dictionaries.md

Dictionaries

Reference:

• https://docs.python.org/3/library/stdtypes.html#dict
• https://docs.python.org/3/library/stdtypes.html#dictionary-view-objects
• https://docs.python.org/3/tutorial/datastructures.html#dictionaries
• https://docs.python.org/3/tutorial/datastructures.html#looping-techniques

Many programming languages provide an "associative array" datatype which represents an object with named attributes. Associative arrays are said to have "key/value" pairs, where the "key" represents the name of the attribute and the "value" represents the attribute's value.

Python's implementation of the associative array concept is known as a "dictionary". A Python dictionary comprises curly braces ({}) containing one or more key/value pairs, with each key separated from its value by a colon (:) and each key/value pair separated by a comma (,).

Example dictionaries:

{}

{"a": 1, "b": 2, "c": 3}

{"a": 1, "b": 2, "c": 3, "fruits": ["apple", "banana", "pear"]} # dictionaries can contain lists, or even other nested dictionaries

{"first_name": "Ophelia", "last_name": "Clark", "message": "Hello Again"}

Each dictionary is similar to a row in a CSV-formatted spreadsheet or a record in a database, where the dictionary's "keys" represent the column names and its "values" represent the cell values.

city	name	league
New York	Yankees	major
New York	Mets	major
Boston	Red Sox	major
New Haven	Ravens	minor

[ # LIST OF DICTIONARIES:
    {"city": "New York", "name": "Yankees", "league":"major"}, 
    {"city": "New York", "name": "Mets", "league":"major"},
    {"city": "Boston", "name": "Red Sox", "league":"major"},
    {"city": "New Haven", "name": "Ravens", "league":"minor"}
]

Operations

Accessing dictionary values by their key, or attribute name:

person = {
    "first_name": "Ophelia",
    "last_name": "Clarke",
    "message": "Hi, thanks for the ice cream!",
    "fav_flavors": ["Vanilla Bean", "Mocha", "Strawberry"]
}

print(person["first_name"]) #> "Ophelia"
print(person["last_name"]) #> "Clark"
print(person["message"]) #> "Hi, thanks for the ice cream!"
print(person["fav_flavors"]) #> ["Vanilla Bean", "Mocha", "Strawberry"]
print(person["fav_flavors"][1]) #> "Mocha" (a list is still a list, even if it exists inside a dictionary!)

Adding, updating, or removing attributes from a dictionary:

person = {
    "first_name": "Ophelia",
    "last_name": "Clarke",
    "message": "Hi, thanks for the ice cream!",
    "fav_flavors": ["Vanilla Bean", "Mocha", "Strawberry"]
}

# adding an attribute:
person["message"] = "New Message" # this is mutating

# updating an attribute:
person["fav_color"] = "blue" # this is mutating

# removing an attribute:
del person["fav_flavors"] # this is mutating

print(person) 
#> {'first_name': 'Ophelia', 'last_name': 'Clark', 'message': 'New Message', 'fav_color': 'blue' }

Accessing a dictionary's keys and/or values seperately, and iterating through them:

person = {
    "first_name": "Ophelia",
    "last_name": "Clarke",
    "message": "Hi, thanks for the ice cream!",
    "fav_flavors": ["Vanilla Bean", "Mocha", "Strawberry"]
}

#
# KEYS:
#

print(person.keys()) #> dict_keys(['first_name', 'last_name', 'message', 'fav_flavors'])
# print(list(person.keys())) #> ['first_name', 'last_name', 'message', 'fav_flavors']

for k in person.keys():
    print("KEY:", k)
    

#
# VALUES:
#

print(person.values()) #> dict_values(['Ophelia', 'Clark', 'Hi, thanks for the ice cream!', ["Vanilla Bean", "Mocha", "Strawberry"]])
# print(list(person.values()) #> ['Ophelia', 'Clark', 'Hi, thanks for the ice cream!', ["Vanilla Bean", "Mocha", "Strawberry"]]

for v in person.values():
    print("VALUE:", v)
    
#
# KEY/VALUE PAIRS:
#

print(person.items()) #> dict_items([('first_name', 'Ophelia'), ('last_name', 'Clark'), ('message', 'Hi, thanks for the ice cream!'), ('fav_flavors', ["Vanilla Bean", "Mocha", "Strawberry"])])
# print(list(person.items())) #> [('first_name', 'Ophelia'), ('last_name', 'Clark'), ('message', 'Hi, thanks for the ice cream!'), ('fav_flavors', ["Vanilla Bean", "Mocha", "Strawberry"])]

# referencing key-value tuple separately as k and v, using a "destructuring" approach:
for k, v in person.items():
    print("KEY:", k, "... VALUE:", v)

NOTE: the dict_keys, dict_values, and dict_items are list-like, so we can convert them to lists by passing them into the list() function, or do list-like things such as looping through them.