Array Methods

In Python, lists are used as arrays in everyday programming. Lists come with built-in methods that make it easy to add, remove, search, count, sort, and copy the data they hold. A method is called with dot notation:

array_name.method_name(arguments)

This lesson walks through each array (list) method with an example, the expected output, and the key detail to remember.

Important: Most Methods Change the Array in Place

Most of these methods modify the original array and return None. Only a few return a value:

A common mistake is writing marks = marks.sort(), which replaces your array with None.

append() — Add an Item to the End

Adds a single element to the end of the array.

colors = ["red", "blue", "green"]
colors.append("yellow")
print(colors)

Expected output:

['red', 'blue', 'green', 'yellow']

clear() — Remove All Elements

Removes every element, leaving an empty array.

numbers = [1, 2, 3, 4]
numbers.clear()
print(numbers)

Expected output:

[]

copy() — Create a Shallow Copy

Returns a new array containing the same elements.

original = ["pen", "pencil", "eraser"]
duplicate = original.copy()

original.append("marker")

print(original)
print(duplicate)

Expected output:

['pen', 'pencil', 'eraser', 'marker']
['pen', 'pencil', 'eraser']

The copy is independent: adding "marker" to original does not change duplicate. Writing duplicate = original would not copy — both names would point to the same array.

count() — Count Occurrences of a Value

Returns how many times a value appears.

scores = [85, 90, 85, 70, 85]
print(scores.count(85))

Expected output:

3

If the value is not present, count() returns 0.

extend() — Add Multiple Elements

Adds each element from another iterable to the end of the array.

list1 = ["HTML", "CSS"]
list2 = ["JavaScript", "Python"]

list1.extend(list2)
print(list1)

Expected output:

['HTML', 'CSS', 'JavaScript', 'Python']

Compare with append(), which would add the whole second list as one nested element: ['HTML', 'CSS', ['JavaScript', 'Python']].

index() — Find the Position of an Element

Returns the index of the first occurrence of a value.

animals = ["cat", "dog", "rabbit", "dog"]
print(animals.index("dog"))

Expected output:

1

"dog" appears at indexes 1 and 3, but only the first position is returned. If the value does not exist, index() raises a ValueError, so check with in first when the value might be missing.

insert() — Add an Element at a Specific Position

Inserts an element before the given index. The existing elements from that position onward shift right.

cities = ["Delhi", "Mumbai", "Chennai"]
cities.insert(1, "Bengaluru")
print(cities)

Expected output:

['Delhi', 'Bengaluru', 'Mumbai', 'Chennai']

pop() — Remove an Element by Index

Removes the element at the given index and returns it.

tasks = ["study", "exercise", "sleep"]
removed_task = tasks.pop(0)

print(removed_task)
print(tasks)

Expected output:

study
['exercise', 'sleep']

Without an index, pop() removes and returns the last element. Calling pop() on an empty array raises an IndexError.

remove() — Remove an Element by Value

Removes the first element that matches the value.

items = ["apple", "banana", "cherry", "banana"]
items.remove("banana")
print(items)

Expected output:

['apple', 'cherry', 'banana']

Only the first "banana" is removed. If the value is not found, remove() raises a ValueError.

reverse() — Reverse the Order

Reverses the elements in place.

nums = [1, 2, 3, 4, 5]
nums.reverse()
print(nums)

Expected output:

[5, 4, 3, 2, 1]

reverse() flips the current order; it does not sort. For a reversed copy that leaves the original untouched, use slicing: nums[::-1].

sort() — Sort the Array

Sorts the elements in ascending order by default.

marks = [88, 72, 95, 60, 83]
marks.sort()
print(marks)

Expected output:

[60, 72, 83, 88, 95]

Sorting in Descending Order

marks.sort(reverse=True)
print(marks)

Expected output:

[95, 88, 83, 72, 60]

Sorting by a Custom Rule

The key parameter accepts a function that decides what to compare:

names = ["Priya", "Al", "Kumaran"]
names.sort(key=len)
print(names)

Expected output:

['Al', 'Priya', 'Kumaran']

The names are sorted by length instead of alphabetically.

Tip: sort() changes the original array. To keep the original and get a sorted copy, use the built-in sorted() function: ordered = sorted(marks).

Practical Example: Managing a Queue of Orders

orders = ["ORD-101", "ORD-102"]

orders.append("ORD-103")         # New order arrives
orders.insert(0, "ORD-100")      # Priority order goes first
next_order = orders.pop(0)       # Process the first order

print("Processing:", next_order)
print("Waiting:", orders)
print("Orders waiting:", len(orders))

Expected output:

Processing: ORD-100
Waiting: ['ORD-101', 'ORD-102', 'ORD-103']
Orders waiting: 3

append() adds to the back of the queue, and pop(0) takes from the front — a "first in, first out" pattern.

Note: For very large queues, pop(0) is slow because every remaining element must shift. The standard library's collections.deque is designed for fast additions and removals at both ends.

Method Summary

Common Mistakes

Related Concepts

• Python Arrays — creating and accessing arrays
• Python Lists and Python List Methods — the full list reference
• Built-in functions such as len(), sorted(), min(), max(), and sum()