# Sum list based on function

Python, Math, List · Nov 2, 2020

Calculates the sum of a list, after mapping each element to a value using the provided function.

- Use
`map()`

with`fn`

to map each element to a value using the provided function. - Use
`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`