How can I add items to an empty set in python

I have the following procedure:

def myProc(invIndex, keyWord):
    D={}
    for i in range(len(keyWord)):
        if keyWord[i] in invIndex.keys():
                    D.update(invIndex[query[i]])
    return D

But I am getting the following error:

Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
TypeError: cannot convert dictionary update sequence element #0 to a sequence

I do not get any error if D contains elements. But I need D to be empty at the beginning.

D = {} is a dictionary not set.

>>> d = {}
>>> type(d)
<type 'dict'>

Use D = set():

>>> d = set()
>>> type(d)
<type 'set'>
>>> d.update({1})
>>> d.add(2)
>>> d.update([3,3,3])
>>> d
set([1, 2, 3])

>>> d = {}
>>> D = set()
>>> type(d)
<type 'dict'>
>>> type(D)
<type 'set'>

What you've made is a dictionary and not a Set.

The update method in dictionary is used to update the new dictionary from a previous one, like so,

>>> abc = {1: 2}
>>> d.update(abc)
>>> d
{1: 2}

Whereas in sets, it is used to add elements to the set.

>>> D.update([1, 2])
>>> D
set([1, 2])