Python 2.7.3 (default, Apr 10 2012, 23:24:47) [MSC v.1500 64 bit (AMD64)] on win32
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>>>

import os, mmap

f = open('etc.test', 'r+b')

mapped = mmap.mmap(f.fileno(), 0, access=mmap.ACCESS_COPY)
print mapped[0]
mapped[0] = 'R'
print mapped.flush()  <--- returns 0 (error on Windows)
mapped.close()
f.close()