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Python Dictionaries

The Valley of Code

Your Web Development Manual

Dictionaries are a very important Python data structure.

While lists allow you to create collections of values, dictionaries allow you to create collections of key / value pairs.

Here is a dictionary example with one key/value pair:

dog = { 'name': 'Roger' }

The key can be any immutable value like a string, a number or a tuple. The value can be anything you want.

A dictionary can contain multiple key/value pairs:

dog = { 'name': 'Roger', 'age': 8 }

You can access individual key values using this notation:

dog['name'] # 'Roger'
dog['age']  # 8

Using the same notation you can change the value stored at a specific index:

dog['name'] = 'Syd'

And another way is using the get() method, which has an option to add a default value:

dog.get('name') # 'Roger'
dog.get('test', 'default') # 'default'

The pop() method retrieves the value of a key, and subsequently deletes the item from the dictionary:

dog.pop('name') # 'Roger'

The popitem() method retrieves and removes the last key/value pair inserted into the dictionary:


You can check if a key is contained into a dictionary with the in operator:

'name' in dog # True

Get a list with the keys in a dictionary using the keys() method, passing its result to the list() constructor:

list(dog.keys()) # ['name', 'age']

Get the values using the values() method, and the key/value pairs tuples using the items() method:

# ['Roger', 8]

# [('name', 'Roger'), ('age', 8)]

Get a dictionary length using the len() global function, the same we used to get the length of a string or the items in a list:

len(dog) #2

You can add a new key/value pair to the dictionary in this way:

dog['favorite food'] = 'Meat'

You can remove a key/value pair from a dictionary using the del statement:

del dog['favorite food']

To copy a dictionary, use the copy() method:

dogCopy = dog.copy()
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