Lists

There are 25 students in a class, and you need to remember each one's grade. Twenty-five separate variables — grade1, grade2, grade3…? That is tiring and awkward. In Python you can keep all the grades in one list instead — as many items as you like under a single name.

Creating a list

Indexes and slices work in lists just as in strings: the first item is [0], the last is [-1], and len() gives the number of items.

fruits = ['apple', 'pomegranate', 'fig', 'grape']
print(fruits)
print(len(fruits))
print(fruits[0], fruits[-1])
print(fruits[1:3])
print('fig' in fruits)

['apple', 'pomegranate', 'fig', 'grape']
4
apple grape
['pomegranate', 'fig']
True

Changing a list

Unlike strings, lists are mutable: you can replace an item by its index, add new items or remove them. Lists have methods for this:

cart = ['bread', 'milk']
cart.append('cheese')
cart.insert(0, 'tea')
print(cart)

cart.remove('milk')
last = cart.pop()
print('Removed:', last)

cart[0] = 'green tea'
print(cart)

['tea', 'bread', 'milk', 'cheese']
Removed: cheese
['green tea', 'bread']

Lists and loops

Lists and the for loop are made for each other. There are also ready-made functions for calculations: sum() gives the total, and min() and max() give the smallest and largest items. Aysel works out the average of her grades for the term:

grades = [5, 4, 5, 3, 4, 5]
average = sum(grades) / len(grades)
print('Grades:', grades)
print('Max:', max(grades), 'Min:', min(grades))
print(f'Average: {average:.2f}')
print('Number of fives:', grades.count(5))

Grades: [5, 4, 5, 3, 4, 5]
Max: 5 Min: 3
Average: 4.33
Number of fives: 3

A loop is also handy for building a new list from an old one: create an empty list [] and append the items that match a condition.

scores = [45, 82, 67, 90, 38]
passed = []
for s in scores:
    if s >= 50:
        passed.append(s)
print(passed)
print(len(passed), 'of', len(scores), 'passed')

[82, 67, 90]
3 of 5 passed

Sorting, split and join

The sort() method changes the list itself, while the sorted() function returns a new sorted list and leaves the old one untouched. reverse=True gives descending order.

numbers = [42, 7, 19, 3]
print(sorted(numbers))
print(numbers)
numbers.sort(reverse=True)
print(numbers)

[3, 7, 19, 42]
[42, 7, 19, 3]
[42, 19, 7, 3]

Two methods act as a bridge between strings and lists: split() breaks a string into words and returns a list, and join() glues the items of a list into one string.

line = 'Baku Ganja Sumgait Lankaran'
cities = line.split()
print(cities)
print(len(cities))
print(' - '.join(cities))

['Baku', 'Ganja', 'Sumgait', 'Lankaran']
4
Baku - Ganja - Sumgait - Lankaran

a = [1, 2, 3]
b = a
b.append(4)
print(a)

c = a.copy()
c.append(5)
print(a, c)

[1, 2, 3, 4]
[1, 2, 3, 4] [1, 2, 3, 4, 5]

scores = [72, 95, 88, 61, 90]
# 1) the highest score
# 2) the lowest score
# 3) the sum of all scores

95
61
406

temps = [22, 31, 27, 19, 33, 25, 29]
hot = []
# 1) append every temperature above 25 to hot
# 2) sort hot from highest to lowest and print it
# 3) print how many hot days there were

[33, 31, 29, 27]
4