When PY_SSIZE_T isn't defined and a format such as "s#" receives an object whose length fits in a Py_ssize_t but not in an int, the buffer length is silently truncated:

>>> s = 'x' * (4 * 1024**3 + 100)
>>> t = zlib.compress(s, 1)
>>> len(t)
12
>>> len(zlib.decompress(t))
100

(from issue8650)