Eryk Sun added the comment:
FYI, you can parse the cookie using struct or ctypes. For example:
class Cookie(ctypes.Structure):
_fields_ = (('start_pos', ctypes.c_longlong),
('dec_flags', ctypes.c_int),
('bytes_to_feed', ctypes.c_int),
('chars_to_skip', ctypes.c_int),
('need_eof', ctypes.c_byte))
In the simple case only the buffer start_pos is non-zero, and the result of
tell() is just the 64-bit file pointer. In Serhiy's UTF-7 example it needs to
also convey the bytes_to_feed and chars_to_skip values:
>>> f.tell()
680564735109527527154978616360239628288
>>> cookie_bytes = f.tell().to_bytes(ctypes.sizeof(Cookie), sys.byteorder)
>>> state = Cookie.from_buffer_copy(cookie_bytes)
>>> state.start_pos
0
>>> state.dec_flags
0
>>> state.bytes_to_feed
16
>>> state.chars_to_skip
2
>>> state.need_eof
0
So a seek(0, SEEK_CUR) in this case has to seek the buffer to 0, read and
decode 16 bytes, and skip 2 characters.
Isn't this solvable at least for the case of truncating, Martin? It could do a
tell(), seek to the start_pos, read and decode the bytes_to_feed, re-encode the
chars_to_skip, seek back to the start_pos, write the encoded characters, and
then truncate.
>>> f = open('temp.txt', 'w+', encoding='utf-7')
>>> f.write(b'+BDAEMQQyBDMENA-'.decode('utf-7'))
5
>>> _ = f.seek(0); f.read(2)
'аб'
>>> cookie_bytes = f.tell().to_bytes(sizeof(Cookie), byteorder)
>>> state = Cookie.from_buffer_copy(cookie_bytes)
>>> f.buffer.seek(state.start_pos)
0
>>> buf = f.buffer.read(state.bytes_to_feed)
>>> s = buf.decode(f.encoding)[:state.chars_to_skip]
>>> f.buffer.seek(state.start_pos)
0
>>> f.buffer.write(s.encode(f.encoding))
8
>>> f.buffer.truncate()
8
>>> f.close()
>>> open('temp.txt', encoding='utf-7').read()
'аб'
Rewriting the encoded bytes is necessary to properly terminate the UTF-7
sequence, which makes me doubt whether this simple approach will work for all
codecs. But something like this is possible, no?
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue26158>
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