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I would rather document it in TextIOBase:
http://docs.python.org/dev/library/io.html#io.TextIOBase
With text I/O streams, tell() returns an arbitrary "position cookie", meaning you can't meaningfully do arithmetic on it: this is why cur-relative seeking and end-relative seeking isn't supported.
Of course, on StringIO the "arbitrary position cookie" is a perfectly well-defined character offset, so we *could* specifically enhance StringIO.tell. Whether it's a good idea to do it (while arbitrary text files would still have the limitation) is left to debate.