IMO, 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.
No there certainly is. This is documented in the official API documentation:
UTF-8 representation is created on demand and cached in the Unicode object.
https://docs.python.org/3/c-api/unicode.html#unicode-objects
https://github.com/python/cpython/blob/main/Objects/unicodeo...
Is it clear enough?
(And the code search seems to be broken; it can't find me the definition of `unicode_fill_utf8` although I'm sure it's obvious enough.)