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Dictionaries in Python Programming

Data Structures in Python

In this lesson, we will understand what is Dictionaries in Python Programming along with some examples.

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.

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Declaration Syntax of a Dictionary in Python

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

Example

# A dictionary containing an employee details
emp = {'id':'e156', 'name':'Allen', 'age':34, 'salary':15000}
Note: In the above example id is a key and its value is e156 similarly name is a key and its value is Allen.

Adding Elements to a Dictionary

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

Syntax of Adding an Element to a Dictionary

variable_name[key] = value

Example

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

Output

{'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.

Example

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

Output

{'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.

Syntax of Accessing an Element of a Dictionary

variable_name = dictionary_name[key]

Example

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

Output

Allen

Deleting an Element from a Dictionary

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

Example

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

Output

{'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.

Example

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

Output

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.

Example

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

Output

{}
{}

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.

Example

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')

Output

{'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.

Example

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

Output

{'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.

Example

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

Output

{}

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.

Example

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

Output

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.

Example

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

Output

{'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.

Example

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

Output

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

keys() Method

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

Example

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

Output

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

values() Method

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

Example 1

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

Output

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

Example 2

Check if a particular value exist in a dictionary or not.

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')

Output

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.

Example

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

Output

{'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.

Example

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

Output

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

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