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True, this could be documented better - flagging as a docs bug. (It is mentioned at the bottom of http://docs.python.org/library/functools.html#partial-objects, but that could be referenced better from the main documentation of the function higher up in the page)
There's actually more to it than just an oversight though - partial() fills in positional arguments from the left, so using one as a method would require specification of a carefully crafted selection of positional and keyword arguments to ensure the instance was bound and passed in at the right point.