[Python] Using list.count to sort a list in-place using .sort() does not work. Why?

I am trying to sort a list by frequency of its elements.

>>> a = [5, 5, 4, 4, 4, 1, 2, 2]
>>> a.sort(key=a.count)
>>> a
[5, 5, 4, 4, 4, 1, 2, 2]


a is unchanged. However:

>>> sorted(a, key=a.count)
[1, 5, 5, 2, 2, 4, 4, 4]


Why does this method not work for .sort()?

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