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> This is because the zero in seek(0) is a "cookie" 
> which contains both the position and the decoder state. 
> Unfortunately, state=0 means 'endianness has been determined:
> native order'.
The problem is maybe that TextIOWrapper._pack_cookie() can create a 
cookie=0. Example to create a non-null value, replace:
 def _pack_cookie(self, position, ...):
 return (position | (dec_flags<<64) | ...
 def _unpack_cookie(self, bigint):
 rest, position = divmod(bigint, 1<<64)
 ...
by
 def _pack_cookie(self, position, ...):
 return (1 | (position<<1) | (dec_flags<<65) | ...
 def _unpack_cookie(self, bigint):
 if not (bigint & 1):
 raise ValueError("invalid cookie")
 bigint >>= 1
 rest, position = divmod(bigint, 1<<64)
 ...
Why the cookie is an integer and not an object with attributes?