qlat.SelectedField

qlat.SelectedField(ctype, fsel, multiplicity=0)

SelectedField(ctype, fsel) with the default multiplicity == 0 creates an empty, uninitialized field that keeps fsel: it is meant to be filled later, e.g. with load_double / float_from_double (see examples-py selected-convert-io.py). Pass a positive multiplicity to allocate now.