Consider codecs that maintain an internal buffer (UTF-7) or other state (ISO-2022). When you call TextIOWrapper.read() and then tell(), the I think the returned number is supposed to hold the _decoder_ state, so you can seek back and read again. But I don’t think the number holds any _encoder_ state, so seek() cannot set up the encoder properly for these more awkward codecs.

I don’t think it is practical to fix this problem using the incremental codec API. You would need to construct the encoder’s state from the decoder. There are a couple of bugs marked as duplicates of this. What are the real-world use cases?