A list is a built-in data structure in Python used to store multiple values in a single variable. Lists are ordered, mutable (modifiable), and allow duplicate values. They can hold different data types, such as integers, strings, and even other lists.
1. Creating a List
Lists are defined using square brackets [ ], with elements separated by commas.
Example: Creating Lists
# Empty list
empty_list = []
# List of integers
numbers = [1, 2, 3, 4, 5]
# List of strings
fruits = ["apple", "banana", "cherry"]
# Mixed data types
mixed = [1, "hello", 3.14, True]
# Nested list (list inside another list)
nested_list = [[1, 2], [3, 4]]
2. Accessing List Elements
You can access elements using indexing and slicing.
(a) Indexing (Zero-based Indexing)
fruits = ["apple", "banana", "cherry"]
print(fruits[0]) # First element
print(fruits[1]) # Second element
print(fruits[-1]) # Last element
Output:
apple
banana
cherry
(b) Slicing (Extracting a Subset)
numbers = [10, 20, 30, 40, 50]
print(numbers[1:4]) # Elements from index 1 to 3
print(numbers[:3]) # First 3 elements
print(numbers[2:]) # Elements from index 2 to end
print(numbers[-3:]) # Last 3 elements
Output:
[20, 30, 40]
[10, 20, 30]
[30, 40, 50]
[30, 40, 50]
3. Modifying a List
Lists are mutable, meaning their elements can be changed.
(a) Changing Elements
numbers = [10, 20, 30, 40]
numbers[1] = 25 # Change second element
print(numbers)
Output:
[10, 25, 30, 40]
(b) Adding Elements
numbers.append(50) # Add to end
numbers.insert(2, 15) # Insert at index 2
print(numbers)
Output:
[10, 25, 15, 30, 40, 50]
(c) Removing Elements
numbers.remove(30) # Remove first occurrence of 30
print(numbers)
del numbers[1] # Delete element at index 1
print(numbers)
popped = numbers.pop() # Remove last element
print(popped)
print(numbers)
Output:
[10, 25, 15, 40, 50]
[10, 15, 40, 50]
50
[10, 15, 40]
4. Looping Through a List
You can use loops to iterate over lists.
(a) Using for Loop
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
print(fruit)
Output:
apple
banana
cherry
(b) Using while Loop
i = 0
while i < len(fruits):
print(fruits[i])
i += 1
Output:
apple
banana
cherry
(c) Using enumerate() (Index + Value)
for index, fruit in enumerate(fruits):
print(index, fruit)
Output:
0 apple
1 banana
2 cherry
5. List Comprehensions
A list comprehension is a concise way to create lists.
Example: Creating a List with for Loop
squares = [x**2 for x in range(1, 6)]
print(squares)
Output:
[1, 4, 9, 16, 25]
Example: Filtering a List
evens = [x for x in range(10) if x % 2 == 0]
print(evens)
Output:
[0, 2, 4, 6, 8]
6. Common List Methods
| Method | Description |
|---|---|
append(x) | Adds x to the end of the list |
insert(i, x) | Inserts x at index i |
remove(x) | Removes the first occurrence of x |
pop([i]) | Removes and returns item at index i (last item if i is omitted) |
index(x) | Returns the index of the first occurrence of x |
count(x) | Returns the count of x in the list |
sort() | Sorts the list in ascending order |
reverse() | Reverses the list |
copy() | Returns a shallow copy of the list |
Example: Using List Methods
numbers = [3, 1, 4, 1, 5, 9]
numbers.sort() # Sort in ascending order
print(numbers)
numbers.reverse() # Reverse order
print(numbers)
print(numbers.count(1)) # Count occurrences of 1
Output:
[1, 1, 3, 4, 5, 9]
[9, 5, 4, 3, 1, 1]
2
7. Copying a List
(a) Using copy()
original = [1, 2, 3]
copy_list = original.copy()
copy_list.append(4)
print(original)
print(copy_list)
Output:
[1, 2, 3]
[1, 2, 3, 4]
(b) Using Slicing ([:])
copy_list = original[:]
(c) Using list()
copy_list = list(original)
8. Joining Lists
Example: Concatenation (+)
a = [1, 2, 3]
b = [4, 5, 6]
c = a + b
print(c)
Output:
[1, 2, 3, 4, 5, 6]
Example: Extending a List (extend())
a.extend(b)
print(a)
Output:
[1, 2, 3, 4, 5, 6]
9. Checking if an Item Exists
fruits = ["apple", "banana", "cherry"]
if "banana" in fruits:
print("Banana is in the list!")
Output:
Banana is in the list!
10. Clearing a List
numbers = [1, 2, 3]
numbers.clear()
print(numbers)
Output:
[]