Dictionaries in Python

Dictionaries are used to store data/value in key-value pairs. A dictionary is a collection which is ordered, and mutable and does not allow duplicate keys, any key of the dictionary is associated (or mapped) to a value. Keys within a dictionary are ‘unique’ and must be from immutable data types such as strings or numbers.

Note

As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered.

Creating Dictionaries

There are a couple of different ways, we can create a dictionary. The most common method is by placing a comma-separated list of key: value pairs within curly braces.

credentials = {"user": "admin","pass": "p@wOrd", "secret": "cisco"}
print(credentials)

{'user': 'admin', 'pass': 'p@wOrd', 'secret': 'cisco'}

Python’s built-in dict() function can create a dictionary. A dict() function accepts a series of keyword arguments (1="one", 2="two"), a list of tuples, or another dictionary.

credentials_list = [
    ("user", "admin"),
    ("pass", "p@wOrd"),
    ("secret", "cisco"),
    ("iOS", 15.2)
]
credentials = dict(credentials_list)
print(credentials)

{'user': 'admin', 'pass': 'p@wOrd', 'secret': 'cisco', 'iOS': 15.2}

Each key must be different in order to maintain uniqueness i.e. to get the correct information about a value.

Accessing dictionaries

Dictionaries are very fast and can access any value in a dictionary via the key. If the key is not found, a KeyError will receive.

credentials = {"user": "admin","pass": "p@wOrd", "secret": "cisco"}
print(credentials["pass"])

p@wOrd

Nested Dictionary

A dictionary within a dictionary is called a nested dictionary. It accumulated multiple dictionaries into one. Each dictionary will have its own key-value pair.

device_list = {
    "cisco": {'r1': 'south', 's1': 'west'},
    'juniper': {'r2': 'east', 's2': 'north'}
    }
    
print(device_list['cisco'])
print(device_list['juniper']["s2"])

{'r1': 'south', 's1': 'west'}
north

Note

Dictionaries are lookup tables. They map from a key to a value.

Modification of Dictionaries

Dictionary elements can be created, accessed, modified or deleted. While a dictionary is created, its elements will contain a key-value pair to be accessed. You can also delete some values or delete a complete dictionary using del and clear respectively.

To add a new item to a dictionary, use the square braces to enter a new key and set it to a value.

credentials = {"user": "admin","pass": "p@wOrd", "secret": "cisco"}
credentials["iOS"] = 15.2
print(credentials)

{'user': 'admin', 'pass': 'p@wOrd', 'secret': 'cisco', 'iOS': 15.2}

Setting a pre-existing key to a new value will overwrite the previous value.

credentials = {"user": "admin","pass": "p@wOrd", "secret": "cisco"}
credentials["iOS"] = 12.2
print(credentials)

{'user': 'admin', 'pass': 'p@wOrd', 'secret': 'cisco', 'iOS': 12.2}

The values in dictionary items can be of any data type.

credentials = {'user': 'admin', 'pass': 'p@wOrd'}
# Adding new key-value as a list
credentials["devices"] = ["cisco", "juniper", "arista"]
print(credentials)

{'user': 'admin', 'pass': 'p@wOrd', 'devices': ['cisco', 'juniper', 'arista']}

Python’s del keyword removes the key-value.

credentials = {'user': 'admin', 'pass': 'p@wOrd'}
del credentials["user"]
print(credentials)

{'pass': 'p@wOrd'}

The clear() method is used to remove all the items from the dictionary.

credentials = {'user': 'admin', 'pass': 'p@wOrd'}
credentials.clear()
print(credentials)

{}

Dictionary Method

As the most data types in Python, dictionaries have also special methods to use.

The get() method to get a value, if the key not found, default value None will return without any error.

credentials = {'user': 'admin', 'pass': 'p@wOrd'}
print(credentials.get("pass"))

p@wOrd

If there is no key is present in dictionary None type will return.

credentials = {'user': 'admin', 'pass': 'p@wOrd'}
print(credentials.get("origin"))

None

The pop() method removes the specified key from the dictionary and return the value of item.

credentials = {'user': 'admin', 'pass': 'p@wOrd'}
print(credentials.pop('user'))

admin

The keys() method returns the keys of the dictionary, as a list.

credentials = {'user': 'admin', 'pass': 'p@wOrd'}
print(credentials.keys())  # print keys

dict_keys(['user', 'pass'])

The values() method returns the values of the dictionary, as a list.

credentials = {'user': 'admin', 'pass': 'p@wOrd'}
print(credentials.values())  # print values

dict_values(['admin', 'p@wOrd'])

The items() method returns the object contains the key-value pairs of the dictionary, as tuples in a list.

credentials = {'user': 'admin', 'pass': 'p@wOrd'}
print(credentials.items())  # print key-value pair

dict_items([('user', 'admin'), ('pass', 'p@wOrd')])