You can also use typing.cast for the handful of cases where your particular
type checker (e.g. MyPy) fails to refine the type properly in a case like the
following:
def lookup_usernames(db, user_ids: Optional[List[int]] = None) -> List[str]:
if user_ids is None:
return []
user_ids = typing.cast(List[int], user_ids)
db.executemany('select name from users where id = ?', [(user_id,) for
user_id in user_ids])
return db.fetchall()
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