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AFAIU the conventions for optional argument in the doc are as follow:
If a function has optional arguments and it accepts keyword arguments, the "func(arg=default)" notation should be used, for example:
 str.splitlines(keepends=False)
If a function has optional arguments but it doesn't accept keyword arguments, the "func([arg1])" notation is used instead. This should apply only to some C functions, for example:
 str.strip([chars])
The notation "func([arg=default])" should never be used, and "func([arg])" should be used only when keyword args are not accepted.
These rules apply to both Python 2 and Python 3.
A thing that is still not clear is what to do in case the default value is a placeholder (like object(), None, -1) and the actual value is then computed in the function.