Sorting list of tuples of tuples

Question

[((D,A),0.0),((D,C),0.0),((D,E),0.5)]

I need to sort the list as:

[((D,E),0.5),((D,A),0.0),((D,C),0.0)]

I have us

Answer 1

Use a tuple as the sort key with a negative on the float to reverse the order:

>>> li=[(('D','A'),0.0),(('D','C'),0.0),(('D','E'),0.5)]
>>> sorted(li, key=lambda t: (-t[-1],t[0]))
[(('D', 'E'), 0.5), (('D', 'A'), 0.0), (('D', 'C'), 0.0)]

If you cannot do negation (say on a string or letter value or something non numeric) then you can take advantage of the fact that the Python sort function is stable and do the sort in two steps:

>>> li=[(('D','A'),'A'),(('D','C'),'A'),(('D','E'),'C')]
>>> sorted(sorted(li), key=lambda t: t[-1], reverse=True)
[(('D', 'E'), 'C'), (('D', 'A'), 'A'), (('D', 'C'), 'A')]

Answer 2

Similarly, you could supply sorted with an iterable that is the result of another sort:

>>> from operator import itemgetter
>>> t = [(('D','A'),0.0),(('D','C'),0.0),(('D','E'),0.5)]
>>> sorted(sorted(t), key=itemgetter(1), reverse=True)
[(('D', 'E'), 0.5), (('D', 'A'), 0.0), (('D', 'C'), 0.0)]