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The design approach (at least for urllib.parse) is to add separate *b APIs that operate on bytes rather than implicitly allowing bytes in the ordinary versions of the function.
Allowing programmers to manipulate correctly encoded (and hence ASCII compatible) bytes to avoid decode/encode overhead when manipulating URLs is fine (and the whole point of adding the new functions). Allowing them to *not know* whether they have encoded data or text suitable for display to the user isn't necessary (and is easy to add later if we decide we want it, while taking it away is far more difficult).
More detail at http://mail.python.org/pipermail/python-dev/2010-September/103828.html