Even the main driver for Python 3, the bytes-Unicode split, has unfortunately turned out to be sub-optimal. Python essentially bet on UTF-32 (with space-saving optimisations), while everyone else has chosen UTF-8.
How so? Python3 strings are unicode and all the encoding/decoding functions default to utf-8. In practice this means all the python I write is utf-8 compatible unicode and I don't ever have to think about it.
While most characters might be encodable as a single code point, Python does not normalize strings, so there is no guarantee that even relatively normal characters are actually stored as single code points.
Try this in Python:
s = "a\u0308"
print(s)
print(s[0])
ä
aIMO, while this may not be optimal, it's far better than the more arcane choice made by other systems. For example, due to reasons only Microsoft can understand, Windows is stuck with UTF-16.
[1] Actually it's more intelligent. For example, Python automatically uses uint8 instead of uint32 for ASCII strings.
>>> x = '日本語'*100000000
>>> import time
>>> t = time.time(); y = x.encode(); time.time() - t # takes nontrivial time
>>> t = time.time(); y = x.encode(); time.time() - t # not cached; not any faster
Python uses either 8, 16 or 32 bits per character according to the maximum code point found in the string; uint8 is thus used for all strings representable in Latin-1, not just "ASCII". (It does have other optimizations for ASCII strings.)
The reason for Windows being stuck with UTF-16 is quite easy to understand: backwards compatibility. Those APIs were introduced before there supplementary Unicode planes, such that "UTF-16" could be equated with UCS-2; then the surrogate-pair logic was bolted on top of that. Basically the same thing that happened in Java.
Languages that use UTF-8 natively don't need those functions at all. And the ones in Python aren't trivial - see, for example, `surrogateescape`.
As the sibling comment says, the only benefit of all this encoding/decoding is that it allows strings to support constant-time indexing of code points, which isn't something that's commonly needed.
It did nothing of the sort. UTF-8 is the default source file encoding and has been the target for many APIs. It likely would have been the default for all I/O stuff if we lived in a world where Windows had functioning Unicode in the terminal the whole time and didn't base all its internal APIs on UTF-16.
I assume you're referring to the internal representation of strings. Describing it as "UTF-32 with space-saving optimizations" is missing the point, and also a contradiction in terms. Yes, it is a system that uses the same number of bytes per character within a given string (and chooses that width according to the string contents). This makes random access possible. Doing anything else would have broken historical expectations about string slicing. There are good arguments that one shouldn't write code like that anyway, but it's hard to identify anything "sub-optimal" about the result except that strings like "I'm learning 日本語" use more memory than they might be able to get away with. (But there are other strings, like "ℍℯℓ℗", that can use a 2-byte width while the UTF-8 encoding would add 3 bytes per character.)