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cloudbonsai • today at 3:43 AM • 3 replies • view on HN

Internally Python holds a string as an array of uint32. A utf-8 representation is created on demand from it (and cached). So pansa2 is basically correct [^1].

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.

Replies

zahlman • today at 7:19 AM

There is no caching of a "utf-8 representation". You may check for example:

  >>> 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
Generally, the only reason this would happen implicitly is for I/O; actual operations on the string operate directly on the internal representation.

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.

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