A crash course in Python - Lesson 4: other data structures

Tuple

A tuple is an immutable list. Technically, you can do on/with tuples whatever you can do on a list that does not involve modifying it.

A tuple can be specified by using parentheses (or nothing) instead of square brackets:

my_tuple = (1, 2)
other_tuple = 3, 4

try:
    my_tuple[1] = 3
except TypeError:
    print "cannot modify a tuple"

A tuple is a convenient way of returning multiple values from functions:

def sum_and_product(x, y):
    return (x + y), (x * y)

sp = sum_and_product(2, 3)  # equals (5,6)
s, p = sum_and_product(5, 10) # s = 15, p = 50

Consequently, tuples can be used for multiple assignments too:

x, y = 1, 2  # x = 1, y = 2
x, y = y, x  # swap: x = 2, y = 1

Set

A set represents a collection of distinct elements. It can be declared, modified and manipulated as follows:

s = set()
s.add(1)  # s = {1}
s.add(2)  # s = {1, 2}
s.add(2)  # s = {1, 2}
x = len(s)  # x = 2
y = 2 in s  # y = True
z = 3 in s  # z = False

Sets perform very well when one has to check if they contain a specific value: in is a very fast operation! So, if we have a large collection of items that we want to use for a membership test, a set is more appropriate than a list.
Moreover, if one builds a set starting from a list, what he obtains is the set of distinct values in that list:

my_list = [1, 2, 3, 2, 1, 3]
item_set = set(my_list)  # {1, 2, 3}
distinct_elements = list(item_set) # [1, 2, 3]