RepeatedComposite delete method

[e-lepilov]

Hi there!
As I understend it RepeatedComposite is list-like container. So list methods like append or extend
work fine.
I am looking for a way to delete the contents of the list (clear the object RepeatedComposite)
The ad contains 4 video files , the print function below shows them

ga_service = client.get_service("GoogleAdsService")
    response = ga_service.search(customer_id=customer_id, query=query)
    ad = response.results[0].ad_group_ad.ad
    
    print(f'{ad.app_ad.youtube_videos}') 
   # [asset: "customers/2309565764/assets/23954537753"
    # , asset: "customers/2309565764/assets/23995613823"
    # , asset: "customers/2309565764/assets/23954549468"
    # , asset: "customers/2309565764/assets/23967118006"
    # ]
    print(f'{type(ad.app_ad.youtube_videos)}') -- > <class 'proto.marshal.collections.repeated.RepeatedComposite'>
     

    # Here I try to delete as a python object
    del ad.app_ad.youtube_videos[:]

    ad_service = client.get_service("AdService")
    ad_operation = client.get_type("AdOperation")

    client.copy_from(ad_operation.update, ad)
    client.copy_from(
        ad_operation.update_mask, protobuf_helpers.field_mask(None, ad._pb)
    )
    ad_service.mutate_ads(
        customer_id=customer_id, operations=[ad_operation]
    )

But it has no effect.
Also try this
del ad.app_ad.youtube_videos
I've also tried clearing the object via an assignment operation
ad.app_ad.youtube_videos = []
but also to no effect.

How to correctly delete linked objects ?

p.s. This code works with the append and extend methods