Comments on: Python dictionaries are not the same as instances

By: Ed Singleton

Ed Singleton — Tue, 26 Jun 2007 10:57:01 +0000

@tazzzzz,

Debugging and Testing is real life use! Testing in particular.

Most of my python is in using TurboGears, and I seem to spend a lot of time passing dictionaries around. I do really like your idea, but I would probably end up reimplementing a lot of the functionality of dictionaries on an instance.

By: tazzzzz

tazzzzz — Tue, 26 Jun 2007 10:37:43 +0000

@Ed: That’s an interesting point. I wonder if the Cookbook has a function to do that with instances… it can certainly be done, but it’s a little complicated.

In real life usage (not debugging/testing), this is not generally a factor, though, right?

@Observer: no worries… happens to everyone at one time or another.

By: Observer

Observer — Tue, 26 Jun 2007 09:51:28 +0000

“Actually what I said was this”

Sorry, misread the intro. Embarrassing. Feel free to remove my comment.

By: Ed Singleton

Ed Singleton — Tue, 26 Jun 2007 05:33:52 +0000

One thing I do like about dictionaries is that it is easy to iterate over their keys and I can’t think off the top of my head how you achieve the same thing with an instance.

By: tazzzzz

tazzzzz — Mon, 25 Jun 2007 15:41:04 +0000

Actually what I said was this:

>>> foo = {‘bar’:'baz’}
>>> isinstance(foo, dict)
True

My point was that while I’m saying that dictionaries are not the same as instances, I do recognize that they are instances of a class themselves.

By: Observer

Observer — Mon, 25 Jun 2007 15:19:46 +0000

“Yes, yes, I know that Python dictionaries are instances of the dict class”

They are?

>>> instance = object()
>>> isinstance(instance, dict)
False

By: tazzzzz

tazzzzz — Sat, 16 Jun 2007 15:46:22 +0000

I’ve made most of the examples more complete… I think I was a little too brief with them. Try them now, and I think you’ll see that it is backwards compatible and quite convenient to use…

You only need fset if the property should be read/write. If it’s read only, just using property as a decorator works well.

By: Jeff Darcy

Jeff Darcy — Fri, 15 Jun 2007 18:37:06 +0000

Well, besides not being backward-compatible, the example as given doesn’t seem to work. Doesn’t there need to be a nested fset etc.?

By: tazzzzz

tazzzzz — Fri, 15 Jun 2007 14:17:46 +0000

not true… that’s what the property() built-in is for.

By: Jeff Darcy

Jeff Darcy — Fri, 15 Jun 2007 12:59:05 +0000

Excellent advice, but I do have one small nit to pick. If you want to replace what was previously a stored value with a computed one, as in your point 2, you have to do it by overriding __getattr__. If you just make a method with the same name, you have to change other code to call that function so it’s not entirely transparent.