Modify item using IN versus Indexing in Python

Question

I'm trying to modify item in a list using IN and Indexing. While modifying via indexing, the item was actually changed. However, when I try to change via IN keyword, it seems that the item in the list does not change.

What's the difference between both method internally?
When using IN, does it mean that you iterate through the list and assign the value of each item in the variable in for statement?

lines = ['hi', 'hello']

for line in lines:
    line += '*'

for i in range(len(lines)):
    lines[i] += '#'

print(lines)

Output:
['hi#', 'hello#']

Does this answer your question? [Scope of python variable in for loop](https://stackoverflow.com/questions/15363138/scope-of-python-variable-in-for-loop) — kcsquared, Sep 29 '21 at 16:50

score 1 · Answer 1 · answered Sep 29 '21 at 17:51

In the first loop, 'line' is merely a placeholder for value received from lines in each iteration.

Whereas, in the second loop, you're directly referring to the individual elements in the list using their index

lines = ['hi', 'hello']

print(f"Outside the loop - id(lines): {id(lines)}\n")

for line in lines:
    line += '*'
    print(f"id(line): {id(line)}")
print("\n")
for i in range(len(lines)):
    lines[i] += '#'
    print(f"id(lines[{i}]: {id(lines[i])}")
    
    
print(f"\n\nid(lines): {id(lines)},id(lines[0]): {id(lines[0])}, id(lines[1]): {id(lines[1])}\n")
print(lines)

Look at the output. You'll notice that id of 'line' is same throughout the loop and it's different from lines[0] and lines[1]. That means, line and lines[0] are not the same and they're located at different locations in the memory.

OUTPUT:

Outside the loop - id(lines): 2831707232384

id(line): 2831707722928
id(line): 2831707722928


id(lines[0]: 2831707976816
id(lines[1]: 2831707977456


id(lines): 2831707232384,id(lines[0]): 2831707976816, id(lines[1]): 2831707977456

['hi#', 'hello#']

Sorry, I posted a similar idea in my answer at almost the same time. I think they are different enough to both be relevant. — Michael Szczesny, Sep 29 '21 at 17:59

Michael Szczesny · Answer 2 · 2021-09-29T18:00:10.610

The pythonic way is to create a new list and don't try to change elements of a list inplace

lines = ['hi', 'hello']
print([f'{line}#' for line in lines])

Output

['hi#', 'hello#']

Both loops can't change a string. They are immutable in python. Your second loop is assigning a new reference to a new string to the given index.

We can verify that by comparing the id of the strings

lines = ['hi', 'hello']

print(*map(id, lines))
for i in range(len(lines)):
    lines[i] += '#'
print(*map(id, lines))

print(lines)

Output

139964185810032 139963627798960
139963551752432 139963317214640
['hi#', 'hello#']

Mutable types (for example a list) behave differently

lines = ['hi', 'hello']

lines = [[*i] for i in lines] # convert strings to mutable lists

print(*map(id, lines))
for line in lines:
    line += ['#']
print(*map(id, lines))

lines = [''.join(i) for i in lines] # convert lists to immutable strings
print(lines)

Output

139963318712752 139963316794224
139963318712752 139963316794224
['hi#', 'hello#']

Modify item using IN versus Indexing in Python

2 Answers2