Py3K is backwards incompatible Python release. I list some tips (You can find more information on http://docs.python.org/3.0/whatsnew/3.0.html):
- Print is Function
- All text is Unicode
- long type is renamed to int. So there is only one built-in type of integers.
- int is a real 'natural' number, because Py3K removes the limit of int type.
- The form of octal literals changes.
PEP 0328, 3102, 3104, 3107, 3109, 3132, 3134, 3110, 3113, 3115.
Some changes are already taking effects in Python 2.6. And other changes can be found in official website http://docs.python.org/3.0/whatsnew/3.0.html.
The performance of Py3K is 10% slower than Python 2.5. But I think there is room for improvement.
Generally speaking, Py3K changes a lot.