Returning Dictionary-length of words in string [duplicate]

Question

I need to build a function that takes as input a string and returns a dictionary.
The keys are numbers and the values are lists that contain the unique words that have a number of letters equal to the keys.
For example, if the input function is as follows:

n_letter_dictionary("The way you see people is the way you treat them and the Way you treat them is what they become")

The function should return:

{2: ['is'], 3: ['and', 'see', 'the', 'way', 'you'], 4: ['them', 'they', 'what'], 5: ['treat'], 6: ['become', 'people']}

The code that I have written is as follows:

def n_letter_dictionary(my_string):
    my_string=my_string.lower().split()
    sample_dictionary={}
    for word in my_string:
        words=len(word)
        sample_dictionary[words]=word
    print(sample_dictionary)
    return sample_dictionary

The function is returning a dictionary as follows:

{2: 'is', 3: 'you', 4: 'they', 5: 'treat', 6: 'become'}

The dictionary does not contain all the words with the same number of letters but is returning only the last one in the string.

Answer 1

Since you only want to store unique values in your lists, it actually makes more sense to use a set. Your code is almost right, you just need to make sure that you create a set if words isn't already a key in your dictionary, but that you add to the set if words is already a key in your dictionary. The following displays this:

def n_letter_dictionary(my_string):
    my_string=my_string.lower().split()
    sample_dictionary={}
    for word in my_string:
        words=len(word)
        if words in sample_dictionary:
            sample_dictionary[words].add(word)
        else:
            sample_dictionary[words] = {word}
    print(sample_dictionary)
    return sample_dictionary

n_letter_dictionary("The way you see people is the way you treat them and the Way you treat them is what they become")

Output

{2: set(['is']), 3: set(['and', 'the', 'see', 'you', 'way']), 
 4: set(['them', 'what', 'they']), 5: set(['treat']), 6: set(['become', 'people'])}

Answer 2

The problem with your code is that you just put the latest word into the dictionary. Instead, you have to add that word to some collection of words that have the same length. In your example, that is a list, but a set seems to be more appropriate, assuming order is not important.

def n_letter_dictionary(my_string):
    my_string=my_string.lower().split()
    sample_dictionary={}
    for word in my_string:
        if len(word) not in sample_dictionary:
            sample_dictionary[len(word)] = set()
        sample_dictionary[len(word)].add(word)
    return sample_dictionary

You can make this a bit shorter by using a collections.defaultdict(set):

    my_string=my_string.lower().split()
    sample_dictionary=collections.defaultdict(set)
    for word in my_string:
        sample_dictionary[len(word)].add(word)
    return dict(sample_dictionary)

Or use itertools.groupby, but for this you have to sort by length, first:

    words_sorted = sorted(my_string.lower().split(), key=len)
    return {k: set(g) for k, g in itertools.groupby(words_sorted, key=len)}

Example (same result for each of the three implementations):

>>> n_letter_dictionary("The way you see people is the way you treat them and the Way you treat them is what they become")
{2: {'is'}, 3: {'way', 'the', 'you', 'see', 'and'}, 4: {'what', 'them', 'they'}, 5: {'treat'}, 6: {'become', 'people'}}