Question:
The standard way is via list list comprehension :
my_list=[1,2,3,4,5,6]
my_list_1=[i*5 for i in my_list]
print(my_list_1)
Why can't I do the same with the original list:
my_list=[1,2,3,4,5,6]
for i in my_list:
i*=5
print(my_list)
This code doesn't work, why?
Answer:
For this design:
for i in my_list:
i*=5
Python creates a new variable i which contains the values from the list in sequence. As a result, you change the values of the variable i , the list remains unchanged.
In [136]: my_list=[1,2,3,4,5,6]
...: for i in my_list:
...: i*=5
...: print(i, my_list)
...:
5 [1, 2, 3, 4, 5, 6]
10 [1, 2, 3, 4, 5, 6]
15 [1, 2, 3, 4, 5, 6]
20 [1, 2, 3, 4, 5, 6]
25 [1, 2, 3, 4, 5, 6]
30 [1, 2, 3, 4, 5, 6]
In the following line of code, you assign the result of a list comprehension to my_list_1 :
my_list_1=[i*5 for i in my_list]
Solution:
my_list[:] = [i*5 for i in my_list]
print(my_list)
#[5, 10, 15, 20, 25, 30]
PS You can read more about assignments to lists and slices in the answers to this question in the English version of SO