Dictionaries in Python Programming

Data Structures in Python

What is Dictionaries in Python?

A Dictionary is an ordered collection of some key:value pairs. The key and its value is separated by a colon (:).

All the key:value pairs are inserted within a curly braces { } separated by commas (,).

Dictionaries are mutable we can add or remove key:value pair in a dictionary once it is created.

Note: Dictionaries are ordered from Python version 3.7 onwards and unordered below version 3.7.

dictionary_name = {key:value, key:value,....}

# A dictionary containing an employee details
emp = {'id':'e156', 'name':'Allen', 'age':34, 'salary':15000}

Adding Elements to a Dictionary

We can add elements (key:value pair) to a dictionary by assignment as per the following syntax.

variable_name[key] = value

emp = {'id':'e156', 'name':'Allen', 'age':34, 'salary':15000}
emp['city']='California'
print(emp)

{'id': 'e156', 'name': 'Allen', 'age': 34, 'salary': 15000, 'city': 'california'}

Updating Element of a Dictionary

We can update the value of an existing key in a dictionary by assigning a new value to the same key.

emp = {'id':'e156', 'name':'Allen', 'age':34, 'salary':15000}
emp['age']=42
print(emp)

{'id': 'e156', 'name': 'Allen', 'age': 42, 'salary': 15000}

In the above example, we can see that the value of the key age has changed from 34 to 42 because we have used assigned a new value to the key.

Accessing an Element of a Dictionary

In a dictionary, an element or value is accessed by its key using the syntax given below.

variable_name = dictionary_name[key]

emp = {'id':'e156', 'name':'Allen', 'age':34, 'salary':15000}
x=emp['name']
print(x)

Allen

Deleting an Element from a Dictionary

We can delete an element (key:value pair) from a dictionary by using del statement as follows.

emp = {'id':'e156', 'name':'Allen', 'age':34, 'salary':15000}
del emp['salary']
print(x)

{'id': 'e156', 'name': 'Allen', 'age': 34}

Traversing All Elements of a Dictionary

We can traverse or access all the elements of a dictionary one by one with the help of for loop as shown below.

emp = {'id':'e156', 'name':'Allen', 'age':34, 'salary':15000}
for key in emp:
    print(key,':',emp[key])

id : e156
name : Allen
age : 34
salary : 15000

Creating an Empty Dictionary

We can also create an empty dictionary that does not contain elements at the time of declaration. See the example given below.

a = {}          # An empty dictionary
b = dict()      # Another way of creating an empty dictionary
print(a)
print(b)

{}
{}

Use of Membership Operator in Dictionary

Both in and not in operators works on Dictionary just like they work for other sequences such as strings, lists, tuples and sets. The in operator tells if a key is present in the dictionary or not and not in does the opposite. See the example given below.

emp = {'id':'e156', 'name':'Allen', 'age':34, 'salary':15000}

print(emp)
if ('name' in emp) == True:
    print('name key is prenest in the dictionary')
else:
    print('name key is not prenest in the dictionary')

if ('city' not in emp) == True:
    print('city key is not prenest in the dictionary')
else:
    print('city key is prenest in the dictionary')

{'id': 'e156', 'name': 'Allen', 'age': 34, 'salary': 15000}
name key is prenest in the dictionary
city key is not prenest in the dictionary

Dictionary Methods and Functions

Dictionary methods in Python are a collection of useful functions used to perform a specific task on a dictionary.

The difference between Function and Method is that a Function is called directly with its name but a Method is called on objects only. Let's see all the important dictionary methods and functions one by one with examples.

copy() Method

The copy() method copies all the key:value pairs from an existing dictionary to a new dictionary.

emp1 = {'id':'e156', 'name':'Allen', 'age':34, 'salary':15000}
emp2=emp1.copy()
print(emp2)

{'id': 'e156', 'name': 'Allen', 'age': 34, 'salary': 15000}

clear() Method

The clear() method removes all the key:value pairs from an existing dictionary. After removing all the key:value pairs the dictionary becomes an empty dictionary.

emp = {'id':'e156', 'name':'Allen', 'age':34, 'salary':15000}
emp.clear()
print(emp)

{}

pop() Method

The pop() method remove and returns the value of a given key from an existing dictionary. Raises KeyError if the key is not found.

emp = {'id':'e156', 'name':'Allen', 'age':34, 'salary':15000}
x=emp.pop('salary')
print('Removed salary',x,'from the dictionary')
print(emp)

Removed salary 15000 from the dictionary
{'id': 'e156', 'name': 'Allen', 'age': 34}

get() Method

The get() returns the value of a given key from an existing dictionary. Return None if the key is not found.

emp = {'id':'e156', 'name':'Allen', 'age':34, 'salary':15000}
print(emp)
x=emp.get('name')
print('Name=',x)
print('City=',emp.get('city'))

{'id': 'e156', 'name': 'Allen', 'age': 34, 'salary': 15000}
Name= Allen
City= None

items() Method

The items() method returns an object that contains all the (key, value) pairs of a dictionary. The pairs are stored as tuples in the object.

emp = {'id':'e156', 'name':'Allen', 'age':34, 'salary':15000}
data=emp.items()
for x in data:
    print(x)

('id', 'e156')
('name', 'Allen')
('age', 34)
('salary', 15000)

keys() Method

The keys() method returns the sequence of keys from a dictionary.

emp = {'id':'e156', 'name':'Allen', 'age':34, 'salary':15000}
print(emp.keys())

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

values() Method

The values() method returns the sequence of values from a dictionary.

emp = {'id':'e156', 'name':'Allen', 'age':34, 'salary':15000}
print(emp.values())

dict_values(['e156', 'Allen', 34, 15000])

emp = {'id':'e156', 'name':'Allen', 'age':34, 'salary':15000}

if ('Allen' in emp.values())==True:
    print('Allen exist in the dictionary')
else:
    print('Allen does not exist in the dictionary')

Allen exist in the dictionary

update() Method

The update() method merge key:value pairs from a new dictionary to an existing dictionary by adding or updating the elements as needed.

emp = {'id':'e156', 'name':'Allen', 'age':34, 'salary':15000}
data={'post':'Engineer','city':'California','age':42}
emp.update(data)
print(emp)

{'id': 'e156', 'name': 'Allen', 'age': 42, 'salary': 15000, 'post': 'Engineer', 'city': 'California'}

len() Function

The len() function returns the total number of key:value pairs present in a dictionary.

emp = {'id':'e156', 'name':'Allen', 'age':34, 'salary':15000}
print(emp)
print('Length=',len(emp))

{'id': 'e156', 'name': 'Allen', 'age': 34, 'salary': 15000}
Length= 4