If someone's going to fix this, perhaps they might consider also adding the following clarification sentence after 'The mapping key selects the value to be formatted from the mapping.'  

The mapping key is interpreted as a string; a key error is raised if the dictionary does not have a matching string key. 

I don't think it's worth adding it, but consider this example:
>>> d = {2:2, '2':'two'}
>>> '%(2)s' % d
'two'