Chapter 3: Data Structures

List #

A data structure that contains items in a contiguous manner:

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

List comprehension #

Lists can also be generated using the list comprehension expression:

squares = [x ** 2 for x in range(1, 10)]

Tuples #

An immutable list data structure:

things = ('apple', 'orange', 'banana')

# Throws an error
things[0] = 'grapes'

Tuples can also be unpacked:

a, b, c = ('apple', 'orange', 'banana')
print(a) # 'apple'
print(b) # 'orange'
print(c) # 'banana'

NOTE: Python will throw an error if there are more items to unpack in the tuple:

# Raises an error
a, b, c = ('apple', 'orange', 'banana', 'grapes')

Sets #

A data structure that contains unique items:

my_set = { 'f', 'o', 'o' }

# Prints { 'f', 'o' }
print(my_set)

NOTE: creating an empty set should be done by calling the set() function with no arguments; defining {} creates an empty dictionary.

Dictionaries #

A data structure of key/value pairs:

person = {
  'name': 'Mohgwyn',
  'type': 'dog',
  'breed': 'Pit Bull'
}

Looping Techniques #

Iterating over dictionaries #

To iterate over dictionaries and retrieving the key/value pairs, use the dictionary's builtin items() method:

for key, value in person.items():
  print(key, value)

Iterating over sequences with their index: #

The enumerate() function returns a tuple with the index and the value of the item in the sequence:

things = ['apple', 'orange', 'banana']

for index, value in enumerate(things):
  print(index, value)

Iterating two or more sequences in parallel #

The zip() function provides a way to iterate over two or more sequences in parallel:

questions = ['name', 'quest', 'favorite color']
answers = ['lancelot', 'the holy grail', 'blue']

for q, a in zip(questions, answers):
    print('What is your {0}?  It is {1}.'.format(q, a))