Add reverse_code optional argument to RunPython

    reduces_to_sql = False

    reversible = False

    def __init__(self, code):

    def __init__(self, code, reverse_code=None):

        # Forwards code

        if isinstance(code, six.string_types):

            # Trim any leading whitespace that is at the start of all code lines

            # so users can nicely indent code in migration files

            # Run the code through a parser first to make sure it's at least

            # syntactically correct

            self.code = compile(code, "<string>", "exec")

            self.is_callable = False

        else:

            self.code = code

            self.is_callable = True

        # Reverse code

        if reverse_code is None:

            self.reverse_code = None

        elif isinstance(reverse_code, six.string_types):

            reverse_code = textwrap.dedent(reverse_code)

            self.reverse_code = compile(reverse_code, "<string>", "exec")

        else:

            self.reverse_code = reverse_code

    def state_forwards(self, app_label, state):

        # RunPython objects have no state effect. To add some, combine this

        # object, representing the versioned models as an AppCache.

        # We could try to override the global cache, but then people will still

        # use direct imports, so we go with a documentation approach instead.

        if self.is_callable:

        if six.callable(self.code):

            self.code(models=from_state.render(), schema_editor=schema_editor)

        else:

            context = {

            eval(self.code, context)

    def database_backwards(self, app_label, schema_editor, from_state, to_state):

        if self.reverse_code is None:

            raise NotImplementedError("You cannot reverse this operation")

        elif six.callable(self.reverse_code):

            self.reverse_code(models=from_state.render(), schema_editor=schema_editor)

        else:

            context = {

                "models": from_state.render(),

                "schema_editor": schema_editor,

            }

            eval(self.reverse_code, context)

    def describe(self):

        return "Raw Python operation"