> Why do strings cache their UTF-8 encoding?

Strings also cache the wide-string representation. For example:

    from ctypes import *
    s = '\241\242\243'
    pythonapi.PyUnicode_AsUnicodeAndSize(py_object(s), None)
    pythonapi.PyUnicode_AsUTF8AndSize(py_object(s), None)

    >>> hex(id(s))
    '0x7ffff69f8e98'

    (gdb) p *(PyCompactUnicodeObject *)0x7ffff69f8e98
    $1 = {_base = {ob_base = {_ob_next = 0x7ffff697f890,
                              _ob_prev = 0x7ffff6a04d40,
                              ob_refcnt = 1, 
                              ob_type = 0x89d860 <PyUnicode_Type>},
                   length = 3,
                   hash = -5238559198920514942,
                   state = {interned = 0,
                            kind = 1,
                            compact = 1,
                            ascii = 0,
                            ready = 1},
                   wstr = 0x7ffff69690a0 L"¡¢£"},
          utf8_length = 6,
          utf8 = 0x7ffff696b7e8 "¡¢£",
          wstr_length = 3}

    (gdb) p (char *)((PyCompactUnicodeObject *)0x7ffff69f8e98 + 1)
    $2 = 0x7ffff69f8ef0 "\241\242\243"

This object uses 4 bytes for the null-terminated Latin-1 string, which directly follows the PyCompactUnicodeObject struct. It uses 7 bytes for the UTF-8 string. It uses 16 bytes for the wchar_t string (4 bytes per wchar_t).