Question

I have a list with nested lists and I want to create a function (modify_list) that gets a tuple and modifies the passed pointer with a passed value argument. The problem is that I’m not sure how to modify a nested value like this programmatically by reference.

Simplified example:

l = [[1, [2,3, [4,5,6]]]]

If I call the function modify_list, these would be how to use it and the expected results:

> l[0][1][2][2]
6

> modify_list((0, 1, 2, 2), 8)
> l
[[1, [2,3, [4,5,8]]]]

> modify_list((0, 1, 1), 14)
> l
[[1, [2,14, [4,5,8]]]]

Thanks

Answer 1

You can determine each sublist by accessing it with the respective index. Use the last index to assign the value:

def set_nested_val(l, indices, val):
  for i in indices[:-1]:
    l = l[i]
  l[indices[-1]] = val

Note that this function operates on an arbitrary list (the first argument), and not only l. If you want to always modify l, use functools.partial:

import functools
l = [[1, [2,3, [4,5,6]]]]
modify_list = functools.partial(set_nested_val, l)

Note that nested lists, and accessing values by indicies, are often a sign of a bad data architecture. Have you considered a dict whose keys are tuples?