Superficial Reflections ⟶ programming

#include <iostream>

int main(int argc, char **argv)
{
    int x = 10;

    // x goes to 0
    while (x --> 0)
        std::cout << x << std::endl;
}

class Spam(object):
    def __init__(self, value):
        self.value = value

    @property
    def spam(self):
        return self.value

    @spam.setter
    def spam(self, value):
        self.value = value

spam = Spam(42)
print spam.spam
spam.spam = 23
print spam.spam

>>> if 0:
...     yield
...
>>> if 1:
...     yield
  File "<input>", line 2
SyntaxError: 'yield' outside function (<input>, line 2)

import inspect, pprint, types, dis, struct, opcode, array
short = struct.Struct('<H')

class Label(object):
    pass

class Code(object):
    @classmethod
    def from_code(cls, code_obj):
        self = cls()
        self.code_obj = code_obj
        self.names = list(code_obj.co_names)
        self.varnames = list(code_obj.co_varnames)
        self.consts = list(code_obj.co_consts)
        ret = []
        line_starts = dict(dis.findlinestarts(code_obj))
        code = code_obj.co_code
        labels = dict((addr, Label()) for addr in dis.findlabels(code))
        i, l = 0, len(code)
        extended_arg = 0
        while i < l:
            op = ord(code[i])
            if i in labels:
                ret.append(('MARK_LABEL', labels[i]))
            if i in line_starts:
                ret.append(('MARK_LINENO', line_starts[i]))
            i += 1
            if op >= opcode.HAVE_ARGUMENT:
                arg, = short.unpack(code[i:i + 2])
                arg += extended_arg
                extended_arg = 0
                i += 2
                if op == opcode.EXTENDED_ARG:
                    extended_arg = arg << 16
                    continue
                elif op in opcode.hasjabs:
                    arg = labels[arg]
                elif op in opcode.hasjrel:
                    arg = labels[i + arg]
            else:
                arg = None
            ret.append((opcode.opname[op], arg))
        self.code = ret
        return self

    def to_code(self):
        code_obj = self.code_obj
        co_code = array.array('B')
        co_lnotab = array.array('B')
        label_pos = {}
        jumps = []
        lastlineno = code_obj.co_firstlineno
        lastlinepos = 0
        for op, arg in self.code:
            if op == 'MARK_LABEL':
                label_pos[arg] = len(co_code)
            elif op == 'MARK_LINENO':
                incr_lineno = arg - lastlineno
                incr_pos = len(co_code) - lastlinepos
                lastlineno = arg
                lastlinepos = len(co_code)

                if incr_lineno == 0 and incr_pos == 0:
                    co_lnotab.append(0)
                    co_lnotab.append(0)
                else:
                    while incr_pos > 255:
                        co_lnotab.append(255)
                        co_lnotab.append(0)
                        incr_pos -= 255
                    while incr_lineno > 255:
                        co_lnotab.append(incr_pos)
                        co_lnotab.append(255)
                        incr_pos = 0
                        incr_lineno -= 255
                    if incr_pos or incr_lineno:
                        co_lnotab.append(incr_pos)
                        co_lnotab.append(incr_lineno)
            elif arg is not None:
                op = opcode.opmap[op]
                if op in opcode.hasjabs or op in opcode.hasjrel:
                    jumps.append((len(co_code), arg))
                    arg = 0
                if arg > 0xffff:
                    co_code.extend((opcode.EXTENDED_ARG,
                        (arg >> 16) & 0xff, (arg >> 24) & 0xff))
                co_code.extend((op,
                    arg & 0xff, (arg >> 8) & 0xff))
            else:
                co_code.append(opcode.opmap[op])

        for pos, label in jumps:
            jump = label_pos[label]
            if co_code[pos] in opcode.hasjrel:
                jump -= pos + 3
            assert jump <= 0xffff
            co_code[pos + 1] = jump & 0xff
            co_code[pos + 2] = (jump >> 8) & 0xff

        return types.CodeType(code_obj.co_argcount, code_obj.co_nlocals,
            code_obj.co_stacksize, code_obj.co_flags, co_code.tostring(),
            tuple(self.consts), tuple(self.names), tuple(self.varnames),
            code_obj.co_filename, code_obj.co_name, code_obj.co_firstlineno,
            co_lnotab.tostring(), code_obj.co_freevars, code_obj.co_cellvars)

    def const_idx(self, val):
        try:
            return self.consts.index(val)
        except ValueError:
            self.consts.append(val)
            return len(self.consts) - 1

def tail_call(func):
    code = Code.from_code(func.func_code)
    func_name = func.__name__
    if func_name in code.varnames:
        raise SyntaxError('"%s" was found as a local variable in the function' %
            func_name)
    try:
        name_idx = code.names.index(func_name)
    except IndexError:
        raise SyntaxError('"%s" not found in function\'s global names' %
            func_name)
    last_idx = 0
    func_start = Label()
    code.code.insert(0, ('MARK_LABEL', func_start))
    while True:
        try:
            lglobal_idx = code.code.index(('LOAD_GLOBAL', name_idx), last_idx)
        except ValueError:
            break

        if code.code[lglobal_idx - 1][0] != 'MARK_LINENO':
            last_idx = lglobal_idx + 1
            continue

        try:
            return_idx = code.code.index(('RETURN_VALUE', None), lglobal_idx)
        except ValueError:
            raise SyntaxError('"return" not found in function after "%s"' %
                func_name)

        if (return_idx != len(code.code) - 1
                and code.code[return_idx + 1][0] != 'MARK_LINENO'):
            last_idx = return_idx + 1
            continue

        if code.code[return_idx - 1][0] in ('CALL_FUNCTION_VAR',
                'CALL_FUNCTION_KW', 'CALL_FUNCTION_VAR_KW'):
            raise SyntaxError('calling with *a and/or **kw is unsupported')

        if code.code[return_idx - 1][0] != 'CALL_FUNCTION':
            last_idx = return_idx + 1
            continue

        if code.code[return_idx - 1][1] & 0xff00:
            raise SyntaxError('calling with keyword arguments is unsupported')

        arg_names, _, _, defaults = inspect.getargspec(func)
        n_args = code.code[return_idx - 1][1]
        if defaults is None:
            defaults = ()
        if n_args + len(defaults) < len(arg_names):
            raise SyntaxError('not enough arguments provided')

        new_bytecode = []
        if n_args < len(arg_names):
            new_bytecode.extend(
                ('LOAD_CONST', code.const_idx(d))
                for d in defaults[n_args - len(arg_names):])
        new_bytecode.extend(
            ('STORE_FAST', code.varnames.index(arg))
            for arg in reversed(arg_names))
        new_bytecode.append(('JUMP_ABSOLUTE', func_start))
        code.code[return_idx - 1:return_idx + 1] = new_bytecode
        del code.code[lglobal_idx]

    func.func_code = code.to_code()
    return func

def factorial(n, acc=1):
    if n <= 0:
        return acc
    return factorial(n - 1, n * acc)

dis.dis(factorial)
factorial = tail_call(factorial)
print
dis.dis(factorial)

print factorial(10000)