How to do an upsert with SqlAlchemy?

SQLAlchemy does have a "save-or-update" behavior, which in recent versions has been built into session.add, but previously was the separate session.saveorupdate call. This is not an "upsert" but it may be good enough for your needs.

It is good that you are asking about a class with multiple unique keys; I believe this is precisely the reason there is no single correct way to do this. The primary key is also a unique key. If there were no unique constraints, only the primary key, it would be a simple enough problem: if nothing with the given ID exists, or if ID is None, create a new record; else update all other fields in the existing record with that primary key.

However, when there are additional unique constraints, there are logical issues with that simple approach. If you want to "upsert" an object, and the primary key of your object matches an existing record, but another unique column matches a different record, then what do you do? Similarly, if the primary key matches no existing record, but another unique column does match an existing record, then what? There may be a correct answer for your particular situation, but in general I would argue there is no single correct answer.

That would be the reason there is no built in "upsert" operation. The application must define what this means in each particular case.

SQLAlchemy supports ON CONFLICT with two methods on_conflict_do_update() and on_conflict_do_nothing().

Copying from the documentation:

from sqlalchemy.dialects.postgresql import insert

stmt = insert(my_table).values(user_email='[email protected]', data='inserted data')
stmt = stmt.on_conflict_do_update(
    index_elements=[my_table.c.user_email],
    index_where=my_table.c.user_email.like('%@gmail.com'),
    set_=dict(data=stmt.excluded.data)
)
conn.execute(stmt)