Tuples and sets

Some data should never change: the days of the week, the geographic coordinates of Baku, the RGB code of a colour. At other times you only want unique values: say you want to know which cities visitors to a website come from, counting each city only once. Python has special types for these two jobs — the tuple and the set.

Tuples

baku = (40.41, 49.87)
print(baku)
print(baku[0])
print(len(baku))

days = ('Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun')
print(days[-1], days.index('Wed'))

(40.41, 49.87)
40.41
2
Sun 2

Indexes, slices, len(), in and the for loop all work with tuples too. But if you write days[0] = 'Monday', Python raises TypeError: 'tuple' object does not support item assignment. This is actually an advantage: no part of your program can change important data by mistake.

The handiest feature of tuples is unpacking: you can put the items into separate variables in one line. The same trick lets you swap two variables without a helper variable.

lat, lon = (40.41, 49.87)
print('Latitude:', lat)
print('Longitude:', lon)

a = 5
b = 8
a, b = b, a
print(a, b)

Latitude: 40.41
Longitude: 49.87
8 5

Unpacking also works in loops. The enumerate() function gives each item together with its number as a (number, item) tuple — the easiest way to number a list. start=1 makes the counting begin at 1.

students = ['Aysel', 'Murad', 'Leyla']
for number, name in enumerate(students, start=1):
    print(f'{number}. {name}')

print(list(enumerate('ab')))

1. Aysel
2. Murad
3. Leyla
[(0, 'a'), (1, 'b')]

Sets

A set throws away duplicates automatically. If you pass a list to the set() function, only one of each value remains. Because a set has no order, we sort it with sorted() to print it neatly.

print({3, 1, 2, 3, 1})

visitors = ['Baku', 'Ganja', 'Baku', 'Shaki', 'Ganja', 'Baku']
unique = set(visitors)
print(len(visitors), len(unique))
print(sorted(unique))

unique.add('Quba')
unique.add('Baku')
print(len(unique))
print('Shaki' in unique)

{1, 2, 3}
6 3
['Baku', 'Ganja', 'Shaki']
4
True

Notice that adding 'Baku' again changed nothing because it was already in the set — the count became 4, not 5. Checking with in is much faster for sets than for lists.

text = 'the sun and the moon and the stars'
words = text.split()
print(len(words), 'words,', len(set(words)), 'unique')

8 words, 5 unique

Set operations

The set operations you know from maths are written with a single symbol in Python. Suppose a school has a chess club and a robotics club. Which students go to both?

chess = {'Aysel', 'Murad', 'Elvin'}
robotics = {'Murad', 'Leyla', 'Elvin', 'Nigar'}

print(sorted(chess & robotics))
print(sorted(chess | robotics))
print(sorted(chess - robotics))
print(sorted(chess ^ robotics))

['Elvin', 'Murad']
['Aysel', 'Elvin', 'Leyla', 'Murad', 'Nigar']
['Aysel']
['Aysel', 'Leyla', 'Nigar']

Which one to choose?

results = [('Aysel', 91), ('Murad', 78), ('Leyla', 85)]
# print: 1. Aysel - 91, then 2. Murad - 78, ...

1. Aysel - 91
2. Murad - 78
3. Leyla - 85

monday = ['Aysel', 'Murad', 'Leyla', 'Aysel', 'Elvin']
tuesday = ['Leyla', 'Nigar', 'Murad', 'Nigar']
# 1) number of different pupils on Monday
# 2) sorted list of pupils who came on both days
# 3) number of different pupils in total

4
['Leyla', 'Murad']
5