How to define a class that can act as dictionary key?

[Lambda]

Hi,

I'd like to define a class to use it as a dictionary key:

class dict_entry:
  def __init__(self, term = "", doc_freq = 0):
    self.term = term
    self.doc_freq = doc_freq

  def __cmp__(self, entry):
    return isinstance(entry, dict_entry) and cmp(self.term,
entry.term)

  def __str__(self):
    return term + ", " + str(docid)

And later...
entry = dict_entry(term, len(index[term]))
self.index[entry] = []

When I run it, it says "TypeError: unhashable instance"

It looks like I can't use the new class object as the dictionary key.
What should I do?