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.