Layout_base.py

class Nested(Control):
    '''
    Base class for all the nested context-manager classes which automatically parent themselves
    '''
    ACTIVE_LAYOUT = None

    def __init__(self, key, *args, **kwargs):
        self.Controls = []
        super(Nested, self).__init__(key, *args, **kwargs)

    def __enter__(self):
        self.__cache_layout = Nested.ACTIVE_LAYOUT
        Nested.ACTIVE_LAYOUT = self
        return self

    def __exit__(self, typ, value, traceback):
        self.layout()
        Nested.ACTIVE_LAYOUT = self.__cache_layout
        self.__cache_layout = None
        cmds.setParent("..")

    def layout(self):
        '''
        this is called at the end of a context, it can be used to (for example) perform attachments
        in a formLayout.  Override in derived classes for different behaviors.
        '''
        return len(self.Controls)

    def add(self, control):
        path_difference = control.Widget[len(self.Widget):].count('|') - 1
        if not path_difference:
            self.Controls.append(control)

        if control.Key and not control.Key[0] == "_":
            if control.Key in self.__dict__:
                raise RuntimeError('Children of a layout must have unique IDs')
            self.__dict__[control.Key] = control

    def remove(self, control):
        self.Controls.remove(control)
        k = [k for k, v in self.__dict__.items() if v == control]
        if k:
            del self.__dict__[k[0]]

    def __iter__(self):
        for item in self.Controls:
            for sub in item:
                yield sub
        yield self

    @classmethod
    def add_current(cls, control):
        if cls.ACTIVE_LAYOUT:
            Nested.ACTIVE_LAYOUT.add(control)


# IMPORTANT NOTE
# this intentionally duplicates redundant property names from Control. That forces the metaclass to re-define the CtlProperties using cmds.layout
# instead of cmds.control. In Maya 2014, using cmds.control to query a layout fails, evem for flags they have in common

class Layout(Nested):

    CMD = cmds.layout
    _ATTRIBS = ['annotation', 'backgroundColor', 'defineTemplate', 'docTag', 'dragCallback', 'dropCallback', 'enable', 'enableBackground', 'exists', 'fullPathName', 'height', 'manage', 'noBackground', 'numberOfPopupMenus', 'parent', 'popupMenuArray', 'preventOverride', 'useTemplate', 'visible', 'visibleChangeCommand', 'width']
    _CALLBACKS = ['dragCallback', 'dropCallback', 'visibleChangeCommand']
    _READ_ONLY = ['isObscured', 'popupMenuArray', 'numberOfPopupMenus', 'childArray', 'numberOfChildren']