Group/Count list of dictionaries based on value

Question

I've got a list of Tokens which looks something like:

[{
    Value: "Blah",
    StartOffset: 0,
    EndOffset: 4
}, ... ]

What I want to do is get a count of how many times each value occurs in the list of tokens.

In VB.Net I'd do something like...

Tokens = Tokens.
GroupBy(Function(x) x.Value).
Select(Function(g) New With {
           .Value = g.Key,
           .Count = g.Count})

What's the equivalent in Python?

Answer 1

IIUC, you can use collections.Counter:

>>> from collections import Counter
>>> tokens = [{"Value": "Blah", "SO": 0}, {"Value": "zoom", "SO": 5}, {"Value": "Blah", "SO": 2}, {"Value": "Blah", "SO": 3}]
>>> Counter(tok['Value'] for tok in tokens)
Counter({'Blah': 3, 'zoom': 1})

if you only need a count. If you want them grouped by the value, you could use itertools.groupby and something like:

>>> from itertools import groupby
>>> def keyfn(x):
        return x['Value']
... 
>>> [(k, list(g)) for k,g in groupby(sorted(tokens, key=keyfn), keyfn)]
[('Blah', [{'SO': 0, 'Value': 'Blah'}, {'SO': 2, 'Value': 'Blah'}, {'SO': 3, 'Value': 'Blah'}]), ('zoom', [{'SO': 5, 'Value': 'zoom'}])]

although it's a little trickier because groupby requires the grouped terms to be contiguous, and so you have to sort by the key first.