Python List Methods: index(), count(), reverse(), sort()

Let’s explore these essential list methods with multiple examples for each.

1. index() Method

Returns the index of the first occurrence of a value.

Examples:

python

# Example 1: Basic usage
fruits = ['apple', 'banana', 'cherry', 'banana']
print(fruits.index('banana'))  # Output: 1

# Example 2: With start parameter
print(fruits.index('banana', 2))  # Output: 3 (starts searching from index 2)

# Example 3: ValueError when not found
try:
    print(fruits.index('orange'))
except ValueError:
    print("Not found")  # Output: Not found

# Example 4: With start and end parameters
numbers = [1, 2, 3, 4, 3, 5]
print(numbers.index(3, 2, 5))  # Output: 4 (finds 3 between indexes 2-5)

2. count() Method

Returns the number of occurrences of a value.

Examples:

python

# Example 1: Basic count
numbers = [1, 2, 3, 1, 4, 1, 5]
print(numbers.count(1))  # Output: 3

# Example 2: Count with strings
words = ['a', 'the', 'a', 'an', 'the']
print(words.count('the'))  # Output: 2

# Example 3: Count with no matches
print(numbers.count(9))  # Output: 0

# Example 4: Count with nested lists (only counts top-level)
nested = [[1, 2], [3, 4], [1, 2]]
print(nested.count([1, 2]))  # Output: 2

3. reverse() Method

Reverses the elements of the list in place (modifies original).

Examples:

python

# Example 1: Basic reverse
nums = [1, 2, 3, 4]
nums.reverse()
print(nums)  # Output: [4, 3, 2, 1]

# Example 2: Reverse strings
colors = ['red', 'green', 'blue']
colors.reverse()
print(colors)  # Output: ['blue', 'green', 'red']

# Example 3: Reverse in place vs reversed()
original = [1, 2, 3]
# reversed() returns an iterator and doesn't modify original
print(list(reversed(original)))  # Output: [3, 2, 1]
print(original)  # Output: [1, 2, 3] (unchanged)

# Example 4: Reverse empty list
empty = []
empty.reverse()
print(empty)  # Output: [] (no error)

4. sort() Method

Sorts the list in place (modifies original).

Examples:

python

# Example 1: Basic sort
nums = [3, 1, 4, 2]
nums.sort()
print(nums)  # Output: [1, 2, 3, 4]

# Example 2: Reverse sort
nums.sort(reverse=True)
print(nums)  # Output: [4, 3, 2, 1]

# Example 3: Sort strings
fruits = ['banana', 'apple', 'cherry', 'date']
fruits.sort()
print(fruits)  # Output: ['apple', 'banana', 'cherry', 'date']

# Example 4: Custom sort with key
words = ['apple', 'Banana', 'cherry', 'Date']
words.sort(key=str.lower)  # Case-insensitive sort
print(words)  # Output: ['apple', 'Banana', 'cherry', 'Date']

# Example 5: Sort vs sorted()
original = [3, 1, 2]
# sorted() returns new list and doesn't modify original
new_list = sorted(original)
print(new_list)  # Output: [1, 2, 3]
print(original)  # Output: [3, 1, 2] (unchanged)

Key Differences:

• reverse() vs reversed(): One modifies in place, one returns new iterator
• sort() vs sorted(): One modifies in place, one returns new list
• index() raises ValueError if not found
• count() returns 0 if no matches found