I agree with Mike, I don't think vars(obj).update(kwargs) is the best way to do
it. Python trusts the programmer enough to let you bypass the middle layers of
abstraction like this, but it's not always a good idea.
Besides, it only takes 2 lines to do it with setattr() instead:
for key, val in kwargs.iteritems():
setattr(obj, key, val)
SQLAlchemy aside, I think the above is better anyway. To me it's a lot clearer
at a glance what it is doing, but they may just be me. I've never seen anyone
do your vars update method.
Cameron Jackson
Engineering Intern
Air Operations
Thales Australia
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From: sqlalchemy@googlegroups.com [mailto:sqlalchemy@googlegroups.com] On
Behalf Of Michael Bayer
Sent: Tuesday, 10 January 2012 10:59 AM
To: sqlalchemy@googlegroups.com
Subject: Re: [sqlalchemy] Strange session.commit behavior
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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