map()
with fn
to map each element to a value using the provided function.sum()
to return the sum of the values.def sum_by(lst, fn): return sum(map(fn, lst))
sum_by([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], lambda v : v['n']) # 20
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