Python Dictionary

What is Dictionary?

Dictionary is a python data type that is used to store 'key-value' pairs.

It enables to quickly retrieve, add, remove, modify, values using a key.

Dictionary is very similar to what we call associative array or hash on other languages.

Dictionaries are mutable.

No index value concept in dictionary

How to create Dictionary?

Dictionaries are represented by enclosed curly braces {}

Each key is separated from its value by a colon (:) and the items are separated by commas(,)

# Empty Dictionary
my_dict = {}
print("Empty dictionary is created",my_dict)

# Dictionary with Integer keys
colors = {1:'red', 2:'green', 3:'Yellow'}
print("Dictionary with integer keys are created ",colors)

# Dictionary with mixed keys
my_dict = {'name': 'vinoth', 1: ['red', 'green']}
print("Dictionary with mixed keys are created",my_dict)

# Checking the Type
print(type(my_dict))

Output:
Empty dictionary is created {}
Dictionary with integer keys are created  {1: 'red', 2: 'green', 3: 'Yellow'}
Dictionary with mixed keys are created {'name': 'vinoth', 1: ['red', 'green']}
<class 'dict'>

Note - No duplicate keys are allowed

When duplicate keys encountered during assignment, the last assigned key will be considered

# Dictionary with duplicate keys
colors = {1:'red', 2:'green', 3:'Yellow', 2:'blue'} # key 2 mentioned twice
print("Key 2 is updated with last assigned value",colors)

Output:
Key 2 is updated with last assigned value {1: 'red', 2: 'blue', 3: 'Yellow'}

How to access elements from a dictionary?

To access dictionary elements, you can use the enclosed square brackets along with the key to obtain its value

# Access the key which is available in the dictionary 
mydetails = {'name': 'Vinoth','age':30, 'address': 'Chennai'}

print(mydetails['name'])
print(mydetails['address'])

Output:
Vinoth
Chennai

# Access the key which is not available in the dictionary - KeyError will be displayed
print(mydetails['phonenumber']) 

Output:
KeyError: Traceback (most recent call last)
<ipython-input-28-355917c6fc04> in <module>()
      1 # Access the key which is not available in the dictionary
----> 2 print(mydetails['phonenumber'])

KeyError: 'phonenumber'

How to access the element using Get Method?

It returns value of key, if key is not found it returns None, instead of throwing KeyError exception

#Create a dictionary
mydetails = {'name': 'Vinoth','age':30, 'address': 'Chennai'}

# Access the key which is available in the dictionary 
print(mydetails.get('name'))

# Access the key which is not available in the dictionary 
print(mydetails.get('phonenumber'))

Output:
Vinoth
None

How to add elements(key) in a dictionary?

Dictionary are mutable. We can add new items or change the value of existing items using assignment operator '='

If the key is already present, value gets updated, else a new key is added

#Create a dictionary
mydetails = {'name': 'Vinoth','age':30, 'address': 'Chennai'}

# Add phonenumber
mydetails['phonenumber'] = '+91-6383544892'

print(mydetails)

Output:
{'name': 'Vinoth', 'age': 30, 'address': 'Chennai', 'phonenumber': '+91-6383544892'}

How to change/update existing element(value) in a dictionary?

# Updating the value
mydetails['age'] = 31

print(mydetails)

Output:
{'name': 'Vinoth', 'age': 31, 'address': 'Chennai', 'phonenumber': '+91-6383544892'}

How to delete an elements in a dictionary?

1. pop() Method

pop() Method - Remove the item from the dictionary, if the key is not found KeyError will be thrown. It will also returns the removed value

#Create a dictionary
mydetails = {'name': 'Vinoth', 'age': 31, 'address': 'Chennai', 'phonenumber': '+91-6383544892'}

#Remove a particular item
print(mydetails.pop('phonenumber'))
print(mydetails)

Output:
+91-6383544892
{'name': 'Vinoth', 'age': 31, 'address': 'Chennai'}

2. popitem() Method

popitem() Method - To remove and return an arbitrary item (key, value)

#Create a dictionary
mydetails = {'name': 'Vinoth', 'age': 31, 'address': 'Chennai', 'phonenumber': '+91-6383544892'}

# It remove the last item in the dictionary
mydetails.popitem()

print(mydetails)

Output:
{'name': 'Vinoth', 'age': 31, 'address': 'Chennai'}

3. del Keyword

del keyword to remove individual item or the entire dictionary itself.

#Sample code to delete a particular item
# Dictionary with cubes of number
cube = {1:1, 2:8, 3:27, 4:64, 5:125}

# Delete particular key
del cube[4]

print(cube)

Output:
{1: 1, 2: 8, 3: 27, 5: 125}

#Sample code to delete the entire dictionary
# Dictionary with cubes of number
cube = {1:1, 2:8, 3:27, 4:64, 5:125}

# Delete the entire dictionary
del cube

# NameError is displayed as the entire dictionary is deleted
print(cube) 

Output:
NameError: Traceback (most recent call last)
<ipython-input-50-7e054bd83a54> in <module>()
      8 
      9 # NameError is displayed as the entire dictionary is deleted
---> 10 print(cube)

NameError: name 'cube' is not defined

4. clear() Method

Clear Method - All the items can be removed at once, without deleting the dictionary

# Dictionary with cubes of number
cube = {1:1, 2:8, 3:27, 4:64, 5:125}

#Remove all items
cube.clear()

print("Empty dictionary is displayed",cube)

Output:
Empty dictionary is displayed {}

Looping items in the dictionary

# Dictionary with cubes of number
cube = {1:1, 2:8, 3:27, 4:64, 5:125}

for num in cube:
    print("Cube of",num,"is",cube[num])

Output:
Cube of 1 is 1
Cube of 2 is 8
Cube of 3 is 27
Cube of 4 is 64
Cube of 5 is 125

Dictionary Methods

1. length - len()

Length of dictionary, i.e. the number of elements in the dictionary.

#Create a dictionary
mydetails = {'name': 'Vinoth', 'age': 31, 'address': 'Chennai', 'phonenumber': '+91-6383544892'}
print("length of the Dictionary is",len(mydetails))

Output:
length of the list is 4

2. Membership - ' in and not in' Operator

#Create a dictionary
mydetails = {'name': 'Vinoth', 'age': 31, 'address': 'Chennai', 'phonenumber': '+91-6383544892'}

# Sample Code for 'in' operator - True if key is in dictionary.
print('name' in mydetails)

# Sample Code for 'not in' operator -True if key is not in dictionary.
print('email' not in mydetails)

Output:
True
True

3. sorted()

The sorted() method sorts the elements of a given dictionary in a specific order - Ascending or Descending.

By default in ascending order

Sorting will be carried by taking key as reference

# Sort the vowels in Ascending order
vowels = {'e': 1, 'a': 2, 'u': 3, 'o': 4, 'i': 5}
print('Sorted Dictionary:', sorted(vowels))

Output:
Sorted Dictionary: ['a', 'e', 'i', 'o', 'u']

# Setting reverse=True sorts the dictionary in the descending order
vowels = {'e': 1, 'a': 2, 'u': 3, 'o': 4, 'i': 5}
print('Sorted Dictionary (in Descending):', sorted(vowels,reverse=True))

Output:
Sorted Dictionary (in Descending): ['u', 'o', 'i', 'e', 'a']

4. keys()

Return keys in the dictionary as tuples

#Create a dictionary
mydetails = {'name': 'Vinoth', 'age': 31, 'address': 'Chennai', 'phonenumber': '+91-6383544892'}
print(mydetails.keys())

Output:
dict_keys(['name', 'age', 'address', 'phonenumber'])

5. values()

Return keys in the dictionary as tuples

#Create a dictionary
mydetails = {'name': 'Vinoth', 'age': 31, 'address': 'Chennai', 'phonenumber': '+91-6383544892'}
print(mydetails.values())

Output:
dict_values(['Vinoth', 31, 'Chennai', '+91-6383544892'])

6. items()

Return a new view of the dictionary items (key, value) as tuples

# Dictionary with cubes of number
cube = {1:1, 2:8, 3:27, 4:64, 5:125}
print(cube.items()) 

Output:
dict_items([(1, 1), (2, 8), (3, 27), (4, 64), (5, 125)])

7. copy()

Return a shallow copy of the dictionary. It doesn't modify the original dictionary.

# Dictionary with cubes of number
cube = {1:1, 2:8, 3:27, 4:64, 5:125}

dictcopy = cube.copy()

print('Original:', cube)
print('New:', dictcopy)

Output:
Original: {1: 1, 2: 8, 3: 27, 4: 64, 5: 125}
New: {1: 1, 2: 8, 3: 27, 4: 64, 5: 125}

8. update()

The update() method inserts the specified items to the dictionary at the end, if the value is not present.

In case if the value is already existing, it will be updated with new value.

The specified items can be a dictionary, or an iterable object.

Inserting the new value


# Dictionary with car details
car = {"brand": "Audi",  "model": "A7",  "year": 2019}

car.update({"color": "White"})
print(car) 

Output:
{'brand': 'Audi', 'model': 'A7', 'year': 2019, 'color': 'White'}

Updating the existing value

# Dictionary with car details
car = {"brand": "Audi",  "model": "A7", "color":"Blue" , "year": 2019}

car.update({"color": "White"})
print(car) 

Output:
{'brand': 'Audi', 'model': 'A7', 'color': 'White', 'year': 2019}

Update with an iterable.

# Dictionary with car details
car = {"brand": "Audi",  "model": "A7", "year": 2019}
   
# Updating with multiple key and values 
car.update(cost = '68000$', color = 'White') 
print("Dictionary after updation:",car) 

Output:
Dictionary after updation:
 {'brand': 'Audi', 'model': 'A7', 'year': 2019, 'cost': '68000$', 'color': 'White'}

9. fromkeys()

The fromkeys() method creates a new dictionary from the given sequence of elements with a value provided by the user.

The fromkeys() method takes two parameters:

sequence - sequence of elements which is to be used as keys for the new dictionary

value (Optional) - value which is set to each each element of the dictionary

#  Creating a dictionary from a sequence of keys with value
# vowels keys
mykeys = {'a', 'e', 'i', 'o', 'u' }
value = 'vowels'

vowels = dict.fromkeys(mykeys, value)
print(vowels)

Output:
{'o': 'vowels', 'a': 'vowels', 'e': 'vowels', 'i': 'vowels', 'u': 'vowels'}

10. setdefault()

The setdefault() method returns the value of a key (if the key is in dictionary). If not, it inserts key with a value to the dictionary.

if key is not in the dictionary.

# Sample code 
employee = {'name': 'Muthu'}

salary = employee.setdefault('salary')
print("Employee details :",employee)
print('salary=',salary)

Output:
Employee details : {'name': 'Muthu', 'salary': None}
salary= None

if key is not in the dictionary and default_value is provided

# Sample code  
employee = {'name': 'Muthu'}

company = employee.setdefault('company', 'CTS')
print("Employee details :",employee)
print('company=',company)

Output:
Employee details : {'name': 'Muthu', 'company': 'CTS'}
company= CTS

11. dir()

Get list of all available methods and attributes of dictionary

mydict = {}
print(dir(mydict))

Output:
['__class__', '__contains__', '__delattr__', '__delitem__', '__dir__', '__doc__', '__eq__',
 '__format__', '__ge__', '__getattribute__', '__getitem__', '__gt__', '__hash__', '__init__',
 '__init_subclass__', '__iter__', '__le__', '__len__', '__lt__', '__ne__', '__new__',
 '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__setitem__', '__sizeof__',
 '__str__', '__subclasshook__', 'clear', 'copy', 'fromkeys', 'get', 'items', 'keys',
 'pop', 'popitem', 'setdefault', 'update', 'values']

Dictionary Comprehension

Dictionary comprehension is an elegant and concise way to create new dictionary from an iterable in Python.

It consists of an expression pair (key: value) followed by for statement inside curly braces {}.

#Using Dictionary comprehension
cubes = {x: x**3 for x in range(10)}
print(cubes)

Output:
{0: 0, 1: 1, 2: 8, 3: 27, 4: 64, 5: 125, 6: 216, 7: 343, 8: 512, 9: 729}

Q. we have a list of fruits and we can use dict comprehension to create a dictionary with fruits, the list elements as the keys and the length of each string as the values

# create list of fruits
fruits = ['apple', 'mango', 'banana']

# dict comprehension to create dict with fruit name as keys
{f:len(f) for f in fruits}

Output:
{'apple': 5, 'mango': 5, 'banana': 6}