This is a minor nit, but the doc string for str.isidentifier() states:

    Use keyword.iskeyword() to test for reserved identifiers such as "def" and "class".

At first glance, I thought that it meant you'd do this (doesn't work):

    'def'.iskeyword()

As opposed to this:

    import keyword
    keyword.iskeyword('def')

Perhaps a clarification that "keyword" refers to the keyword module could be added.   Or better yet, just make 'iskeyword()` a string method ;-).