What is the difference between these two methods;

# Solution 1
list1 = ['a', 'b', 'c', 'd']
temp_list1 = list1
for i in range(len(list1)):
    temp_list1[i] = "1"
print list1;              # ['1', '1', '1', '1']
print temp_list1;         # ['1', '1', '1', '1']

# Solution 2
list2 = ['a', 'b', 'c', 'd']
temp_list2 = list2
temp_list2 = ['1', '1', '1', '1']
print list2;              # ['a', 'b', 'c', 'd']
print temp_list2;         # ['1', '1', '1', '1']

By using solution1, want to update the temp_list1 with some other value at the same time want to retain the old value in list1. But this updated the both lists.

Because your temporary list is the same thing as your other list. You said temp_list1 = list1 . This means that the value for temp_list1 will be the same address in memory as the value for list1, i.e. they reference the same thing. Modifying one modifies the other because they are just 2 different names for the same data.

I forgot to mention that in solution #2, you assign temp_list2 to the same address of memory as list2, but then that's undone as you create a new list (and new address in memory for it) consisting of four ones. Therefore, temp_list2 no longer refers to the same data as list2.

If you want to copy the list over without making the new name reference the same data, how about

list1 = ['a', 'b', 'c', 'd']
list2 = [x for x in list1]

That'll copy all the indices of list1 into a new address for list2, and thus, they are independent of each other.

Simply use list():

list1 = ['a', 'b', 'c', 'd']
# this will create a new list with a different mem ref
temp_list1 = list(list1)
for i in range(len(list1)):
    temp_list1[i] = "1"
print list1;              # ['a', 'b', 'c', 'd']
print temp_list1;         # ['1', '1', '1', '1']

For nested lists you need to use deepcopy() from module copy.

Comments
Well I learned something new about using list() :P

Here is an example for the use of deepcopy with a nested list. First without ...

# a nested list
list1 = ['a', 'b', 'c', 'd', [1, 2, 3]]
list2 = [x for x in list1]

# now replace 2 with 77 in list1
list1[4][1] = 77

print(list1)  # ['a', 'b', 'c', 'd', [1, 77, 3]]
print(list2)  # ['a', 'b', 'c', 'd', [1, 77, 3]] oops!!!

now with ...

import copy

# a nested list
list1 = ['a', 'b', 'c', 'd', [1, 2, 3]]
list2 = copy.deepcopy(list1)

# now replace 2 with 77 in list1
list1[4][1] = 77

print(list1)  # ['a', 'b', 'c', 'd', [1, 77, 3]]
print(list2)  # ['a', 'b', 'c', 'd', [1, 2, 3]]   okay!!!

In the first example the list comprehension simply copied the inner list as an alias list which shares the memory reference.

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