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The documentation looks contradictory. "The *data* argument must be", but "The *data* argument may also be". "must be a bytes object", but "If *data* is a buffer".
Why not write just "The data argument must be a bytes-like object, an iterable of bytes-like objects, or None"? It doesn't depend if url is a string or a Request object.
AFAIK the data argument of Request can be an iterable of bytes-like objects in additional to a bytes-like object or None.
The note about the application/x-www-form-urlencoded format is applied not only to a bytes object, but to an iterable of bytes-like objects too. I.e. to any acceptable value except None.