Imagine storing the marks of 60 students in 60 separate variables — impossible to manage! A list stores all of them in one place, in order, and lets you add, remove, sort and search them easily. Lists are the most used data structure in Python, and they appear in almost every coding interview. Let's master them.
Watch the video first, then use this blog as your notes. Every section has runnable examples, and at the end you will find a "Predict the Output" quiz, practice challenges, a self-check list, interview questions and a quick summary.
How to use this blog: Open the Python shell (type python in your terminal) or any online Python editor and type every example yourself. Change the values, break the code and see what happens — that is how concepts really stick.
A list is an ordered, mutable collection of items written inside square brackets [ ] and separated by commas.
marks = [85, 92, 78, 90]
fruits = ["apple", "mango", "banana"]
mixed = [1, "hello", 3.5, True, [1, 2]] # any types, even another list
empty = [] # or list()
print(type(marks)) # <class 'list'>| Property | Meaning |
|---|---|
| Ordered | Items keep the order in which you added them |
| Mutable | You can change, add and remove items after creation |
| Allows duplicates | [1, 1, 2] is perfectly valid |
| Heterogeneous | Items can be of different types |
| Indexed | Each item has a position starting from 0 |
List: [ 10, 20, 30, 40, 50 ]
Index: 0 1 2 3 4
Negative: -5 -4 -3 -2 -1nums = [10, 20, 30, 40, 50]
print(nums[0]) # 10 first item
print(nums[-1]) # 50 last item
print(nums[2]) # 30
print(len(nums)) # 5
# print(nums[5]) # IndexError: list index out of rangeSyntax: list[start : stop : step] — stop is not included.
l = [10, 20, 30, 40, 50]
print(l[1:4]) # [20, 30, 40]
print(l[:3]) # [10, 20, 30]
print(l[-2:]) # [40, 50]
print(l[::2]) # [10, 30, 50] every 2nd item
print(l[::-1]) # [50, 40, 30, 20, 10] reversed
print(l[:]) # a full copycolors = ["red", "green", "blue"]
colors[1] = "yellow"
print(colors) # ['red', 'yellow', 'blue']
colors[0:2] = ["black", "white"]
print(colors) # ['black', 'white', 'blue']l = [1, 2, 3]
l.append(4) # add at the end
print(l) # [1, 2, 3, 4]
l.insert(1, 15) # insert 15 at index 1
print(l) # [1, 15, 2, 3, 4]
l.extend([5, 6]) # add many items
print(l) # [1, 15, 2, 3, 4, 5, 6]append vs extend: [1, 2, 3].append([4, 5]) gives [1, 2, 3, [4, 5]] (one item: a list). [1, 2, 3].extend([4, 5]) gives [1, 2, 3, 4, 5].
l = [10, 20, 30, 40, 50]
l.remove(30) # remove by VALUE (first match)
print(l) # [10, 20, 40, 50]
last = l.pop() # remove and return the last item
print(last, l) # 50 [10, 20, 40]
l.pop(0) # remove by INDEX
print(l) # [20, 40]
del l[0] # delete by index
print(l) # [40]
l.clear() # remove everything
print(l) # []| Method | Removes by | Returns item? | Error if missing |
|---|---|---|---|
| remove(x) | Value | No | ValueError |
| pop(i) | Index (default last) | Yes | IndexError |
| del l[i] | Index / slice | No | IndexError |
| clear() | Everything | No | — |
nums = [5, 2, 9, 1, 5]
print(5 in nums) # True
print(nums.index(9)) # 2
print(nums.count(5)) # 2
print(max(nums), min(nums), sum(nums)) # 9 1 22
print(sorted(nums)) # [1, 2, 5, 5, 9] new list
print(nums) # [5, 2, 9, 1, 5] original unchanged
nums.sort(reverse=True) # sorts IN PLACE
print(nums) # [9, 5, 5, 2, 1]
nums.reverse()
print(nums) # [1, 2, 5, 5, 9]Trap: x = nums.sort() makes x equal to None, because sort() changes the list in place and returns nothing. Use sorted() when you need a new list.
fruits = ["apple", "mango", "kiwi"]
for fruit in fruits:
print(fruit)
for i, fruit in enumerate(fruits):
print(i, fruit) # 0 apple, 1 mango, 2 kiwi
names = ["Aman", "Riya"]
marks = [88, 95]
for name, mark in zip(names, marks):
print(name, mark) # Aman 88, Riya 95Syntax: [expression for item in iterable if condition]
# Normal loop
squares = []
for x in range(1, 6):
squares.append(x ** 2)
# Same thing with list comprehension
squares = [x ** 2 for x in range(1, 6)]
print(squares) # [1, 4, 9, 16, 25]
evens = [x for x in range(10) if x % 2 == 0]
print(evens) # [0, 2, 4, 6, 8]
words = ["python", "java", "c"]
print([w.upper() for w in words]) # ['PYTHON', 'JAVA', 'C']matrix = [
[1, 2, 3],
[4, 5, 6],
[7, 8, 9]
]
print(matrix[1][2]) # 6 (row 1, column 2)
for row in matrix:
print(*row)
# 1 2 3
# 4 5 6
# 7 8 9a = [1, 2, 3]
b = a # NOT a copy - both names point to the same list
b.append(4)
print(a) # [1, 2, 3, 4]
c = a.copy() # or a[:] or list(a) - a real (shallow) copy
c.append(99)
print(a) # [1, 2, 3, 4] unchangedFor nested lists, use copy.deepcopy() — a shallow copy still shares the inner lists.
print([1, 2] + [3, 4]) # [1, 2, 3, 4] concatenation
print([0] * 5) # [0, 0, 0, 0, 0] repetition
print(list("abc")) # ['a', 'b', 'c']
print(" ".join(["I", "love", "Python"])) # I love Python
print("a,b,c".split(",")) # ['a', 'b', 'c']| Method | Description |
|---|---|
| append(x) | Add x at the end |
| extend(iterable) | Add all items of an iterable |
| insert(i, x) | Insert x at index i |
| remove(x) | Remove first occurrence of x |
| pop(i) | Remove and return item at i (default last) |
| clear() | Remove all items |
| index(x) | Index of first x |
| count(x) | Number of times x appears |
| sort() | Sort in place |
| reverse() | Reverse in place |
| copy() | Shallow copy |
marks = [78, 92, 65, 88, 95, 54, 81]
print("Total students:", len(marks))
print("Highest:", max(marks))
print("Lowest:", min(marks))
print(f"Average: {sum(marks) / len(marks):.2f}")
print("Passed (>= 60):", [m for m in marks if m >= 60])
print("Top 3:", sorted(marks, reverse=True)[:3])Total students: 7
Highest: 95
Lowest: 54
Average: 79.00
Passed (>= 60): [78, 92, 65, 88, 95, 81]
Top 3: [95, 92, 88]Don't run the code yet! Write your answers in a notebook, then check the Answers section at the end of this blog.
Q1. What is the output?
l = [10, 20, 30, 40]
print(l[-1], l[1:3])Q2. What is the output?
l = [1, 2, 3]
l.append([4, 5])
print(len(l))Q3. What is the output?
x = [3, 1, 2]
print(x.sort())Q4. What is the output?
print([1, 2] * 2)Q5. What is the output?
a = [1, 2, 3]
b = a
b[0] = 100
print(a)Q6. What is the output?
print([x * 2 for x in range(4) if x != 2])Q7. What is the output?
l = [5, 6, 7]
print(l.pop(), l)Solve these from easy to tricky:
Solutions for challenges 2, 3, 4 and 6 are in the Answers section.
Tick off each skill once you are confident:
Q: Difference between append() and extend()?
append adds its argument as one item; extend adds every element of an iterable.
Q: Difference between sort() and sorted()?
sort() is a list method that changes the list and returns None. sorted() is a built-in that returns a new sorted list and works on any iterable.
Q: Difference between remove(), pop() and del?
remove deletes by value, pop deletes by index and returns the item, del deletes by index or slice (or the whole variable).
Q: What is the time complexity of common list operations?
Index access and append: O(1). insert/pop at the beginning, in search and remove: O(n). sort: O(n log n).
Q: What is the difference between a shallow copy and a deep copy?
A shallow copy creates a new outer list but shares the inner objects; a deep copy (copy.deepcopy) copies everything recursively.
| Concept | Syntax / Key point |
|---|---|
| Create | l = [1, 2, 3] |
| Access | l[0], l[-1] |
| Slice | l[1:3], l[::-1] |
| Add | l.append(x) |
| Remove | l.remove(x), l.pop() |
| Sort | l.sort(), sorted(l) |
| Length | len(l) |
| Comprehension | [x*2 for x in l] |
# Challenge 2
nums = [12, 45, 2, 41, 45, 30]
unique = sorted(set(nums), reverse=True)
print(unique[1]) # 41
# Challenge 3
items = [3, 1, 3, 2, 1, 5]
result = []
for x in items:
if x not in result:
result.append(x)
print(result) # [3, 1, 2, 5]
# Challenge 4
print([n for n in range(1, 51) if n % 7 == 0]) # [7, 14, 21, 28, 35, 42, 49]
# Challenge 6
nested = [[1, 2], [3, 4], [5, 6]]
flat = [x for row in nested for x in row]
print(flat) # [1, 2, 3, 4, 5, 6]Lists are your Swiss-army knife in Python. You can now create, access, modify, sort and transform them like a pro. Next, learn if / elif / else to make your programs take decisions.
If this blog helped you, watch the full video above, subscribe to the channel and share it with a friend who is learning Python. Happy coding!