The default format depends on the style.  From 3.8:

class Formatter()
   <docstring>
    def __init__(self, fmt=None, datefmt=None, style='%'):
        ...
        self._fmt = self._style._fmt

It appears that the class docstring was not updated when the style parameter was added, in 3.2.  So I think more change is needed.  I will try to make a proposal later.