Python Dictionaries

 Unordered collection of items.

Every dictionary item is a Key-value pair.

Creating a Dictionary

Created by enclosing items within {curly} brackets

Each item in dictionary has a key - value pair separated by a comma.

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dict_a = {

"name": "Teja",

"age": 15

}

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Key - Value Pairs

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dict_a = {

"name": "Teja",

"age": 15

}

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In the above dictionary, the

• keys are 
  name
   and 
  age
• values are 
  Teja
   and 
  15

Collection of Key-Value Pairs

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dict_a = { "name": "Teja",

"age": 15 }

print(type(dict_a))

print(dict_a)

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Output

<class 'dict'>

{'name': 'Teja','age': 15}

Immutable Keys

Keys must be of immutable type and must be unique.

Values can be of any data type and can repeat.

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dict_a = {

"name": "Teja",

"age": 15,

"roll_no": 15

}

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Creating Empty Dictionary

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dict_a = dict()

print(type(dict_a))

print(dict_a)

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Output

<class 'dict'>

{}

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dict_a = {}

print(type(dict_a))

print(dict_a)

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Output

<class 'dict'>

{}

Accessing Items

To access the items in dictionary, we use square bracket

[ ]

along with the

key

to obtain its value.

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dict_a = {

'name': 'Teja',

'age': 15

}

print(dict_a['name'])

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Output

Teja

Accessing Items - Get

The

get()

method returns

None

if the key is not found.

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dict_a = {

'name': 'Teja',

'age': 15

}

print(dict_a.get('name'))

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Output

Teja

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dict_a = {

'name': 'Teja',

'age': 15

}

print(dict_a.get('city'))

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Output

None

KeyError

When we use the square brackets

[]

to access the key-value, KeyError is raised in case a key is not found in the dictionary.

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dict_a = {'name': 'Teja','age': 15 }

print(dict_a['city'])

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Output

KeyError: 'city'

Quick Tip

If we use the square brackets 

[]

, 

KeyError

 is raised in case a key is not found in the dictionary. On the other hand, the 

get()

 method returns 

None

 if the key is not found.

Membership Check

Checks if the given key exists.

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dict_a = {

'name': 'Teja',

'age': 15

}

result = 'name' in dict_a

print(result)

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Output

True

Operations on Dictionaries

We can update a dictionary by

• Adding a key-value pair
• Modifying existing items
• Deleting existing items

Adding a Key-Value Pair

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dict_a = {'name': 'Teja','age': 15 }

dict_a['city'] = 'Goa'

print(dict_a)

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Output

{'name': 'Teja', 'age': 15, 'city': 'Goa'}

Modifying an Existing Item

As dictionaries are mutable, we can modify the values of the keys.

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dict_a = {

'name': 'Teja',

'age': 15

}

dict_a['age'] = 24

print(dict_a)

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Output

{'name': 'Teja', 'age': 24}

Deleting an Existing Item

We can also use the

del

keyword to remove individual items or the entire dictionary itself.

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dict_a = {

'name': 'Teja',

'age': 15

}

del dict_a['age']

print(dict_a)

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Output

{'name': 'Teja'}

Dictionary Views

They provide a dynamic view on the dictionary’s entries, which means that when the dictionary changes, the view reflects these changes.

Dictionary Methods

• dict.keys()
  • returns dictionary Keys
• dict.values()
  • returns dictionary Values
• dict.items()
  • returns dictionary items(key-value) pairs

The objects returned by

keys()

,

values()

&

items()

are View Objects .

Getting Keys

The

keys()

method returns a view object of the type dict_keys that holds a list of all keys.

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dict_a = {

'name': 'Teja',

'age': 15

}

print(dict_a.keys())

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Output

dict_keys(['name', 'age'])

Getting Values

The

values()

method returns a view object that displays a list of all the values in the dictionary.

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dict_a = {

'name': 'Teja',

'age': 15

}

print(dict_a.values())

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Output

dict_values(['Teja', 15])

Getting Items

The

items()

method returns a view object that displays a list of dictionary's (key, value) tuple pairs.

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dict_a = {

'name': 'Teja',

'age': 15

}

print(dict_a.items())

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Output

dict_items([('name', 'Teja'), ('age', 15)])

Iterate over Dictionary Views

Example - 1

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dict_a = {

'name': 'Teja',

'age': 15

}

for key in dict_a.keys():

print(key)

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Output

name

age

Example - 2

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dict_a = {

'name': 'Teja',

'age': 15

}

keys_list = list(dict_a.keys())

print(keys_list)

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Output

['name', 'age']

Example - 3

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dict_a = {

'name': 'Teja',

'age': 15

}

for value in dict_a.values():

print(value)

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Output

Teja

15

Example - 4

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dict_a = {

'name': 'Teja',

'age': 15

}

for key, value in dict_a.items():

pair = "{} {}".format(key,value)

print(pair)

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Output

name Teja

age 15

Dictionary View Objects

keys()

,

values()

&

items()

are called Dictionary Views as they provide a dynamic view on the dictionary’s items.

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dict_a = {

'name': 'Teja',

'age': 15

}

view = dict_a.keys()

print(view)

dict_a['roll_no'] = 10

print(view)

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Output

dict_keys(['name', 'age'])

dict_keys(['name', 'age', 'roll_no'])

Converting to Dictionary

dict(sequence)

takes any number of key-value pairs and converts to dictionary.

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list_a = [

("name","Teja"),

["age",15],

("roll_no",15)

]

dict_a = dict(list_a)

print(dict_a)

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Output

{'name': 'Teja', 'age': 15, 'roll_no': 15}

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list_a = ["name", "Teja", 15]

dict_a = dict(list_a)

print(dict_a)

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Output

ValueError: dictionary update sequence element #0 has length 4; 2 is required

Type of Keys

A dictionary key must be of a type that is immutable.