Dictionary ¶

Lists allow us to store ordered collections of data which can be flexibly accessed via indices. However, frequently we need an alternative access pattern—looking up values by a descriptive key rather than numerical index. This is enabled in Python using dictionaries.

Dictionaries provide a flexible mapping of unique keys to associated values, like a real world dictionary maps words to definitions. Defining a dictionary uses braces with colons separating keys and values.

In [18]:

person_favorites = {
    "color": "blue",
    "food": ["Chinese", "Thai", "American"],
    "number": 32,
}
print(person_favorites)

person_favorites = { "color": "blue", "food": ["Chinese", "Thai", "American"], "number": 32, } print(person_favorites)

{'color': 'blue', 'food': ['Chinese', 'Thai', 'American'], 'number': 32}

Dictionaries have some key capabilities:

• Store mappings of objects to easy retrieval by descriptive keys;
• High performance lookup time even for large data sets;
• Keys can use many immutable types: strings, numbers, tuples;
• Values can be any Python object;
• Extensible structure allowing easy growth.

In [19]:

print(person_favorites.keys())

print(person_favorites.keys())

dict_keys(['color', 'food', 'number'])

In [20]:

print(person_favorites["color"])
print(person_favorites["food"])
print(person_favorites["number"])

print(person_favorites["color"]) print(person_favorites["food"]) print(person_favorites["number"])

blue
['Chinese', 'Thai', 'American']
32

In [21]:

person_favorites["color"] = "red"
print(person_favorites)

person_favorites["color"] = "red" print(person_favorites)

{'color': 'red', 'food': ['Chinese', 'Thai', 'American'], 'number': 32}

In [22]:

person_favorites["city"] = "Pittsburgh"
print(person_favorites)

person_favorites["city"] = "Pittsburgh" print(person_favorites)

{'color': 'red', 'food': ['Chinese', 'Thai', 'American'], 'number': 32, 'city': 'Pittsburgh'}

In [23]:

person_favorites["food"][2] = "Italian"
print(person_favorites)

person_favorites["food"][2] = "Italian" print(person_favorites)

{'color': 'red', 'food': ['Chinese', 'Thai', 'Italian'], 'number': 32, 'city': 'Pittsburgh'}