# 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:

```python
{}

{"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  |

```python
[
    {"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

Access individual object attributes by their key:

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

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

Add or update or remove attributes from an object:

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

person["message"] = "New Message" # this is mutating

person["fav_color"] = "blue" # this is mutating

del person["fav_flavors"] # this is mutating

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

Its possible to separate the dictionaries keys from its values, and also to iterate through each pair:

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

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

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

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

for k, v in person.items():
    print("KEY:", k, "... VALUE:", v)

#> KEY: first ... VALUE: Ophelia
#> KEY: last ... VALUE: Clark
#> KEY: message ... VALUE: Hi, thanks for the ice cream!
#> KEY: fav_flavors ... VALUE: ["Vanilla Bean", "Mocha", "Strawberry"]
```