Order a list of dictionary in Python with numeric keys

Question

I have a list of arrays that contains a dictionary in python. Here there is my example:

[[(0, 1), 'up'], [(1, 2), 'right'], [(3, 2), None], [(0, 0), 'up'], [(3, 0), 'left'], [(3, 1), None], [(2, 1), 'up'], [(2, 0), 'left'], [(2, 2), 'right'], [(1, 0), 'left'], [(0, 2), 'right']]

What I need to do is to order it base on the key (x, y) first on x and after on y: is it possible?

Here there is what I want as output:

[[(0, 0), 'up'], [(0, 1), 'up'], [(0, 2), 'right'], [(1, 0), 'left'],[(1, 2), 'right'], [(2, 0), 'left'], ....]

I have tried using that code:

s = sorted(s, key = lambda x: (x[0][0][0], x[0][0][1]))

But it returns:

TypeError: 'int' object has no attribute '__getitem__'

Answer 1

You're going too deep in the subscripting. In your case just sort on the tuple, the lexicographical order of the tuple suits your problem:

sorted(s,key = lambda x : x[0])

or without lambda with operator.itemgetter :

sorted(s,key = operator.itemgetter(0))

result:

[[(0, 0), 'up'], [(0, 1), 'up'], [(0, 2), 'right'], [(1, 0), 'left'], [(1, 2), 'right'], [(2, 0), 'left'], [(2, 1), 'up'], [(2, 2), 'right'], [(3, 0), 'left'], [(3, 1), None], [(3, 2), None]]

The interesting thing about this dataset is that you cannot use sorted(s) without a key because of your last argument, which can be a string or None .

On those particular values, it seems to work, but in the case of a tie (2 identical tuples, and the other terms being a string and None ), sort would have compare the string and None , and that doesn't work in Python 3.

But could be solved by using a key function like lambda x : (x[0],x[1] or "") so if x[1] is None it changes it into an empty string and the comparison works (if None should be first, else use lambda x : (x[0],-(x[1] is not None),x[1] or "" to put None after any string).