tell() is opaque when opening a text file: you can't interpret the output, its only use is for input to seek().

From the docs https://docs.python.org/3/tutorial/inputoutput.html#methods-of-file-objects:

"f.tell() returns an integer giving the file object’s current position in the file represented as number of bytes from the beginning of the file when in binary mode and an opaque number when in text mode."

Does the value returned from tell() not work in seek()? That would be the only bug here.