Efficient mutable bytestrings for Python 2 and 3 in less than 20 lines of code!

mutable_bytes.py

from ctypes import CDLL, cast, c_char, c_char_p, c_void_p, c_int
from ctypes.util import find_library

libc = CDLL(find_library('c'))

class mutable_bytes(bytes):
    def __setitem__(self, idx, val):
        if isinstance(idx, slice):
            indices = range(*idx.indices(len(self)))
        else:
            indices = [idx]
        assert len(indices) <= len(val)
        si = 0
        for i in indices:
            dest = c_void_p(cast(c_char_p(self), c_void_p).value + i)
            libc.memset(dest, c_char(val[si]), c_int(1))
            si += 1

x = mutable_bytes(b'hello world')
print("%d: %s" % (id(x), x.decode('utf-8')))
x[0:5] = b'  bye'
print("%d: %s" % (id(x), x.decode('utf-8')))
x[7] = b'y'
print("%d: %s" % (id(x), x.decode('utf-8')))
x[1:10:2] = b'xxxxx'
print("%d: %s" % (id(x), x.decode('utf-8')))