TypeError: '<' not supported between instances of 'tuple' and 'str'

Question

I have a method that build huffman tree which is as follows:

def buildTree(tuples) :
    while len(tuples) > 1 :
        leastTwo = tuple(tuples[0:2])                  # get the 2 to combine
        theRest  = tuples[2:]                          # all the others
        combFreq = leastTwo[0][0] + leastTwo[1][0]     #enter code here the branch points freq
        tuples   = theRest + [(combFreq,leastTwo)]     # add branch point to the end
        tuples.sort()                                  # sort it into place
    return tuples[0]            # Return the single tree inside the list

but while I feed the function with following parameter:

[(1, 'b'), (1, 'd'), (1, 'g'), (2, 'c'), (2, 'f'), (3, 'a'), (5, 'e')]

I get the error as

  File "<stdin>", line 7, in buildTree
    tuples.sort()
TypeError: '<' not supported between instances of 'tuple' and 'str'

While debugging I found the error was in tuples.sort().

Answer 1

The error is thrown because you are creating inner nodes in (priority, (node, node)) form. For equal priorities, Python then tries to compare a symbol from a leaf node (so the second element in a (priority, symbol) node tuple) with the (node, node) tuple from an inner node:

>>> inner = (combFreq, leastTwo)
>>> inner
(2, ((1, 'b'), (1, 'd')))
>>> theRest[1]
(2, 'c')
>>> theRest[1] < inner
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: '<' not supported between instances of 'str' and 'tuple'

For building a huffman tree, if you want to sort your array of nodes, you only really need to sort on the priority, ignoring the rest of the tuples (symbols or child nodes):

tuples.sort(key=lambda t: t[0])

With that correction, your buildTree() function produces a tree:

>>> buildTree([(1, 'b'), (1, 'd'), (1, 'g'), (2, 'c'), (2, 'f'), (3, 'a'), (5, 'e')])
(15, ((6, ((3, 'a'), (3, ((1, 'g'), (2, 'c'))))), (9, ((4, ((2, 'f'), (2, ((1, 'b'), (1, 'd'))))), (5, 'e')))))

Personally, I'd use a priority queue instead, avoiding sorting each time. See How to implement Priority Queues in Python?