py translation of pseudocode in https://source.opennews.org/en-US/learning/sane-data-updates-are-harder-you-think/

pseudo.py

all_records_that_are_new_or_changed_since_your_last_import = get_all_records_that_are_new_or_changed_since_your_last_import()

for record in all_records_that_are_new_or_changed_since_your_last_import:
    found = find_the_record_in_your_old_data(record)
    if not found:
        add_the_record(record)
    if found:
        change_whatever_bits_of_the_record_are_different(record)

all_your_existing_data = get_all_your_existing_data()

for datum in all_your_existing_data:
    if not in_source_data_any_longer(datum):
        delete_datum(datum)