list.append() / list.extend() / list.insert()

# Adds a single element to the end of the list (returns None).
list.append(element)

# Adds all elements from another iterable to the end of the list (returns None).
list.extend(iterable)

# Inserts an element at the specified index position (returns None).
list.insert(index, element)

# Use append() to add an element to the end of the list.
fruits = ["apple", "orange"]
fruits.append("grape")
print(fruits)  # ['apple', 'orange', 'grape']

# append() adds a list as a single nested element (watch out for nesting).
more = ["strawberry", "peach"]
fruits.append(more)  # The entire list is added as one element.
print(fruits)  # ['apple', 'orange', 'grape', ['strawberry', 'peach']]
print(len(fruits))  # 4 (four elements total)

# Use extend() to add elements from another list individually.
fruits2 = ["apple", "orange"]
more2 = ["strawberry", "peach"]
fruits2.extend(more2)
print(fruits2)  # ['apple', 'orange', 'strawberry', 'peach']
print(len(fruits2))  # 4

# Passing a string to extend() adds it one character at a time.
letters = ["a", "b"]
letters.extend("cde")  # A string is treated as an iterable.
print(letters)  # ['a', 'b', 'c', 'd', 'e']

# Use insert() to add an element at the beginning (index 0).
nums = [2, 3, 4]
nums.insert(0, 1)
print(nums)  # [1, 2, 3, 4]

# Use insert() to add an element in the middle.
nums.insert(2, 99)
print(nums)  # [1, 2, 99, 3, 4]

# A typical pattern: building a list by appending in a loop.
squares = []
for i in range(1, 6):
    squares.append(i ** 2)
print(squares)  # [1, 4, 9, 16, 25]

# The difference between the + operator and extend().
# + returns a new list (the original is unchanged).
a = [1, 2, 3]
b = a + [4, 5]
print(a)  # [1, 2, 3] (unchanged)
print(b)  # [1, 2, 3, 4, 5] (new list)

# extend() modifies the original list in place.
a.extend([4, 5])
print(a)  # [1, 2, 3, 4, 5]

Although append() and extend() look similar, they handle their arguments differently. When you pass a list, append() nests the entire list as a single element, whereas extend() unpacks the list and adds each element individually. Use extend() when you want to merge the elements of one list into another.

insert() is useful when you need to add an element at the beginning (index 0) or somewhere in the middle of a list. Keep in mind that inserting at the beginning requires shifting all existing elements, so it can be costly for large lists. If you frequently add to the front, consider using collections.deque and its appendleft() method instead.

All three methods return None. Writing new_list = my_list.append(x) assigns None to new_list, not the updated list. For removing elements, see list.pop() / list.remove().