sample impl of tracking marked columns and also columns with default values

update_with_defaults.py

def get_marked(obj):
    """Returns the set of marked columns for an object"""
    return set(_obj_tracking[obj]["marked"])


def get_marked_or_default(obj):
    marked = get_marked(obj)
    for column in obj.Meta.columns:
        if column.default is not missing:
        marked.add(column)
    return marked

update_with_defaults_sometimes_eager.py

_obj_tracking = WeakDefaultDictionary(lambda: {
    "marked": set(),
    "snapshot": None,
    "new_object": True,
})


def is_new(obj):
    return _obj_tracking[obj]["new_object"]


def clear_new(obj):
    _obj_tracking[obj]["new_object"] = False


def get_marked_or_default(obj):
    marked = get_marked(obj)
    # Since this is a new object never persisted to DynamoDB before,
    # eagerly evaluate all default Columns as well
    if is_new(obj):
        for column in obj.Meta.columns:
            if column.default is not missing:
                marked.add(column)
    return marked


# deleted/loaded/saved fire after a sync to/from Dynamo
def on_object_[deleted|loaded|saved](...):
    ...  # Whatever those methods already do
    clear_new(obj)