Have you ever heard or used python counters? They are very useful to count the number of occurrences of “simple” items. Basically:

> from collections import Counter
> colors = ['red', 'blue', 'red', 'green']
> Counter(colors)
Counter({'red': 2, 'blue': 1, 'green': 1})

However, if you try to use it on non hashable types it doesn’t work.

> colors = [['red', 'warm'], ['blue', 'cold'], ['red', 'warm']]
> Counter(colors)
[...]
TypeError: unhashable type: 'list'

What do we do then?

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