You might be right about the “reconstruction algorithm”. This text was added in revision 0bba533c0959; maybe Antoine can comment whether we should clarify or remove it.

I think the text added for TextIOBase.seek() in revision 03e61104f7a2 (Issue 12922) is closer to the truth. Though I would probably drop the bit about tell() not usually returning a byte position; for many codecs it does seem to.

This illustrates the only four cases of seeking I understand are allowed for text streams:

>>> text = TextIOWrapper(BytesIO(), "utf-7")
>>> text.write("привет")
6
>>> text.seek(0)  # 1: Rewind to start
0
>>> text.read(1)
'п'
>>> saved = text.tell()
>>> text.read()
'ривет'
>>> text.seek(saved)  # 2: Seek to saved offset
340282368347045388720132684115559317504
>>> text.read(1)
'р'
>>> text.seek(0, SEEK_CUR)  # 3: No movement
680564735267983852183507291547327528960
>>> text.read(1)
'и'
>>> text.seek(0, SEEK_END)  # 4: Seek to end
18
>>> text.read()  # EOF
''

If the “slow reconstruction algorithm” was clarified or removed, and the documentation explained that you cannot seek to arbitrary characters without having previously called tell(), would that work?