black magic

from inspect import currentframe
import ctypes


__all__ = ['slots']


PyFrame_LocalsToFast = ctypes.pythonapi.PyFrame_LocalsToFast
PyFrame_LocalsToFast.argtypes = [ctypes.py_object, ctypes.c_int]
PyFrame_LocalsToFast.restype = None


class SlotContext(object):
    def __init__(self, datum, frame):
        self._datum = datum
        self._frame = frame
        self._g_update = {}
        self._g_add = []
        self._l_update = {}

    def __enter__(self):
        _globals = self._frame.f_globals
        _locals = self._frame.f_locals

        for key, val in self._datum.iteritems():
            if key in _locals:
                self._l_update[key] = _locals[key]
                _locals[key] = val
            elif key in _globals:
                self._g_update[key] = _globals[key]
                _globals[key] = val
            else:
                self._g_add.append(key)
                _globals[key] = val

        # black magic
        PyFrame_LocalsToFast(self._frame, 0)

    def __exit__(self, type, value, traceback):
        _globals = self._frame.f_globals
        _locals = self._frame.f_locals

        for key, val in self._g_update.iteritems():
            _globals[key] = val

        for key in self._g_add:
            del _globals[key]

        for key, val in self._l_update.iteritems():
            _locals[key] = val

        # black magic
        PyFrame_LocalsToFast(self._frame, 0)

        # avoid possible memory leaks
        del self._frame

        # XXX check me
        if type is None:
            return True


def slots(datum):
    # Get caller frame using black magic
    frame = currentframe().f_back

    return SlotContext(datum, frame)