Python多维列表：如何不用链式索引实现类似splat运算符的索引赋值？

Question #

I have a 2D list a = [[1, 2, 3], [4, 5, 6]] and an index list index = [1, 2]. I want to assign a value to the element at the position specified by index using a syntax similar to a[*index] = 9 (or a[**index] = 9), but neither of these works. I'm looking for a way to do this without chained indexing like a[index[0]][index[1]] and without importing external libraries.

Answer #

Great question! Python doesn’t natively support unpacking index lists directly in square brackets for assignment, but there are a couple of clean, built-in approaches to get the job done:

1. Manual Index Traversal (No Imports Needed) #

This is the most straightforward method—we just walk through the nested list step-by-step to reach the parent of our target element, then assign using the final index:

a = [[1, 2, 3], [4, 5, 6]]
index = [1, 2]

# Navigate to the parent list of the target element
current_level = a
for idx in index[:-1]:
    current_level = current_level[idx]

# Assign the value to the final position
current_level[index[-1]] = 9

print(a)  # Output: [[1, 2, 3], [4, 5, 9]]

This works for any depth of nested lists, not just 2D—just adjust the index list to match the depth you need.

2. Use functools.reduce (Standard Library, No External Installs) #

If you prefer a more concise approach, you can use Python’s built-in reduce function from the functools module (which is part of the standard library, so no extra packages required):

from functools import reduce

a = [[1, 2, 3], [4, 5, 6]]
index = [1, 2]

# Use reduce to navigate to the parent list
parent_list = reduce(lambda lst, idx: lst[idx], index[:-1], a)
parent_list[index[-1]] = 9

print(a)  # Output: [[1, 2, 3], [4, 5, 9]]

The reduce function applies the lambda repeatedly to each index (except the last one), starting from the top-level list a, until we reach the parent of our target element.

Both methods avoid chained indexing like a[index[0]][index[1]] and stick to Python’s built-in tools.

内容的提问来源于stack exchange，提问作者dylnan