Why isn't SQLAlchemy default column value available before object is committed?

one would prefer default over server default for one of four reasons:

1. you'd like to run a Python function, not a SQL function, for the default (or a SQL expression that needs some per-INSERT Python state also).

2. the default is part of a primary key column. the ORM can't load a row back without the primary key, so server_default is generally not useful for a PK column when using the ORM.

3. the SQL expression you want to run isn't supported by the database as a "server default".

4. You're dealing with a schema you can't/don't want to change.

In this case, when you'd like "foo" to be "0" in your application independent of database operations, the choices are:

1. use __init__(). It's python!

2. use events.

Here's __init__():

class TestModel(Base):   # ...   def __init__(self):       self.foo = 0

Here's events (specifically the init event):

from sqlalchemy import event@event.listens_for(Foo, "init")def init(target, args, kwargs):    target.foo = 0

You can use force_instant_defaults listener from sqlalchemy_utils to change this behavior:

from sqlalchemy_utils import force_instant_defaultsforce_instant_defaults()class TestModel(Base):    __tablename__ = 'tmodel'    id = sa.Column(sa.Integer, primary_key=True)    foo = sa.Column(sa.Integer, default=0)model = TestModel()assert model.foo == 0

You can use the init event to fill defaults. This event listener will do it:

from sqlalchemy import eventfrom sqlalchemy.orm import mapperfrom sqlalchemy.inspection import inspectdef instant_defaults_listener(target, args, kwargs):    for key, column in inspect(target.__class__).columns.items():        if column.default is not None:            if callable(column.default.arg):                setattr(target, key, column.default.arg(target))            else:                setattr(target, key, column.default.arg)event.listen(mapper, 'init', instant_defaults_listener)