parameters: (self, field_mask=None)
docstring: Initializes the tree by FieldMask.
docstring_summary: Initializes the tree by FieldMask.
docstring_tokens: Initializes the tree by FieldMask .
function: def __init__(self, field_mask=None):
      """Initializes the tree by FieldMask."""
      self._root = {}
      if field_mask:
        self.MergeFromFieldMask(field_mask)
function_tokens: def __init__ ( self , field_mask = None ) : self . _root = { } if field_mask : self . MergeFromFieldMask ( field_mask )