Python Dictionaries

A dictionary is a built-in data type in Python that stores data in key-value pair format. It is similar to a list or an array in that it stores a key-value pair at each index rather than a single value. A dictionary's keys must be unique, and they are used to access the corresponding values.

Create a new dictionary

We can create a python dictionary using the open and close curly brackets, with a key value combination representing each item in the dictionary. Here's an example of how you can create and use a dictionary in Python:

dict = {
    "name" : "John",
    "age" : 40,
    "department": "Software Engineering"
}
print(dict)

Adding a new key value pair

We can add items to an existing dictionary by wrapping the new item key in a square bracket along with the items value. dict[key] = value

This is an example:

dict = {
    "name" : "John",
    "age" : 40
}
dict["job title"] = "Lead Engineer"
print(dict)

You can access the values in a dictionary using the keys, which are unique within a dictionary. You can also use the len() function to get the number of key-value pairs in a dictionary, and the in operator to check if a key is in a dictionary.

Furthermore, you can also use the items(), keys(), and values() methods to get a view of the key-value pairs, keys, or values in a dictionary, respectively

Fetch items using key

To fetch specific items from a python dictionary, we use the following pattern dict[key]

# fetch item using key
name = dict["name"]
age = dict["age"]
department = dict["department"]
job_title = dict["job title"]

fetch item using the .get()

To fetch specific items from a python dictionary, we use the following pattern dict[key]

dict = {
    "name" : "John",
    "age" : 40
}
name = dict.get("name")
age = dict.get("age")
print(name)
print(age)

Deleting items from a dictionary using the del keyword

We can delete items from a dictionary using the del keyword following the key enclosed in a square bracket. This is an example of how to delete items in a dictionary.

dict = {
    "name" : "John",
    "age" : 40,
    "department": "Engineering"
}
del dict["name"]
print(dict)

Iterating through a dictionary

Giving the fact that a dictionary is a iter (an iterable type), we can iterate over the items .items() while accessing the key and value following the pattern for key, value in dictionary.items().

In this example, we are iterating over a list of dictionaries:

array_of_dict = [
    {
        "id" : '09812hAA',
        "name" : "John",
        "age" : 40,
        "department": "Software Engineering"
    },
    {
        "id" : '09812hAB',
        "name" : "Emmanuella",
        "age" : 16,
        "department": "Software Engineering"
    },
]

# getting the key and the value using .items
for dictionary in array_of_dict: # iterating over the array
    for key, value in dictionary.items(): # iterating over the dictionary items
        print(f'{key} -> {value}')

Dictionary comprehension

A dictionary comprehension is a concise way to create a dictionary. It is similar to a list comprehension, but it returns a dictionary instead of a list.

stocks = {
    'AAPL': 121,
    'AMZN': 3380,
    'MSFT': 219,
    'BIIB': 280,
    'QDEL': 266,
    'LVGO': 144
}

new_stocks = {name: value * 1.2 for (name, value) in stocks.items()}

print(new_stocks)

Dictionaries are useful for storing data that needs to be quickly retrieved using a unique key. They are also used to implement many other data structures in Python, such as sets and graphs.