Assigning to nested list with list of indices

Given a nested list, you can access the elements with something like

data = [[12, 15], [21, 22], [118, 546], [200, 1200]]

assert data[0][0] == 12

The issue is that in this case, I want to index into a nested list with a "list of indexes", with dynamic at runtime length. For example, the "list of indexes" for above would be [0,0]

I want this general type of function

def nested_list_assignment(nested_list, list_of_indices, value):

and would work something like

# passes this basic test
data = [[12, 15], [21, 22], [118, 546], [200, 1200]]

assert data[0][0] == 12
nested_list_assignment(data, [0, 0], 0)
assert data[0][0] == 0

I have a working impl like

def nested_list_assignment(nested_list, list_of_indices, value):
    # ill handle this case later
    assert len(list_of_indices) > 0

    if len(list_of_indices) == 1:
        nested_list[list_of_indices[0]] = value
    else:
        nested_list_assignment(nested_list[list_of_indices[0]], list_of_indices[1:], value)

but I’m curious if Python gives any constructs for this, or just a standard library function for this in general.

>Solution :

There’s no standard Python function or operator for this, but something like this works generically as long as you’re sure that everything being indexed is itself indexable with the elements of indices:

def list_assignment(indexable, indices, value):
    for i in indices[:-1]:
        indexable = indexable[i]
    indexable[indices[-1]] = value


data = [[12, 15], [21, 22], [118, 546], [200, 1200]]
list_assignment(data, [0, 0], 0)
print(data)

Output:

[[0, 15], [21, 22], [118, 546], [200, 1200]]

This works for anything you can index:

d = { 
    'a': {
        'b': 1,
        'c': 2,
    },
    'd': 3
}

list_assignment(d, ['a', 'c'], 42)
print(d)

Output:

{'a': {'b': 1, 'c': 42}, 'd': 3}

Keep in mind that this can go wrong in several ways:

• an empty list of indices fails
• a list with too many indices, or missing indices, fails