Dictionary

The dictionary is an unordered collection of items. An item is a key-value pair.

The key in a dictionary allows retrieving the value optimally.

​​​​​​​Creating  a dictionary

The dictionary is created by placing key-value pairs inside curly braces where a key-value pair is separated by commas. The key-value pair is separated by a colon.

A key must be immutable type, however value can be immutable or mutable. The key must be unique in a dictionary.

_dict = {1:'a', 2:'b' , 3:'c', 4:'d'}

print(_dict)

_dict = {'a':1,'b':[1,2,3] , 'c':(1,2,3), 'd':4}

print(_dict)

_dict = {"id":123, "emp name":"ramesh", "age": 24}

print(_dict)

#Creating an empty dictionary

_dict= {}

print(_dict)

Output

{1: 'a', 2: 'b', 3: 'c', 4: 'd'}

{'a': 1, 'b': [1, 2, 3], 'c': (1, 2, 3), 'd': 4}

{'id': 123, 'emp name': 'ramesh', 'age': 24}

{}

Creating  dictionary using dict() function

There are various ways to create the dictionary these are

Create Dictionary Using keyword arguments

point = dict(x1=0,y1=0, x2=1,y2=1)

print(point)

Output

{'x1': 0, 'y1': 0, 'x2': 1, 'y2': 1}

Create  a dictionary using an Iterable

# Create dictionary using a list of tuples

point = dict( [('x1',0),('y1',0)])

print(point)

# Create dictionary using a list of tuples and keyword argument

point3d = dict( [('x',0),('y',0)] , z=0)

print(points)

# Create dictionary using a lists for key and values and zip

point3d = dict(zip(['x', 'y', 'z'], [1, 2, 3]))

print(point3d)

Output

{'x1': 0, 'y1': 0}

{'x': 0, 'y': 0, 'z': 0}

{'x': 1, 'y': 2, 'z': 3}

Create a dictionary using Mapping

_dict=  dict({'x1': 0, 'y1': 0}) 

print(_dict)

Output

{'x1': 0, 'y1': 0}

Accessing items from a dictionary

The key is used to retrieve the values from a dictionary. The get() function and [] square are used to retrieve a value.

_dict = {"id":123, "emp name":"ramesh", "age": 24}

print(_dict.get("id"))

print(_dict["age"])

Output

123

24

​​​​​​​Note: The difference between using get() and the square bracket is, square brackets raises a KeyError if the key is not found in the dictionary, whereas the get() function returns None if the key is not found.

Add or modify  an item to a dictionary

Dictionaries are mutable, it is possible to add a new item or change the value of an existing item using an assignment operator.

Note: If the key is already exists,  then the existing value  gets updated otherwise, a new (key: value) pair (item) is added to the dictionary.

_dict = {"id":123, "emp name":"ramesh", "age": 24}

# Modify the value of the key age

_dict["age"]= 27

print(_dict)

# Add an item to a dictionary

_dict["Band"]= 'E'

print(_dict)

Output

{'id': 123, 'emp name': 'ramesh', 'age': 27}

{'id': 123, 'emp name': 'ramesh', 'age': 27, 'Band': 'E'}

Removing item(s) from a dictionary

There are various ways to remove item(s) from a dictionary

1. A particular item is removed by using the pop() function. The pop(key) function removes an item and returns its value.
2. The popitem() function removes and returns an arbitrary (key, value)  pair from a dictionary.
3. The clear() function removed all the items from the dictionary.
4. The del  removes the  dictionary  or an item

vowels = {'a':1, 'e':2,'i': 3, 'o':4, 'u':5, 'A':6}

print("The removed value is ", vowels.pop('A'))

print(vowels)

print("The removed item is ",vowels.popitem())

vowels = {'a':1, 'e':2,'i': 3, 'o':4, 'u':5, 'A':6}

del vowels['a']

print("Dictionary after removing key 'a'' ", vowels)

vowels.clear()

print("Dictionary after clearing all the items ", vowels)

vowels = {'a':1, 'e':2,'i': 3, 'o':4, 'u':5, 'A':6}

del vowels

print(vowels)

Output

The removed value is  6

{'a': 1, 'e': 2, 'i': 3, 'o': 4, 'u': 5}

The removed item is  ('u', 5)

Dictionary after removing key 'a''  {'e': 2, 'i': 3, 'o': 4, 'u': 5, 'A': 6}

Dictionary after clearing all the items  {}

Traceback (most recent call last):

  File "dice.py", line 748, in <module>

    print(vowels)

NameError: name 'vowels' is not defined

Iterating through a Dictionary

vowels = {'a':1, 'e':2,'i': 3, 'o':4, 'u':5}

#items function return dictionary contains a list of

# key value pair in tuples

for item in vowels.items():

    print(item)

 #printing all the key of a dictionary

for key in vowels:

    print(key)

#printing all the values

for key in vowels:

            print(vowels.get(key))

Output

('a', 1)

('e', 2)

('i', 3)

('o', 4)

('u', 5)

a

e

i

o

u

1

2

3

4

5

How to retrieve all the keys from  a dictionary

The keys() function  returns an iterable of keys for example

_dict = {1:'a',2:'b', 3:'c'}

#print all the keys in a dictionary

for key in  _dict.keys():

           print(key)

Output

1

2

3

How to retrieve all the values from  a dictionary

The values() function  returns an iterable of values for example

# print all the values in a dictionary

for value in  _dict.values():

           print(value)

Output

a

b

c

Membership Test

The ‘in’ and ‘not in’  operators  check the existence  of a key in a dictionary  for example

_dict = {1:'a',2:'b', 3:'c', 4:'d'}

print( 1 in _dict)

print( 5 in _dict)

print( 4 not in _dict)

print( 5 not in _dict)

Output

True

False

False

True

Concatenate dictionaries

The update() function is used to concatenate the dictionary

Note: If the keys are same in both the dictionary the value is overwritten for that key

_dict1 = {1:2, 2:3}

_dict2= {3:4, 4:5}

_dict1.update(_dict2)

print("Dictionay after update " ,_dict1)

_dict1 = {1:2, 2:3}

_dict2 = {1:10, 4:5}

_dict1.update(_dict2)

print("Dictionay after update value is overwritten",_dict1)

Output

Dictionay after update  {1: 2, 2: 3, 3: 4, 4: 5}

Dictionay after update value is overwritten  {1: 10, 2: 3, 4: 5}