Skip to main content

Identity Comparison vs Comparing Identities

An explaination of the virtues of foo is not None over foo != None lead to an explaination of identity comparison with the is operator. A fellow equated this to, roughly, id(a.x) == id(b.x), which I told him was roughly correct but probably not actually correct. It only took a little bit of through to see how uncorrect it was.

The following code creates a simple class with one property descriptor (read-only). It solves the requirements that with to instances of this class, a and b, id(a.x)==id(b.x) can be true while a.x is not b.x! How does this happen?

class foo(object):
x = property(lambda s: id(s))
a = foo()
b = foo()
assert id(a.x) == id(b.x)
assert a.x is not b.x

How on earth does this code prove what it does? a.x and b.x are created on the fly, passed to the id function, and then destroyed with no references left. Because they are both created in the right order with the id(a.x)==id(b.x) expression, they just happen to get the same memory addresses, which in CPython is used for the id function's result. This leads to misleading results, so don't rely on them in such ways. Identification is what it is, and you shouldn't try to break it down.

Comments

Anonymous said…
Pretty neat experiment there.
Anonymous said…
wow, ironfroggy. for a complex problem, that's a pretty clean and elegant example. nice.
-j

Popular posts from this blog

Why I Switched From Git to Microsoft OneDrive

I made the unexpected move with a string of recent projects to drop Git to sync between my different computers in favor of OneDrive, the file sync offering from Microsoft. Its like Dropbox, but "enterprise."

Feeling a little ashamed at what I previously would have scoffed at should I hear of it from another developer, I felt a little write up of the why and the experience could be a good idea. Now, I should emphasize that I'm not dropping Git for all my projects, just specific kinds of projects. I've been making this change in habit for projects that are just for me, not shared with anyone else. It has been especially helpful in projects I work on sporadically. More on why a little later.

So, what drove me away from Git, exactly?

On the smallest projects, like game jam hacks, I just wanted to code. I didn't want to think about revisions and commit messages. I didn't need branching or merges. I didn't even need to rollback to another version, ever. I just …

Respect and Code Reviews

Code Reviews in a development team only function best, or possible at all, when everyone approaches them with respect. That’s something I’ve usually taken for granted because I’ve had the opportunity to work with amazing developers who shine not just in their technical skills but in their interpersonal skills on a team. That isn’t always the case, so I’m going to put into words something that often exists just in assumptions.
You have to respect your code. This is first only because the nature and intent of code reviews are to safeguard the quality of your code, so even having code reviews demonstrates a baseline of respect for that code. But, maybe not everyone on the team has the same level of respect or entered a team with existing review traditions that they aren’t acquainted with.
There can be culture shock when you enter a team that’s really heavy on code reviews, but also if you enter a team or interact with a colleague who doesn’t share that level of respect for the process or…

On Pruning Your Passions

We live in a hobby-rich world. There is no shortage of pastimes to grow a passion for. There is a shortage of one thing: time to indulge those passions. If you're someone who pours your heart into that one thing that makes your life worthwhile, that's a great deal. But, what if you've got no shortage of interests that draw your attention and you realize you will never have the time for all of them?

If I look at all the things I'd love to do with my life as a rose bush I'm tending, I realize that careful pruning is essential for the best outcome. This is a hard lesson to learn, because it can mean cutting beautiful flowers and watching the petals fall to the ground to wither. It has to be done.

I have a full time job that takes a lot of my mental energy. I have a wife and a son and family time is very important in my house. I try to read more, and I want to keep up with new developments in my career, and I'm trying to make time for simple, intentional relaxing t…