List Methods — sort(), reverse(), copy(), and del

What you will learn

Organize · Duplicate · Delete Lists

This lesson focuses on organizing data, creating safe copies, and deleting elements. These methods are essential for interviews because they test your understanding of memory (id()), mutation, and shallow copies.

Tool	Role
copy()	Independent list object
sort()	Arrange values in order
reverse()	Flip existing order in place
del	Delete item, slice, or variable name

Unlike File 02 — mutation basics and File 03 — search and merge, this lesson emphasizes memory behavior and in-place vs new object.

Learning roadmap

Memory formula: Copy → Sort → Reverse → Delete

1. copy() — create an independent list

The biggest beginner mistake is confusing copy with alias.

Alias vs copy (visual)

a = [10, "RS"]
b = a.copy()
print(a is b)  # False

After modifying one list:

a.append("Python")
b.insert(1, "NumPy")
print(a)  # [10, 'RS', 'Python']
print(b)  # [10, 'NumPy', 'RS']

Statement	Memory
b = a	Same object (alias)
b = a.copy()	Different object

copy() performs a shallow copy — only the outer list is duplicated. See Aliasing and Copying for nested lists.

2. reverse() — flip the order

Syntax: listobj.reverse()

reverse() changes the same list in place.

lst = [10, 20, 30, 40]
lst.reverse()
print(lst)  # [40, 30, 20, 10]

x = lst.reverse()
print(x)  # None

reverse() returns nothing. It only mutates the list.

3. sort() — arrange data

Syntax: listobj.sort() / listobj.sort(reverse=True)

Sorting works best on same-type homogeneous data.

Ascending sort

nums = [10, 2, 13, -12, 56, 3]
nums.sort()
print(nums)  # [-12, 2, 3, 10, 13, 56]

Descending order

nums.sort(reverse=True)
print(nums)  # [56, 13, 10, 3, 2, -12]

String sorting

names = ["Zoom", "Rossum", "Anil", "Rajesh", "Babu"]
names.sort()
print(names)  # ['Anil', 'Babu', 'Rajesh', 'Rossum', 'Zoom']

Mixed data?

data = [10, "Python", 20]
data.sort()  # TypeError — cannot compare int with str

sort() vs reverse()

Arranges values logically

Simply flips existing order

Feature	sort()	reverse()
Arranges values	Yes	No
Reverses order	No	Yes
Mutates list	Yes	Yes
Returns	None	None

4. del — delete elements

Unlike methods, del is a Python statement — not a list method.

Delete one element

lst = ["A", "B", "C", "D"]
del lst[2]
print(lst)  # ['A', 'B', 'D']

Delete a slice

lst = [10, "RS", 45.67, "Python", True, "HYD", 23.45]
del lst[3:6]
print(lst)  # [10, 'RS', 45.67, 23.45]

Delete the whole variable

lst = [1, 2, 3]
del lst
print(lst)  # NameError: name 'lst' is not defined

The variable name disappears; the object becomes eligible for garbage collection.

Use	Syntax
Delete one item	del lst[i]
Delete slice	del lst[a:b]
Delete name	del lst

Reverse copy using slicing

Sometimes you want the original list untouched.

lst = [10, 20, 30]
rev = lst[::-1]
print(lst)  # [10, 20, 30]
print(rev)  # [30, 20, 10]

[::-1] creates a new list, while reverse() modifies the original.

Mini project — leaderboard ranking

scores = [88, 95, 72, 95, 81]

backup = scores.copy()          # Step 2: backup
scores.sort(reverse=True)       # Step 3: sort descending

for rank, score in enumerate(scores, start=1):
    print(rank, score)

Visual leaderboard

This combines copy + sort + enumerate into one practical program. backup still holds the original unsorted scores.

One-page revision sheet

Task	Best choice
Sort ascending	sort()
Sort descending	sort(reverse=True)
Reverse same list	reverse()
Reverse without changing original	[::-1]
Create independent list	copy()
Delete one item	del lst[i]
Delete range	del lst[a:b]
Delete variable	del lst

Common interview mistakes

Mistake 1 — saving sort() result:

x = nums.sort()  # x is None — wrong pattern
nums.sort()
print(nums)      # correct

Mistake 2 — alias instead of copy:

b = a            # same object — both change together
b = a.copy()     # independent list — correct

Practice questions

Test yourself before File 05 — Nested Lists:

1. What is the difference between reverse() and [::-1]?
2. Why does sort() return None?
3. What happens after b = a and a.append(5)?
4. Which statement creates an independent list?
5. How do you delete indices 2 through 5?

Knowledge check

Q1: reverse() vs [::-1]?
Answer: reverse() mutates the same list; [::-1] returns a new reversed list.

Q2: Why does sort() return None?
Answer: It sorts in place — the list object is modified directly.

Q3: After b = a and a.append(5), what is b?
Answer: Same as a — alias shares one object.

Q4: Practice Q5 — delete indices 2 through 5?
Answer: del lst[2:6] (slice end is exclusive).