Question

Let’s say I have a list like:

my_list = [[1,2,3],[4,5,6],[7,8,9]] 

How do I alter every value in the list without doing?:

for x in range(0, 3):     for y in range(0, 3):         my_list[x][y] = -my_list[x][y] 

I have tried to simplify this by doing

my_list = [[[-a, -b, -c] for [a, b, c] in d] for d in my_list] 

but the values remain the same.

Answer 1

Many answers are about creating altered copy of list, but literal meaning of question is about in-place modification of list.

Here is my version of best-of-breed in-place list altering solution:

def alter_elements(lst, func):   for i, item in enumerate(lst):     if isinstance(item, list):       alter_elements(item, func)     else:       lst[i] = func(item) 

Test run:

>>> sample = [[1,2,3],[4,5,6],[7,8,9]] >>> alter_elements(sample, lambda x: -x) >>> print sample >>> [[-1, -2, -3], [-4, -5, -6], [-7, -8, -9]] 

No list copies. No hardcoded bounds. No list comprehensions with side-effects.