class_docstr: Singleton metaclass for :class:`fiftyone.core.frame.Frame`.
  
  This metaclass maintains a weakref dictionary of all in-memory
  :class:`fiftyone.core.frame.Frame` instances keyed by
  ``[collection name][sample ID][frame number]``.
docstr: Clears the values for the given fields (i.e., sets them to None)
  on all in-memory frames in the collection.
docstr_tok: ["Clears","the","values","for","the","given","fields","(","i.e.",",","sets","them","to","None",")","on","all","in-memory","frames","in","the","collection","."]
code: def _clear_fields(cls, collection_name, field_names):
      """Clears the values for the given fields (i.e., sets them to None)
      on all in-memory frames in the collection.
      """
      if collection_name not in cls._instances:
          return
  
      for frames in cls._instances[collection_name].values():
          for frame in frames.values():
              for field_name in field_names:
                  frame._doc._data[field_name] = None
  
imports: from collections import defaultdict
  import weakref
label_desc: Use image node_id 7 for calling the FrameSingleton obj's underlying member method code with example usage: obj._clear_fields(cls, collection_name, field_names) without return types