def __len__(self):
        """The total number of samples that will be imported across all group
        slices.

        Raises:
            TypeError: if the total number is not known
        """
        raise TypeError(
            "The number of samples in this %s is not known a priori"
            % type(self)
        )
The total number of samples that will be imported across all group
        slices.

        Raises:
            TypeError: if the total number is not known
        
fiftyone/utils/data/importers.py