One more nit on all_equal()

[tkim602]

I missed something during the code review. There is an extra unnecessary next call in the first for-loop. This is easily solved by adding another return True.

def all_equal(iterable, key=None):
    iterator = groupby(iterable, key)
    for first in iterator:
        for second in iterator:
            return False
        return True                          # <== This was missed
    return True

For example, all_equal('a') calls the groupby.__next__ three times.

This is can be made visible by replacing groupby with verbose_groupby:

class verbose_groupby(itertools.groupby):

    def __next__(self):
        print('N')
        return super().__next__()