Dictionary

A dictionary as a set of key: value pairs, with the requirement that the keys are unique (within one dictionary).

Dictionaries are indexed by keys, which can be any immutable type; strings and numbers or tuples with immutable objects. The values of a dictionary can be of any type.

Each key is separated from its value by a colon (:), the items are separated by commas, and the whole thing is enclosed in curly braces. 

Creating a Dictionary

A pair of braces creates an empty dictionary: {}

Dictionary can also be created by the built-in function dict(). An empty dictionary can be created by just placing to curly braces{}.

Note – Dictionary keys are case sensitive, same name but different cases of Key will be treated distinctly.

Example to illustrate different ways to create a Dictionary

python"># Creating an empty Dictionary 

Dict = {} 

print("Empty Dictionary: ") 

print(Dict)

# Creating a Dictionary  with key:value pair

student_details = {'id':1234, "name":"John", 'course':'computer science','year' : 3 }

print "Student details: "

print student_details

# Creating a Dictionary with dict() method 

Dict = dict({'id':1234, "name":"John", 'course' : 'computer science', 'year' : 3 })

print("\nDictionary with the use of dict(): ") 

print(Dict) 

# Creating a Dictionary with each item as a Pair 

Dict = dict([(1, 'Geeks'), (2, 'For')]) 

Dict = dict([('id', 1234), ("name", "John"), ('course', 'computer science'), ('year', 3)])

print("\nDictionary with each item as a pair: ") 

print(Dict) 

When the above code is executed, it produces the following result −

python">Empty Dictionary: 

{}

Student details:

{'course': 'computer science', 'year': 3, 'id': 1234, 'name': 'John'}


Dictionary with the use of dict(): 

{'course': 'computer science', 'id': 1234, 'name': 'John', 'year': 3}


Dictionary with each item as a pair: 

{'course': 'computer science', 'year': 3, 'id': 1234, 'name': 'John'}

Updating a Dictionary

In Python Dictionary, Addition of elements can be done in multiple ways. 

One value at a time can be added to a Dictionary by defining value along with the key e.g. Dict[Key] = ‘Value’. For Example:

python">student_details = {'id': 1234, 'course': 'computer science', 'year': 3, 'name': 'John'}

student_details["gender"] = "female" # add a new entry

print student_details

When the the above code is executed it returns the fowllowing updated student details: – 

python">{'id': 1234, 'course': 'computer science', 'year': 3, 'name': 'John', “gender”: “female” }

You can modify an existing entry in a dictionary. For example, suppose we want to change the student course:

python">student_details[‘course’] = "Software Engineering" # update an existing entry

print student_details

The output of the above code when executed will be:

python">{'id': 1234, 'course': “Software Engineering”, 'year': 3, 'name': 'John', “gender”: “female” }

Updating an existing value in a Dictionary can be done by using the built-in update() method. Nested key values can also be added to an existing Dictionary.

Acc essing elements from a Dictionary

To access the items of a dictionary we refer to its key name. Key can be used inside square brackets.There is also a method called get() that will also help in acessing the element from a dictionary.

For Example to access student course and gender:

python">student_details = {'id': 1234, 'course': 'computer science', 'year': 3, 'name': 'John'}

print student[‘course’]
print student[‘gender’]

Result of the above code when executed:

"Software Engineering"
"female"

Using the get() method to access the student name

python">print student_details.get('name')

Output of the above code

"John"

Delete Dictionary Elements

You can either remove individual dictionary elements or clear the entire contents of a dictionary. You can also delete entire dictionary in a single operation.

To explicitly remove an entire dictionary, just use the del statement. For example −

python">dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'}

del dict['Name'] # remove entry with key 'Name'

dict.clear()     # remove all entries in dict

del dict         # delete entire dictionary


print "dict['Age']: ", dict['Age']

print "dict['School']: ", dict['School']

Output: Note that an exception is raised because after del dictdictionary does not exist any more 

python">dict['Age']:

Traceback (most recent call last):

   File "test.py", line 8, in 

      print "dict['Age']: ", dict['Age'];

TypeError: 'type' object is unsubscriptable

Looping Technique

When looping through dictionaries, the key and corresponding value can be retrieved at the same time using the items() method.

For example to print student_details key and val

python">students = {'course': 'computer science', 'id': 1234, 'name': 'John', 'year': 3}

for key, val in student.items():

    print key, val

Output of the above code when executed: 

course computer science

id 1234

name John

year 3

Built-in Dictionary Functions & Methods

Python includes the following dictionary functions −

Python includes following dictionary methods −