On Jan 7, 2012, at 6:05 PM, Pavel Ponomarev wrote:
> Hi,
>
> Heard lots of good things about sqlalchemy and decided to give it a
> try.
> But almost immediately was confused by strange session.commit()
> behavior, please look through following snippets.
>
> Update is pretty straightforward:
>
> \And it works great, but when I need to update bunch of attrs from dict
> first thought would be built-in vars function:
>
>>>> obj.attr
> bar
>>>> kwargs = {'attr':'foo'}
>>>> vars(obj).update(kwargs)
>>>> obj.attr
SQLAlchemy uses descriptors (see
http://docs.python.org/reference/datamodel.html#implementing-descriptors) to
intercept attribute set/get/delete events. These events then feed into the
unit of work implementation and result in SQL statements to emit when the
pending state is flushed. This usage of descriptors is mentioned in passing
at
http://www.sqlalchemy.org/docs/orm/tutorial.html#create-an-instance-of-the-mapped-class
When you use vars(obj), you're essentially dealing with obj.__dict__ directly.
This bypasses the class in use and any behavior defined on it, essentially
writing data directly to the underlying storage (arguably not as pythonic, wont
work with __slots__ for example). So you need to use setattr() or other
methods that don't bypass instrumentation when setting attributes.
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