why_python3.rst

Why Python 3?

Python 2.7 is the final Python 2.x release. Python 3.x is the future. The Python ecosystem needs to consolidate. A split or schism between different incompatible versions is not healthy for growing the community.
Function annotations
Decimal module 100x faster. As fast as floats.
Easier to learn. (Less cruft in language and stdlib, more consistency, better docstrings, etc.)
Much safer handling of unicode text and encodings: fewer bugs.
More memory efficiency (shared dict keys (PEP 412) and space-efficient Unicode representation (PEP 393))
Exception chaining

it is not the default string type (you have to prefix the string with a u to get Unicode);
it is missing some functionality, e.g. casefold;
there are two distinct implementations, narrow builds and wide builds;
wide builds take up to four times more memory per string as needed;
narrow builds take up to two times more memory per string as needed;
worse, narrow builds have very naive (possibly even "broken") handling of code points in the Supplementary Multilingual Planes.

The unicode string type in Python 3 is better because:

it is the default string type;
it includes more functionality;
starting in Python 3.3, it gets rid of the distinction between narrow and wide builds;
which reduces the memory overhead of strings by up to a factor of four in many cases;
and fixes the issue of SMP code points.

(quote from a mailing list post by Steve D'Aprano on 2014-01-17).