elegant way to reduce a list of dictionaries?

Question

I have a list of dictionaries and each dictionary contains exactly the same keys. I want to find the average value for each key and I would like to know how to do it using r

Answer 1

You could use a Counter to do the summing elegantly:

from itertools import Counter

summed = sum((Counter(d) for d in folds), Counter())
averaged = {k: v/len(folds) for k, v in summed.items()}

If you really feel like it, it can even be turned into a oneliner:

averaged = {
    k: v/len(folds)
    for k, v in sum((Counter(d) for d in folds), Counter()).items()
}

In any case, I consider either more readable than a complicated reduce(); sum() itself is an appropriately specialized version of that.

An even simpler oneliner that doesn't require any imports:

averaged = {
    k: sum(d[k] for d in folds)/len(folds)
    for k in folds[0]
}

Interestingly, it's considerably faster (even than pandas?!), and also the statistic is easier to change.

I tried replacing the manual calculation by statistics.mean() function in Python 3.5, but that makes it over 10 times slower.