[Python] brainf**k interpreter with no AST

bf.py

import sys

class Engine(object):
    """Brainf**k interpreter.
    usage:
      bf = Engine()
      bf("++++++++++++++.")
    """
    def __init__(self):
        self.insts = {
            ">": self._pinc,
            "<": self._pdec,
            "+": self._vinc,
            "-": self._vdec,
            ".": self._putc,
            ",": self._getc,
            "[": self._jump,
            "]": self._loop,
            }
        pass

    def __call__(self, code="",
                 input=sys.stdin, output=sys.stdout, bufsize=30000):
        env = _Env(input, output, bufsize)
        ip = 0
        while ip < len(code):
            inst = self.insts.get(code[ip], self._noop)
            ip = inst(env, code, ip)
            pass
        return env

    def _noop(self, env, code, ip): return ip + 1
    def _pinc(self, env, code, ip): env.pinc(); return ip + 1
    def _pdec(self, env, code, ip): env.pdec(); return ip + 1
    def _vinc(self, env, code, ip): env.vinc(); return ip + 1
    def _vdec(self, env, code, ip): env.vdec(); return ip + 1
    def _putc(self, env, code, ip): env.putc(); return ip + 1
    def _getc(self, env, code, ip): env.getc(); return ip + 1
    def _jump(self, env, code, ip):
        if env.value() != 0: return ip + 1
        opens = 0
        while opens >= 0:
            ip += 1
            w = code[ip]
            if w == "]": opens -= 1
            if w == "[": opens += 1
            pass
        return ip + 1
    def _loop(self, env, code, ip):
        if env.value() == 0: return ip + 1
        closes = 0
        while closes >= 0:
            ip -= 1
            w = code[ip]
            if w == "]": closes += 1
            if w == "[": closes -= 1
            pass
        return ip + 1
    pass

class _Env(object):
    def __init__(self, input, output, bufsize):
        self.input = input
        self.output = output
        self.buf = [0] * bufsize
        self.index = 0
        pass
    def value(self):
        return self.buf[self.index]

    def pinc(self): self.index += 1
    def pdec(self): self.index -= 1
    def vinc(self): self.buf[self.index] += 1
    def vdec(self): self.buf[self.index] -= 1
    def putc(self): self.output.write(chr(self.buf[self.index]))
    def getc(self): self.buf[self.index] = ord(self.input.read(1))
    pass


if __name__ == "__main__":
    code = """
    +++++++++[>++++++++>+++++++++++>+++++<<<-]>.>++.+++++++..+++.>-.
    ------------.<++++++++.--------.+++.------.--------.>+.
    >++++++++++.
    """ # "Hello, world!\n" from Wikipedia
    if len(sys.argv) > 1: code = sys.argv[1]

    engine = Engine()
    engine(code)
    pass