Add new keys to a dictionary while incrementing existing values

Question

I am processing a CSV file and counting the unique values of column 4. So far I have coded this three ways. One uses "if key in dictionary", the second traps the KeyError and the third uses "DefaultDictionary". For example (where x[3] is the value from the file and "a" is a dictionary): First way: ``` if x[3] in a: a[x[3]] += 1 else: a[x[3]] = 1 ``` Second way: ``` try: b[x[3]] += 1 except KeyError: b[x[3]] = 1 ``` Third way: ``` from collections import defaultdict c = defaultdict(int) c[x[3]] += 1 ``` My question is: which way is more efficient... cleaner... better... etc. Or is there a better way. Both ways work and give the same answer, but I thought I would tap the hive mind as a learning case. Thanks -

Accepted Answer

Use `collections.Counter`. `Counter` is syntactic sugar for `defaultdict(int)`, but what's cool about it is that it accepts an iterable in the constructor, thus saving an extra step (I assume all of your examples above are wrapped in a for-loop.)

```
from collections import Counter
count = Counter(x[3] for x in my_csv_reader)
```

Prior to the introduction of `collections.Counter`, `collections.defaultdict` was the most idiomatic for this task, so for users < 2.7, use `defaultdict`.

```
from collections import defaultdict
count = defaultdict(int)
for x in my_csv_reader:
    count[x[3]] += 1
```

Add new keys to a dictionary while incrementing existing values

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