Dictionary Data Structure in Python

1. What is a Dictionary? Creating a Dictionary in Python

A Dictionary Data Structure in Python is used to store data in the form of keys and values. The elements in the Dictionary are ordered, changeable and do not allow 2 elements with the same key. The key and value pairs in the Dictionary are declared within curly braces {}. The values in the Dictionary can have any data type.

# empty dictionary
my_dict = {}
print(my_dict)
# dictionary with integer keys
my_dict = {
    1: 'apple',
    2: 'banana',
    3: 'lemon'
}
print(my_dict)
# dictionary with mixed keys
my_dict = {
  "name": "John",
  1: [2, 3, 4],
  2: "12A1"
}
print(my_dict)
# dictionary with mixed values
my_dict = {
  "name": "John",
  "yearofbirth": 2000,
  "class": "12A1",
  "subject": ["math", "literature", "english"]
}
print(my_dict)
# using dict() to create dictionary
my_dict = dict({1: 'apple', 2: 'banana', 3: 'lemon'})
print(my_dict)
# from sequence having each item as a pair
my_dict = dict([(1,'apple'), (2,'banana'), (3, 'lemon')])
print(my_dict)

Output

{}
{1: 'apple', 2: 'banana', 3: 'lemon'}
{'name': 'John', 1: [2, 3, 4], 2: '12A1'}
{'name': 'John', 'yearofbirth': 2000, 'class': '12A1', 'subject': ['math', 'literature', 'english']}
{1: 'apple', 2: 'banana', 3: 'lemon'}
{1: 'apple', 2: 'banana', 3: 'lemon'}

Do not allow 2 elements with the same key in Dictionary

The Dictionary will get the latest value of the duplicate keys for that key.

my_dict = {
  "name": "John",
  "yearofbirth": 2000,
  "class": "12A1",
  "class": "IT02"
}
print(my_dict)

Output

{'name': 'John', 'yearofbirth': 2000, 'class': 'IT02'}

Number of elements in Dictionary

Use the len() function to return the number of elements in the Dictionary.

my_dict = {
  "name": "John",
  "yearofbirth": 2000,
  "class": "12A1"
}
print("Number of items in my_dict:", len(my_dict))
my_dict = {
  "name": "John",
  "yearofbirth": 2000,
  "class": "12A1",
  "class": "IT02"
}
print("Number of items in my_dict:", len(my_dict))

Output

Number of items in my_dict: 3
Number of items in my_dict: 3

2. Accessing elements in Dictionary

We can access elements in the Dictionary by using the key in square brackets []. Additionally, we can also use the get() method.

my_dict = {
  "name": "John",
  "yearofbirth": 2000,
  "class": "12A1"
}
print("Get value of 'class' key using []:", my_dict["class"])
my_dict = {
  "name": "John",
  "yearofbirth": 2000,
  "class": "12A1"
}
print("Get value of 'class' key using get():", my_dict.get("class"))

Output

Get value of 'class' key using []: 12A1
Get value of 'class' key using get(): 12A1

2.1. Get all keys in Dictionary

The keys() method returns a list of all keys in the Dictionary.

my_dict = {
  "name": "John",
  "yearofbirth": 2000,
  "class": "12A1"
}
all_keys = my_dict.keys()
print("All keys in my_dict:", all_keys)

Output

All keys in my_dict: dict_keys(['name', 'yearofbirth', 'class'])

2.2. Get all values in Dictionary

The values() method returns a list of all values in the Dictionary.

my_dict = {
  "name": "John",
  "yearofbirth": 2000,
  "class": "12A1"
}
all_values = my_dict.values()
print("All values in my_dict:", all_values)

Output

All values in my_dict: dict_values(['John', 2000, '12A1'])

2.3. Get all elements in Dictionary

The items() method returns each element in the Dictionary as a tuple in a list.

my_dict = {
  "name": "John",
  "yearofbirth": 2000,
  "class": "12A1"
}
all_items = my_dict.items()
print("All items in my_dict:", all_items)

Output

All items in my_dict: dict_items([('name', 'John'), ('yearofbirth', 2000), ('class', '12A1')])

2.4. Check if a key exists in the Dictionary Data Structure in Python

Use the keyword in to determine if a key exists in the Dictionary.

my_dict = {
  "name": "John",
  "yearofbirth": 2000,
  "class": "12A1"
}
# check yearofbirth key
if "yearofbirth" in my_dict:
  print("'yearofbirth' is one of the keys in the my_dict dictionary")
else:
  print("'yearofbirth' isnot one of the keys in the my_dict dictionary")
# check subject key
if "subject" in my_dict:
  print("'subject' is one of the keys in the my_dict dictionary")
else:
  print("'subject' isnot one of the keys in the my_dict dictionary")

Output

'yearofbirth' is one of the keys in the my_dict dictionary
'subject' isnot one of the keys in the my_dict dictionary

3. Adding, Removing, and Changing Elements in the Dictionary

3.1. Adding elements to the Dictionary Data Structure in Python

To add an element to the Dictionary, declare a new key and assign a value to it.

my_dict = {
  "name": "John",
  "yearofbirth": 2000,
  "class": "12A1"
}
my_dict["math"] = 9.5
print("Add an item to my_dict:", my_dict)

Output

Add an item to my_dict: {'name': 'John', 'yearofbirth': 2000, 'class': '12A1', 'math': 9.5}

Python supports the update() function to add the elements of one Dictionary to another Dictionary.

my_dict = {
  "name": "John",
  "yearofbirth": 2000,
  "class": "12A1"
}
my_dict.update({"math": 9.5, "english": 9.9})
print("Add an item to my_dict:", my_dict)

Output

Add an item to my_dict: {'name': 'John', 'yearofbirth': 2000, 'class': '12A1', 'math': 9.5, 'english': 9.9}

3.2. Removing elements from the Dictionary Data Structure in Python

Using the pop() function

The pop() function removes an element in the Dictionary with a predetermined key.

my_dict = {
  "name": "John",
  "yearofbirth": 2000,
  "class": "12A1"
}
my_dict.pop("class")
print("Remove an item of my_dict using pop():", my_dict)

Output

Remove an item of my_dict using pop(): {'name': 'John', 'yearofbirth': 2000}

Using the popitem() function

The popitem() function removes the last element added to the Dictionary.

my_dict = {
  "name": "John",
  "yearofbirth": 2000,
  "class": "12A1"
}
my_dict.popitem()
print("Remove an item of my_dict using popitem():", my_dict)
my_dict["class"] = "12A2"
print("Add an item to my_dict:", my_dict)
my_dict.popitem()
print("Remove an item of my_dict using popitem():", my_dict)

Output

Remove an item of my_dict using popitem(): {'name': 'John', 'yearofbirth': 2000}
Add an item to my_dict: {'name': 'John', 'yearofbirth': 2000, 'class': '12A2'}
Remove an item of my_dict using popitem(): {'name': 'John', 'yearofbirth': 2000}

Using the clear() function

The clear() function removes all elements in the Dictionary to make it an empty Dictionary.

my_dict = {
  "name": "John",
  "yearofbirth": 2000,
  "class": "12A1"
}
print("my_dict:", my_dict)
my_dict.clear()
print("Clear my_dict:", my_dict)

Output

my_dict: {'name': 'John', 'yearofbirth': 2000, 'class': '12A1'}
Clear my_dict: {}

Using the del keyword

The del keyword will remove all elements in the Dictionary and also delete the reference variable to that Dictionary.

my_dict = {
  "name": "John",
  "yearofbirth": 2000,
  "class": "12A1"
}
print("my_dict:", my_dict)
del my_dict
# NameError: name 'my_dict' is not defined
#print("my_dict:", my_dict)

3.3. Changing elements in the Dictionary Data Structure in Python

We can change the value of a key in the Dictionary by reassigning the value to that key.

my_dict = {
  "name": "John",
  "yearofbirth": 2000,
  "class": "12A1"
}
my_dict["class"] = "1A2"
print("Change an item of my_dict:", my_dict)

Output

Change an item of my_dict: {'name': 'John', 'yearofbirth': 2000, 'class': '1A2'}

To change the value of a key in the Dictionary, use the update() function with the existing key and the new value as arguments.

my_dict = {
  "name": "John",
  "yearofbirth": 2000,
  "class": "12A1"
}
my_dict.update({"class": "1A2"})
print("Change an item of my_dict using update():", my_dict)

Output

Change an item of my_dict using update(): {'name': 'John', 'yearofbirth': 2000, 'class': '1A2'}