Question

I would like to be able to make a Python dictionary with strings as keys and sets of strings as the values. E.g.: { "crackers" : ["crunchy", "salty"] } It must be a set, not a list.

However, when I try the following:

  word_dict = dict()
  word_dict["foo"] = set()
  word_dict["foo"] = word_dict["foo"].add("baz")                                    
  word_dict["foo"] = word_dict["foo"].add("bang")

I get:

Traceback (most recent call last):
  File "process_input.py", line 56, in <module>
    test()
  File "process_input.py", line 51, in test
    word_dict["foo"] = word_dict["foo"].add("bang")
AttributeError: 'NoneType' object has no attribute 'add'

If I do this:

  word_dict = dict()
  myset = set()
  myset.add("bar")
  word_dict["foo"] = myset
  myset.add("bang")
  word_dict["foo"] = myset

  for key, value in word_dict:                                                       
      print key,                                                                
      print value

I get:

Traceback (most recent call last):
  File "process_input.py", line 61, in <module>
    test()
  File "process_input.py", line 58, in test
    for key, value in word_dict:
ValueError: too many values to unpack

Any tips on how to coerce Python into doing what I’d like? I’m an intermediate Python user (or so I thought, until I ran into this problem.)

Answer 1

set.add() does not return a new set, it modifies the set it is called on. Use it this way:

word_dict = dict()
word_dict["foo"] = set()
word_dict["foo"].add("baz")                                    
word_dict["foo"].add("bang")

Also, if you use a for loop to iterate over a dict, you are iterating over the keys:

for key in word_dict:
   print key, word_dict[key]

Alternatively you could iterate over word_dict.items() or word_dict.iteritems():

for key, value in word_dict.items():
   print key, value