List of dict => dict of dict aggregator.

agg.py

import statistics


class Field:
    def __init__(self, field, agg_func=lambda l: l[0]):
        self.field = field
        self.agg_func = agg_func

    def aggregate(self, l):
        return self.agg_func(l)


class Mean(Field):
    def aggregate(self, l):
        l = [e if e else 0 for e in l]
        return statistics.mean(l)


class Median(Field):
    def aggregate(self, l):
        l = [e if e else 0 for e in l]
        return statistics.median(l)


class Count(Field):
    def aggregate(self, l):
        return len(l)


def agg(data, key, **fields):
    fields = {
        n: Field(f) if isinstance(f, str) else f
        for n, f in fields.items()
    }
    agged = {}
    for d in data:
        if d[key] in agged:
            for name, field in fields.items():
                agged[d[key]][name].append(d[field.field])
        else:
            agged[d[key]] = {
                name: [d[field.field]]
                for name, field in fields.items()
            }
    for k, d in agged.items():
        for n, f in fields.items():
            d[n] = f.aggregate(d[n])
    return agged