Of the things you mentioned, only Unicode has been a problem, and that's exactly what 3 is fixing, so that's to be expected. The rest was automatically handled by 2to3 with only a cursory review.
We tried 2to3 and it gave us poor results. But probably because we deviated too far from being pythonic.
The str/unicode misery is one of the biggest gripes I have with Python. I'm glad this unpleasant knot has been mostly untied in Python 3. I came to the conclusion that the transition would have been much easier if Python 3 just concentrated on the separation of bytes and (unicode) strings. The other features could have been in Python 4.
It's a bit like IPv6. If it would just solve the address space problem, most would have moved to it already. Instead it comes with a lot more baggage. And each additional feature has it's own uphill battle for acceptance. So nearly everyone is dragging their feet, citing their pet peeve with the technology.
I'm not sure that's true because, as I said, most of it is automatic and worked well with 2to3, leaving us to deal pretty much only with Unicode. I'd certainly prefer to only have to do this upgrade once.
How do you automatically go from sort( ... some elaborate compare function ...) to sort(key=some completely different function). Yes, there's a wrapper, but it makes the code more convoluted instead of transforming it to the key-paradigm. And if you want to sort by several keys, now you will have to call sort several times.
How do you automatically infer the intention of somedict.keys()? Is it going to be used as a list or as an iterator?
Those are just off the top of my head. I don't remember all the cases where 2to3 tripped over and produced garbage. But there were too many cases to put actual faith in automatic conversion.
It might work if your code is kind of new and homogenic. But looking at how much trouble Dropbox had, even with all the tooling and Guidos they could muster, I have the feeling that your positive experience with 2to3 might rather be the exception than the rule for old and big code bases.
> How do you automatically go from sort( ... some elaborate compare function ...) to sort(key=some completely different function). Yes, there's a wrapper
Yes, there's a wrapper. You use it, add a comment "this is wrapped in the migration to 3" and move on.
> How do you automatically infer the intention of somedict.keys()? Is it going to be used as a list or as an iterator?
If it's being iterated on first thing, it's an iterator. If list methods are called on to it, it's a list. This just hasn't been a problem for us, sure, it took some looking at, but it wasn't more than 30 seconds per case.
> I have the feeling that your positive experience with 2to3 might rather be the exception than the rule for old and big code bases.
Maybe so, but the codebase was ten years old and hundreds of thousands of lines.
