TypeError; Must use key word argument or key function in python 3.x

Question

I am new to python, trying to port a script in 2.x to 3.x i am encountering the error TypeError; Must use key word argument or key function in python 3.x. Below is the piece of code: Please help

def resort_working_array( self, chosen_values_arr, num ):
    for item in self.__working_arr[num]:
        data_node = self.__pairs.get_node_info( item )

        new_combs = []
        for i in range(0, self.__n):
            # numbers of new combinations to be created if this item is appended to array
            new_combs.append( set([pairs_storage.key(z) for z in xuniqueCombinations( chosen_values_arr+[item], i+1)]) - self.__pairs.get_combs()[i] )
        # weighting the node
        item.weights =  [ -len(new_combs[-1]) ]    # node that creates most of new pairs is the best
        item.weights += [ len(data_node.out) ] # less used outbound connections most likely to produce more new pairs while search continues
        item.weights += [ len(x) for x in reversed(new_combs[:-1])]
        item.weights += [ -data_node.counter ]  # less used node is better
        item.weights += [ -len(data_node.in_) ] # otherwise we will prefer node with most of free inbound connections; somehow it works out better ;)

    self.__working_arr[num].sort( key = lambda a,b: cmp(a.weights, b.weights) )

Answer 1

Looks like the problem is in this line.

self.__working_arr[num].sort( key = lambda a,b: cmp(a.weights, b.weights) )

The key callable should take only one argument. Try:

self.__working_arr[num].sort(key = lambda a: a.weights)

Answer 2

The exact same error message appears if you try to pass the key parameter as a positional parameter.

Wrong:

sort(lst, myKeyFunction)

Correct:

sort(lst, key=myKeyFunction)

Python 3.6.6