Dictionaries

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Part 1

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What is it?

A dictionary is an unordered mapping of key-value pairs.
Dictionaries are indexed by keys of any immutable type.

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Creating A Dictionary

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Colon Separated Key Value Pairs

empty_dict = {}
phone_numbers = {
    'mom': '302-123-4567', 
    'dad': '302-123-4567', 
    'my_boy_blue': '302-654-9876'
}

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dict Function

empty_dict = dict()
items_to_prices = dict(milk=4.5, bread=2.55, beef=7.50)

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Accessing Elements From A Dictionary

Use keys to look up corresponding values.

english_to_spanish = {
    'one': 'uno', 
    'two': 'dos', 
    'three': 'tres'
}
english_to_spanish['one']
# 'uno'
english_to_spanish['four']
# Traceback (most recent call last):
#   File "<stdin>", line 1, in <module>
# KeyError: 'four'

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Dictionaries

Part 2

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The In Operator - Keys

The in operator can be used to check if a key is present in the dictionary.

english_to_spanish = {
    'one': 'uno', 
    'two': 'dos', 
    'three': 'tres'
}
'one' in english_to_spanish
# True
'four' in english_to_spanish
# False

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The In Operator - Values

english_to_spanish = {
    'one': 'uno', 
    'two': 'dos', 
    'three': 'tres'
}
english_to_spanish.keys()
# dict_keys(['one', 'two', 'three'])
english_to_spanish.values()
# dict_values(['uno', 'dos', 'tres'])
'uno' in english_to_spanish.values()
# True

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Updating Items In A Dictionary

If the item already exist, it will be updated. If the item does not exist, it will be added.

spells = {}
spells['Expelliarmus'] = 'The only spell Harry knows.'
print(spells)
# {'Expelliarmus': 'The only spell Harry knows.'}
spells['Expelliarmus'] = 'A useless spell that can be ignored if you own a Nintendo Wii hand strap.'
print(spells)
# {'Expelliarmus': 'A useless spell that can be ignored if you own a Nintendo Wii hand strap.'}

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Deleting Items From Dictionary

nums_to_words = {1: 'one', 2: 'two', 3: 'three'}
print(nums_to_words)
# {1: 'one', 2: 'two', 3: 'three'}

del

del nums_to_words[1]
print(nums_to_words)
# {2: 'two', 3: 'three'}

pop

item = nums_to_words.pop(3)
print(item)
# three
print(nums_to_words)
# {2: 'two'}

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Dictionaries

Part 3

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Dictionary Methods - get

Returns the value for key if key is in the dictionary, else returns a default item.
This method avoids KeyError.

xmen_powers = {'Phoenix': 'Telepathic and telekinetic', 'Wolverine': 'Healing and claws'}
result = xmen_powers.get('Wolverine')
print(result)
# Healing and claws

Default value provided

result = xmen_powers.get('Nightcrawler', 'Unknown Powers')
print(result)
# Unknown Powers

Default value not provided

result = xmen_powers.get('Captain Underpants') 
print(result)
# None

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Dictionary Iteration

xmen_powers = {
    'Phoenix': 'Telepathic and telekinetic', 
    'Wolverine': 'Healing and claws'
}
for key in xmen_powers:
    print("key: {0:20s} \tValue: {1:s}".format(key, xmen_powers[key]))

# key: Phoenix              	Value: Telepathic and telekinetic
# key: Wolverine            	Value: Healing and claws

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Dictionary Methods - items

The items() method in dictionaries returns a sequence of tuples where each tuple is a key-value pair.

simple_dict = {'a': 1, 'b': 2, 'c': 3}
simple_dict.items()
# dict_items([('a', 1), ('b', 2), ('c', 3)])

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Iterating Dictionaries With items Method

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

# a 1
# b 2
# c 3

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zip Built-in Function Revisited

keys = ['a', 'b', 'c']
values = [1, 2, 3]
my_dict = dict(zip(keys, values))
print(my_dict)
# {'a': 1, 'b': 2, 'c': 3}

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Dictionaries as kwargs - Gather

An arbitrary number of keyword arguments can be passed to a function. A parameter that beings with ** gathers keyword arguments into a dictionary.

def print_team(**kwargs):
    print(type(kwargs))
    for key, value in kwargs.items():
        print(f"{key}: {value}")

print_team(bait="Scooby-Doo", lovable_coward="Shaggy", leader="Fred", brain="Velma", investor="Daphne")
# <class 'dict'>
# bait: Scooby-Doo
# lovable_coward: Shaggy
# leader: Fred
# brain: Velma
# investor: Daphne

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Dictionaries as kwargs - Scatter

team = {
    "bait": "Scooby-Doo",
    "lovable_coward": "Shaggy",
    "leader": "Fred",
    "brain": "Velma",
    "investor": "Daphne"
}

def print_team(leader, brain, investor, **kwargs):
    print(f"leader: {leader}")
    print(f"brain: {brain}")
    print(f"investor: {investor}")
    print(kwargs)

print_team(**team)
# leader: Fred
# brain: Velma
# investor: Daphne
# {'bait': 'Scooby-Doo', 'lovable_coward': 'Shaggy'}

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Sets

Part 1

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What is it?

A set works much like a list but without allowing any duplicates.

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Why

Common uses include:

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Creating A Set - Using Curly Braces

my_set = {1, 2, 3, 4}
print(my_set)
# {1, 2, 3, 4}
type(my_set)
# <class 'set'>

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Creating A Set - Using Built-in Function

my_other_set = set(['a', 'a', 'b', 'c'])
print(my_other_set)
# {'b', 'a', 'c'}
yet_another_set = set()
print(yet_another_set)
# set()

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Membership Testing

Use the in keyword as with other types to determine membership.

zoo_animals = set(['lion', 'elephant', 'rhino'])
'pigeon' in zoo_animals
# False

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Sets

Part 2

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add

If the item being added already exists, nothing happens. Otherwise the new item is added.

abcs = {'a', 'b', 'c'}
abcs.add('d')
print(abcs)
# {'a', 'b', 'c', 'd'}
 
abcs.add('a')
print(abcs)
# {'a', 'b', 'c', 'd'}

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update

Update the set, adding elements form all others.

set_1 = {1, 2, 3}
set_2 = set()
set_2.update(set_1)
set_2
# {1, 2, 3}

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remove

Removes an item from the set. If the item isn’t present in the set, it raises a KeyError.

abcs = {'a', 'b', 'c'}
abcs.remove('a')
print(abcs)
# {'b', 'c'}

abcs.remove('z')
# Traceback (most recent call last):
#  File "<stdin>", line 1, in <module>
# KeyError: 'z'

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discard

Just like remove but does not raise exception.

abcs = {'a', 'b', 'c'}
abcs.discard('z')

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pop

Arbitrarily removes and returns an item from the set.

abcs = {'a', 'b', 'c'}

removed_item = abcs.pop()
print(removed_item)
# a 

removed_item = abcs.pop()
print(removed_item)
# b

print(abcs)
# {'c'}

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clear

Removes all items in one shot.

abcs = {'a', 'b', 'c'}
abcs.clear()
print(abcs)
# set()

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The End

Parrot