A **Python dictionary** is an unordered collection of items where every item is a `key/value` pair.


##   
### Creating a dictionary
* To create a python dictionary, simply place #key# items: key:`value/pairs` #key# in curly brackets {}, separated by commas.

* Keys must be immutable for an item in a dictionary, whereas values can be of any datatypes.

Below are different ways to create a dictionary.




# empty dictionary
empty_dict = {}
print("Empty dictionary --> "+ str(empty_dict))

# dictionary with items
fruits = {1:"Apple",2:"Orange"}
print("fruits dictionary --> "+ str(fruits))

# using built in dict() function
pets = dict({1:"Cat",2:"Dog"})
print("pets dictionary --> "+ str(pets))

##   
### Accessing elements from a dictionary
Python dictionary uses `Keys` to access items such as:
* Square brackets `[]`
* The `get()` method

>Interpreter may throw `KeyError`, if a key is not found in dictionary when we use square brackets `[]`. However, the `get()` method returns `None`.

It's always safer to use the `get()` method.

fruits = {1:"Apple",2:"Orange"}
print(fruits[1])
print(fruits.get(1))

# returns None
print(fruits.get(3))

# Raises KeyError since 3 is not a key in fruits dictionary
print(fruits[3])

##    
### Adding or removing items
We can add new items or change the value of existing items using the assignment `=` operator.

Use the `pop()` method to remove an item from a dictionary. It takes `key` as a parameter and returns the value of the item that is removed.

fruits = {1:"Apple",2:"Orange"}
print("Original fruits dict --> "+str(fruits))

# add new item
fruits[3] = "Banana"
print("Added 'Banana' to fruits dict --> "+str(fruits))


# change value of an item
fruits[2] = "Grapes"
print("Change Value at index 2 in fruits dict --> "+str(fruits))

# removes item and returns it
fruits.pop(1)
print("Remove element at index 1 fruits dict --> "+str(fruits))

##   
### When to use python dictionaries
We have other data structures like `List`, `Set` etc., in Python. Therefore, it is important to know when to use dictionaries.

* Python dictionaries are optimized versions of HashMap.
* HashMaps are developed for faster lookups.
* Use Python dictionaries when there is a need to search elements faster.

> Time complexity to set, get and delete an item in dictionary is 
$O(n)$


What are python dictionaries?

Creating a dictionary

Accessing elements from a dictionary

Adding or removing items

When to use python dictionaries