Python Dictionaries

Overview

In this lesson we'll be learning about Python dictionaries! Like objects in JavaScript, dictionaries in Python are a powerful data type that allows us to store keys and values.

Objectives

Learn how create and format dictionaries
Learn how to modify and read dictionaries

Getting Started

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Dictionaries

Dictionaries are a collection of unique key-value pairs that are easily accessible. They allow us to keep a fast and easy to read record of information. We've seen dictionaries before in the form of javascript objects:

let myObj = {
  key: 'value'
}

Now let's see what python dictionaries look like:

my_dict = {
    "key":"value"
}

Notice the difference here? In the javascript object, the key is not wrapped in quotes, in a python dictionary, it is. Python is an interpreted language meaning that anything that doesn't have an explicit type or key word is interpreted as a variable.

Key's can also be integers and tuples! Key's must be an immutable type!

Working With Dictionaries

Accessing Values

Let's say we have a dictionary with a key of "name" and a value of "Jane", in javascript we would normally access it like so:

let people = {
  name: 'Jane'
}
people.name // Returns Jane

In python however, we use bracket [] notation:

people = {
    "name":"Jane"
}
people["name"] # Returns Jane

Another way of accessing a value by key is using the .get() method:

people = {
    "name":"Jane"
}
people.get("name")

And what if we wanted all of the values? There's a method for that too:

person = {
    "name":"Jane",
    "age":16,
    "birth_place":"London"
}

person.values() # Returns dict_values(['Jane', 16, 'London'])

Another way to declare a dictionary is by using the dict constructor:

my_dict = dict()

There are many more dictionary methods available in python,heres a list:

Dictionary Methods	Description
`mydict.clear`	removes all items
`mydict.get`	returns the value of a specified key
`mydict.pop`	removes and returns element with a given key from the dictionary
`mydict.update`	updates the dictionary
`mydict.copy`	returns shallow copy of a dictionary
`mydict.items`	returns a list of the dictionary's key value pairs as tuples
`mydict.popitem`	removes and returns the last element (key, value) pair inserted into the dictionary
`mydict.values`	returns a list of all values in dictionary
`mydict.fromkeys`	creates dictionary from given sequence of keys
`mydict.keys`	returns a list of all keys in a dictionary
`mydict.setdefault`	returns the value of a key if the key is in dictionary. If not, it inserts key with a value to the dictionary.

Modifying Dictionaries

Adding items to a dictionary is as simple as using [] notation:

my_dict ={
    "name":"Jane"
}
my_dict["age"] = 6

print(my_dict) # Returns {'name': 'Jane', 'age': 6}

What if we wanted to update a record in our dictionary? Use the update method:

my_dict = {
    "name": "Jane"
}
my_dict["age"] = 6


my_dict.update({"age": 10})
print(my_dict)
 # Returns {'name': 'Jane', 'age': 10}

We can also use update to add something to a dictionary if it doesn't exist:

my_dict = {
    "name": "Jane"
}
my_dict["age"] = 6


my_dict.update({"age": 10})
my_dict.update({"home_town": "London"})
print(my_dict) # Returns {'name': 'Jane', 'age': 10, 'home_town': 'London'}

Pretty neat! We can also merge dictionaries together using the update method:

dict_1 = {
    "town": "Riverdale"
}

dict_2 = {
    "state": "New York"
}

dict_1.update(dict_2)

print(dict_1) # Returns {'town': 'Riverdale', 'state': 'New York'}

Iterating Through Dictionaries

It's time to use our friend the for loop again!

Dictionaries are an iterable data type:

snoops_profile = {
    "id": "1574083",
    "username": "snoopdogg",
    "full_name": "Snoop Dogg",
    "profile_picture": "http://distillery.s3.amazonaws.com/profiles/profile_1574083_75sq_1295469061.jpg",
    "bio": "This is my bio",
    "website": "http://snoopdogg.com",
}

for key in snoops_profile:
    print(key)
# Returns
# id
# username
# full_name
# profile_picture
# bio
# website

As you can see, utilizing a loop, the key variable only returns the dictionaries keys, however utilizing bracket notation we can gain access to the values as well:

snoops_profile = {
    "id": "1574083",
    "username": "snoopdogg",
    "full_name": "Snoop Dogg",
    "profile_picture": "http://distillery.s3.amazonaws.com/profiles/profile_1574083_75sq_1295469061.jpg",
    "bio": "This is my bio",
    "website": "http://snoopdogg.com",
}

for key in snoops_profile:
    print(snoops_profile[key])
# Returns
# 1574083
# snoopdogg
# Snoop Dogg
# http://distillery.s3.amazonaws.com/profiles/profile_1574083_75sq_1295469061.jpg
# This is my bio
# http://snoopdogg.com

We can also use list comprehension to accomplish the same thing as above:

snoops_values = [snoops_profile[key] for key in snoops_profile]
print(snoops_values) # Returns a list with the dictionary values:
# ['1574083', 'snoopdogg', 'Snoop Dogg', 'http://distillery.s3.amazonaws.com/profiles/profile_1574083_75sq_1295469061.jpg', 'This is my bio', 'http://snoopdogg.com']

You Do

Work in the main.py file and follow the instructions provided. Test your work using python3 dict_test.py.

Recap

In this lesson, we learned about the syntax, manipulation, and iteration of dictionaries in Python. A few key things to note:

Dictionaries in python work similarly to objects in JavaScript, but their syntax requires keys to be wrapped in quotes. Example:
```
my_dict = {
  "hello": "world"
}
```
Dictionaries have access to powerful methods like update that can be used to manipulate their values, add in new key value pairs, or even merge dictionaries together!
Dictionaries are iterable, meaning that we can loop through their keys and values with for loops and list comprehension

nicklope/u4_lesson_python_dictionaries-1