Change: Assign, Insert, Replace a Slice

By the end of this lesson you can:

• Change one item in a list by assigning to its index.
• Replace a whole slice with a different number of items.
• Copy a list properly, and explain what goes wrong when you do not.

Two attempts to change the first item. One of them fails. Which, and why?

word = "cat"
word[0] = "b"

names = ["cat", "dog"]
names[0] = "bat"

Traceback (most recent call last):
TypeError: 'str' object does not support item assignment

A whiteboard, not a printed page. You rub out one word and write another; it is the same board, and everyone looking at it sees the change.

menu = ["pizza", "noodles", "pancake"]

menu[1] = "fried rice"
print(menu)

Output

['pizza', 'fried rice', 'pancake']

No new variable, no reassignment of menu — the list itself changed. Note you can only assign to a position that already exists; menu[9] = "x" raises IndexError. Growing a list is lesson 3.

Assigning to a slice

Put a slice on the left of the = and you replace that whole run at once.

scores = [10, 20, 30, 40, 50]

scores[1:3] = [99, 98]
print(scores)

scores[0:2] = [1]
print(scores)

scores[1:1] = [7, 8]
print(scores)

Output

[10, 99, 98, 40, 50]
[1, 98, 40, 50]
[1, 7, 8, 98, 40, 50]

• Same size — a straight swap of two items for two.
• Fewer — two items become one, and the list gets shorter.
• An empty slice [1:1] selects nothing, so the new items are inserted at that point without removing anything.

Two names, one list

This is the trap, and it catches everyone once.

original = ["Mia", "Leo"]
backup = original

backup[0] = "CHANGED"

print("backup:  ", backup)
print("original:", original)

Output

backup:   ['CHANGED', 'Leo']
original: ['CHANGED', 'Leo']

backup = original did not copy anything. It gave the same list a second name — like writing another label on the same box. Editing through one name changes what the other sees.

original = ["Mia", "Leo"]
backup = original[:]

backup[0] = "CHANGED"

print("backup:  ", backup)
print("original:", original)

Output

backup:   ['CHANGED', 'Leo']
original: ['Mia', 'Leo']

original[:] is a slice of everything, and a slice always builds a new list. list(original) and original.copy() do the same job.

Why it matters. Any program that edits data needs to change items in place, and any program that keeps an "before" copy needs a real one. Both are one line — and one of them is silently wrong if you get it out of order.

One school day, changed three times, with a real copy kept so the changes can be shown.

# timetable.py - edit a day in place, keep a true copy of the original

monday = ["Spanish", "Mathematics", "Science", "Rehat", "Sejarah", "PJK"]
original = monday[:]

print("Before:")
for i in range(len(monday)):
    print(" ", i, "-", monday[i])

# 1. One swap: PJK is cancelled, Coding Club instead
monday[5] = "Coding Club"

# 2. A double period: Science becomes two slots of Physics
monday[2:3] = ["Physics", "Physics Lab"]

# 3. An assembly inserted at the very start, removing nothing
monday[0:0] = ["Perhimpunan"]

print()
print("After:")
for i in range(len(monday)):
    print(" ", i, "-", monday[i])

print()
print("Periods before:", len(original))
print("Periods after: ", len(monday))
print("Original untouched:", original[5])

Output

Before:
  0 - Spanish
  1 - Mathematics
  2 - Science
  3 - Rehat
  4 - Sejarah
  5 - PJK

After:
  0 - Perhimpunan
  1 - Spanish
  2 - Mathematics
  3 - Physics
  4 - Physics Lab
  5 - Rehat
  6 - Sejarah
  7 - Coding Club

Periods before: 6
Periods after:  8
Original untouched: PJK

Three different edits, three different tools:

• Index assignment when one item becomes one other item.
• Slice assignment when one becomes two — the list grows by itself.
• Empty slice [0:0] to insert at the front without deleting.

Lists are mutable, so items[i] = value edits in place. Assigning to a slice replaces that run, and the replacement may be a different length — an empty slice inserts without removing. b = a gives one list two names; b = a[:] makes a real copy.