How to clean the database, dropping all records using sqlalchemy?

If you models rely on the existing DB schema (usually use autoload=True), you cannot avoid deleting data in each table. MetaData.sorted_tables comes in handy:

for tbl in reversed(meta.sorted_tables):
    engine.execute(tbl.delete())

If your models do define the complete schema, there is nothing simpler than drop_all/create_all (as already pointed out by @jadkik94).

Further, TRUNCATE would anyways not work on the tables which are referenced by ForeignKeys, which is limiting the usage significantly.