
        t = []
        if e[0] == "(":
            d = 0
            r = ""
            for c in e:
                if c == "(":
                    d += 1
                elif c == ")":
                    d -= 1
                r += c
                if d == 0:
                    t.append(self.__parse(r[1:-1]))
                    if len(e[len(r):]):
                        t.extend(self.__parse(e[len(r):]))
                    break
        else:
            if " " in e:
                t.append(e[:e.index(" ")])
                t.extend(self.__parse(e[e.index(" ") + 1:]))
            else:
                return [e]
        if t:
                return t
        else:
            return "nil"

if __name__ == "__main__":
    l = Lisp()
    for arg in sys.argv[1:]:
        print(l.eval(l.parse(arg)))