'Accesses a box collection and associated fields.
This function returns specified field with object; if no field is specified,
it returns the box coordinates.
Args:
field: this optional string parameter can be used to specify
a related field to be accessed.
Returns:
a tensor representing the box collection or an associated field.
Raises:
ValueError: if invalid field'

def get_field(self, field):

    if (not self.has_field(field)):
        raise ValueError((('field    ' + str(field)) + '    does    not    exist'))
    return self.data[field]
