How can I add items to an empty set in python

I have the following procedure:

    def myProc(invIndex, keyWord):
        for i in range(len(keyWord)):
            if keyWord[i] in invIndex.keys():
        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])