https://docs.python.org/3.5/library/io.html?highlight=stringio#id3 :

Also, TextIOWrapper.tell() and TextIOWrapper.seek() are both quite slow due to the reconstruction algorithm used.

What is reconstruction algorightm ? Experiments show, that seek() and tell() returns values of count of bytes (not letters).


#!/usr/bin/python3.5
import tempfile

with tempfile.TemporaryFile(mode='r+t') as f:
    l = f.write('привет')
    print(l, f.tell()) # "6 12"
    f.seek(3)
    f.write('прекол42')
    f.seek(0)
    print(f.read()) # raise UnicodeDecodeError

So, please reopen. Issue is still here.