Use a list as the key in a Python dict

Question

I have two lists and I need to rank their elements with the Borda positional ranking. so I made this function, but I have this error:

TypeError: unhashable type: 'list'.

As indicated by other answers The problem is that I can't use a list as the key in a dict, since dict keys need to be immutable.So I used a tuple instead, but the error remains.

The files of the two lists look like this
list1 = [([('diritti', 'S'), ('umani', 'A')]), ([('violazioni', 'S'), ('dei', 'E'), ('diritti', 'S'), ('umani', 'A')]), ([('forze', 'S'), ('di', 'E'), ('sicurezza', 'S')]), ([('violazioni', 'S'), ('dei', 'E'), ('diritti', 'S')]), ([('Nazioni', 'SP'), ('Unite', 'SP')]), ([('anni', 'S'), ('di', 'E'), ('carcere', 'S')])] 
list2 = [([('anni', 'S'), ('di', 'E'), ('carcere', 'S')]), ([('diritti', 'S'), ('umani', 'A')]), ([('forze', 'S'), ('di', 'E'), ('sicurezza', 'S')]), ([('violazioni', 'S'), ('dei', 'E'), ('diritti', 'S'), ('umani', 'A')]), ([('violazioni', 'S'), ('dei', 'E'), ('diritti', 'S')]), ([('Nazioni', 'SP'), ('Unite', 'SP')]), ([('uso', 'S'), ('eccessivo', 'A'), ('della', 'E'), ('forza', 'S')])]

I open the two files as list in this way

list1 = codecs.open('/home/list1', 'r', 'utf-8').read()
list2 = codecs.open('/home/list2', 'r', 'utf-8').read()
li = ast.literal_eval(list1)
lii = ast.literal_eval(list2)

def borda_sort(lists):
###Borda’s positional ranking combines ranked lists using information of the ordinal ranks of the elements in each list.Given lists t1, t2, t3 ... tk, for each candidate c and list ti, the score B ti (c) is the number of candidates ranked below c in ti. So The total Borda score is B(c) = ∑ B ti (c) The candidates are then sorted by descending Borda scores. Given the lists = [ ['a', 'c'], ['b', 'd', 'a'], ['b', 'a', 'c', 'd'] ], the output will be ['b', 'a', 'c', 'd']
    scores = {}
    for l in lists:
        for idx, elem in enumerate(reversed(l)):
            if not elem in scores:
                scores[elem] = 0
            scores[elem] += idx
    return sorted(scores.keys(), key=lambda elem: scores[elem], reverse=True)

lists = zip(li, lii)
print borda_sort(lists)

Can someone help?

Lists can't be used as keys for dict. Maybe casting lists to strings will solve your issue? — Ihor Pomaranskyy, May 16 '15 at 09:54
Duplicate of [this](http://stackoverflow.com/questions/7257588/why-cant-i-use-a-list-as-a-dict-key-in-python). — Ami Tavory, May 16 '15 at 09:56
You say that you used a tuple, but I don't see one anywhere. Can you tell us which line the error is coming from? — cdarke, May 16 '15 at 11:12
Could you give some sample input in list1 and list2 and expected results? — cdarke, May 16 '15 at 11:26
unrelated: you could use `json` to store the lists instead of Python literals (`ast.literal_eval()`). — jfs, May 17 '15 at 14:08

NegativeFeedbackLoop · Answer 1 · 2015-05-16T12:28:56.990

2

Use a tuple instead of lists. They can be used as keys in dictionaries. Assuming that the lists have a fixed number of elements. I.e. len(list) is same for all lists and constant.

scores = {}
for l in lists:
    for idx, elem in enumerate(reversed(l)):
        if not elem in scores:
            scores[tuple(elem)] = 0    #Notice that i have converted list to a tuple.
        scores[tuple(elem)] += idx     #Notice that i have converted list to a tuple.
return sorted(scores.keys(), key=lambda elem: scores[tuple(elem)], reverse=True) #Notice that i have converted list to a tuple.

edited May 16 '15 at 12:28

answered May 16 '15 at 11:40

NegativeFeedbackLoop

140
1
7

Traceback (most recent call last): File "/home/", line 48, in print borda_sort(lists) File "/home/fat.py", line 42, in borda_sort if not elem in scores: TypeError: unhashable type: 'list' – sss May 16 '15 at 12:01
Forgot to change one list to tuple. I'll update the code above. However the main point is that you must change all lists that are supposed to be keys to tuples. – NegativeFeedbackLoop May 16 '15 at 12:25
the error remains: TypeError: unhashable type: 'list' – sss May 16 '15 at 19:22

Alex I · Accepted Answer · 2015-05-17T09:59:08.730

Since your lists have complex elements (tuple of list of tuples of strings), they have to be converted to contain tuples of tuples of strings so they are hashable. Like this:

list1a = [ tuple(x) for x in list1 ]
list2a = [ tuple(x) for x in list2 ]

print borda_sort([list1a, list2a])

# prints [(('diritti', 'S'), ('umani', 'A')), (('forze', 'S'), ('di', 'E'), ('sicurezza', 'S')), (('violazioni', 'S'), ('dei', 'E'), ('diritti', 'S'), ('umani', 'A')), (('anni', 'S'), ('di', 'E'), ('carcere', 'S')), (('violazioni', 'S'), ('dei', 'E'), ('diritti', 'S')), (('Nazioni', 'SP'), ('Unite', 'SP')), (('uso', 'S'), ('eccessivo', 'A'), ('della', 'E'), ('forza', 'S'))]

OLD ANSWER If you have list1 and list2, the way to call this function is borda_sort([list1, list2]) (or borda_sort((list1, list2)), either works the same). If your lists contain individual elements in the same order that you want to use for borda, the answer is to make a list of lists, not to zip your lists together. I think zip() doesn't do what you think it does. zip makes a list of pairs from a pair of lists, like this:

>>> zip((1, 2, 3), ('a', 'b', 'c'))
[(1, 'a'), (2, 'b'), (3, 'c')]

Use a list as the key in a Python dict

2 Answers2

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