SQLAlchemy commit changes to object modified through __dict__

Question

I am developing a multiplayer game. When I use an object from inventory, it should update the user creature\'s stats with the values of the attributes of an object.

Answer 1

The problem does not reside in SQLAlchemy but is due to Python's descriptors mechanism. Every Column attribute is a descriptor: this is how SQLAlchemy 'hooks' the attribute retrieval and modification to produce database requests.

Let's try with a simpler example:

class Desc(object):
    def __get__(self, obj, type=None):
        print '__get__'
    def __set__(self, obj, value):
        print '__set__'

class A(object):
    desc = Desc()

a = A()
a.desc                  # prints '__get__'
a.desc = 2              # prints '__set__'

However, if you go through a instance dictionary and set another value for 'desc', you bypass the descriptor protocol (see Invoking Descriptors):

a.__dict__['desc'] = 0  # Does not print anything !

Here, we just created a new instance attribute called 'desc' with a value of 0. The Desc.__set__ method was never called, and in your case SQLAlchemy wouldn't get a chance to 'catch' the assignment.

The solution is to use setattr, which is exactly equivalent to writing a.desc:

setattr(a, 'desc', 1)   # Prints '__set__'