Python lists

A list holds many values in order

marks = [78, 91, 65, 88]
names = ["Asha", "Ravi", "Meena"]
mixed = ["Asha", 15, True, 91.5]     # different types are allowed
empty = []

Lists use square brackets. Items keep their order and duplicates are allowed. Indexing and slicing work exactly as they do for strings:

print(marks[0])      # 78
print(marks[-1])     # 88
print(marks[1:3])    # [91, 65]
print(len(marks))    # 4

Lists are mutable

Unlike strings, you can change a list in place:

marks[2] = 70
print(marks)         # [78, 91, 70, 88]

Adding and removing items

del names[1] also removes the item at position 1.

Sorting, searching and counting

nums = [40, 10, 30, 10]
nums.sort()                     # changes nums: [10, 10, 30, 40]
nums.sort(reverse=True)         # [40, 30, 10, 10]
ordered = sorted(nums)          # a new sorted list; nums is unchanged
nums.reverse()                  # reverse in place

print(30 in nums)               # True
print(nums.index(30))           # position of the first 30
print(nums.count(10))           # 2
print(sum(nums), max(nums), min(nums))

list.sort() returns None, so nums = nums.sort() loses your list. Use sorted() when you need a new list.

Looping over a list

marks = [78, 91, 65, 88, 42]

total = 0
for m in marks:
    total += m
print("Average:", total / len(marks))

passed = []
for m in marks:
    if m >= 50:
        passed.append(m)
print(passed)          # [78, 91, 65, 88]

When you need the position too, use enumerate():

for i, name in enumerate(["Asha", "Ravi"], start=1):
    print(i, name)

Finding the topper without max()

marks = [78, 91, 65, 88]
best = marks[0]
for m in marks:
    if m > best:
        best = m
print(best)     # 91

This "keep the best so far" idea is used everywhere, from leaderboards to finding the cheapest price.

Lists of lists (2D lists)

A list can hold other lists, which gives you rows and columns like a table:

seats = [
    ["Asha", "Ravi", "Meena"],
    ["Kiran", "Zoya", "Amit"],
]
print(seats[1][2])          # row 1, column 2: Amit

for row in seats:
    for name in row:
        print(name, end="  ")
    print()

Copying lists: b = a does NOT make a copy

a = [1, 2, 3]
b = a           # b is another name for the SAME list
b.append(4)
print(a)        # [1, 2, 3, 4]  -- a changed too!

c = a.copy()    # a real copy (a[:] or list(a) also work)
c.append(5)
print(a)        # [1, 2, 3, 4]  -- unchanged

Example: shopping cart

cart = []
while True:
    item = input("Add item (or 'done'): ")
    if item == "done":
        break
    cart.append(item)
print("You bought", len(cart), "items:", ", ".join(cart))

Common mistakes

• IndexError from using len(list) as an index: the last index is len(list) - 1.
• Writing nums = nums.sort(), which sets nums to None.
• Expecting b = a to copy a list.
• Removing items from a list while looping over the same list.

Key points

• Lists are ordered, changeable and allow duplicates.
• append, insert and extend add; remove, pop and del take away.
• sort() changes the list; sorted() returns a new one.
• list[row][col] reads a 2D list; copy() makes a real copy.

Practice questions

nums = [3, 1, 3, 2, 1]
unique = []
for n in nums:
    if n not in unique:
        unique.append(n)
print(unique)    # [3, 1, 2]

nums = sorted([45, 12, 89, 67])
print(nums[-2])     # 67

for row in [[1, 2, 3], [4, 5, 6]]:
    print(sum(row))     # 6, then 15