1.Why two methods?

• to_python(): Converts any input (database value, form data, manual assignment) into your Python object. It's the "universal converter" for the field.
• from_db_value(): Only converts values fresh from the database (e.g., after a SELECT query). It never handles form data or manual assignments.

to_python() is called in all contexts (DB reads, form validation, direct assignment like obj.field = "xyz").
from_db_value() is only called for DB reads (and only if defined—older Django versions relied solely on to_python() for DB values).

2.Why does to_python() check for Hand instances?

If you do obj.hand = Hand(...) in code, Django calls to_python() on that Hand instance. The check avoids re-parsing an already-correct object. However, from_db_value() never gets a Hand instance. Database values are always raw strings (e.g., "AKQJ..."). So it skips this check.