Inherited custom widget and theme apply callback

Micropython
,
1

I have created a simple widget that is derived from a lv.label

class MyLabel(lv.label):
    def __init__(self, parent):
        super().__init__(parent)
        self.set_user_data(self)

When applying a custom theme to style the new widget, the obj received does not point to the MyLabel but rather the underlying lv.label

newTheme = lv.theme_t()
newTheme.set_apply_cb(onThemeApply)
...
def onThemeApply(self, th: lv.lv_theme_t, obj: lv.obj):        
  if (isinstance(obj, MyLabel)): ...  #this never returns true

When checking the id() of the created MyLabel and of the object the theme callback is being called for, the IDs differ. Is there a way in the theme callback to determine the derived class type?