· python

Python: Counter - ValueError: too many values to unpack

I recently came across Python's Counter tool which makes it really easy to count the number of occurrences of items in a list.

In my case I was trying to work out how many times words occurred in a corpus so I had something like the following:

>> from collections import Counter
>> counter = Counter(["word1", "word2", "word3", "word1"])
>> print counter
Counter({'word1': 2, 'word3': 1, 'word2': 1})

I wanted to write a for loop to iterate over the counter and print the (key, value) pairs and started with the following:

>>> for key, value in counter:
...   print key, value
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: too many values to unpack

I'm not sure why I expected this to work but in fact since Counter is a sub class of dict we need to call iteritems to get an iterator of pairs rather than just keys.

The following does the job:

>>> for key, value in counter.iteritems():
...   print key, value
word1 2
word3 1
word2 1

Hopefully future Mark will remember this!

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