Dictionaries

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What we will cover…

1. Motivation for dictionaries
2. Creating and using dictionaries
3. KeyErrors

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Why dictionary?

Lists and tuples are both collections of elements.

We saw that with destructuring and tuples, we could conveniently access pairs (or triples, etc.) of elements.

But sometimes we don’t want to remember, or commit to, the order of the elements.

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Dictionaries

user = {'name': 'Foo', 'country': 'es'}

user['country'] # 'es'
user['name'] # 'Foo'

Dictionaries are associative data structures.

Like the eponymous dictionary, they associate a key with a value.

This allows us to access a value via its key.

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Creating dictionaries

{'score': 55, 'country': 'es' }

To create a dictionary:

1. Start with curly brackets {}
2. Add a key ('score')
3. Add a colon :
4. Add the value (55)
5. Commas separate additional key/value pairs ,

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Creating dictionaries

user = {'name': 'foo',
        'countries': ['es', 'dk', 'uk'],
        'address': {'street': 'diagonal'}}

keys can be strings or numbers.

values can be any data type!

keys must be unique.

Style: write on multiple lines to make more legible!

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Getting/Setting values

user = {'name': 'Foo', 'score': 55}

user['score'] = 100

print(user['score']) # 100

user['country'] = 'es'

print(user['country]) # 'es'

Values can be retrieved via their keys with square brackets [], much like lists.

Values can also be set via square brackets [], much like lists!

Unlike lists, however, a value can be set via square brackets [] even if that key did not exist.

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KeyError

user = {'name': 'Foo', 'score': 55}

try:
    country = user['country']
except KeyError:
    country = 'No country!'

print(country)

Dictionaries raise a KeyError exception when you try to access the value of a key that does not exist in the dictionary.

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Review

1. Motivation for dictionaries
2. Creating and using dictionaries
3. KeyErrors