CODEWITHSUNDEEP
pythonpython-3.8
Jongkeun Choi
Jongkeun Choi

Reputation: 43

What does this KeyError mean? in Accumulation key value of dictionary in python

(1) : This does work, but it doesn't accumulate.

word_dict = {'now': 3, 'this': 3, 'now': 2, 'this': 2}
new_word_dict = {}

for word in word_dict:
    n = word_dict.get(word)

    new_word_dict[word] = n
    print(new_word_dict)

(1) the result

    {'now': 2}
    {'now': 2, 'this': 2}

(2) : This doesn't work. What the KeyError is ?

word_dict = {'now': 3, 'this': 3, 'now': 2, 'this': 2}
new_word_dict = {}

for word in word_dict:
    n = word_dict.get(word)

    new_word_dict[word] += n
    print(new_word_dict)

(2) the result

  ---->  new_word_dict[word] += n
  
 KeyError: 'now'

Upvotes: 0

Views: 361

Answers (2)

JJ Hassan
JJ Hassan

Reputation: 395

The KeyError is because the key you're trying to increment doesn't exist, but I think there's more than just this issue in your example.

Assuming that the result you want is new_word_dict to be an accumulation of word_dict you first need to change the type of word_dict to a list of tuples (because, as @deceze pointed out in a comment, duplicate keys aren't allowed in dicts). So I think you want to end up with something like this:

words == [("now", 3), ("this", 3), ("now", 2), ("this", 2)]
words_accumulated == {"now": 5, "this": 5}

then your loop could be changed to

words = [("now", 3), ("this", 3), ("now", 2), ("this", 2)]
words_accumulated = {}
for word, val in words: # unpack each tuple into two variables
  existing_val = words_accumulated.setdefault(word, 0) # if it doesn't exist yet, set to 0. This solves your KeyError
  words_accumulated[word] = existing_val + val

For further research on how to do this better, look up defaultdict and maybe some of the tools in itertools

Upvotes: 1

Faulheit
Faulheit

Reputation: 146

new_word_dict[word] += n need the latest value of new_word_dict[word] if it's not initialize it should crash.

to fix this, you can check if new_word_dict exist like new_word_dict[word] = n if new_word_dict[word] else new_word_dict[word] + n

You can use too word_dict.items() like for key, value in word_dict.items()

Upvotes: 0

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