Advertisement

---

Top

< Prev

Next >

In our last few Python article, we have explained the collection data types such as - tuple and list. Moving on in this tutorial, we are going to explain another collection data type - dictionary, which is used to store items in the form of key:value pairs, where both key and value are objects.

As of Python 3.7.0 version, dictionary is an ordered sequence of items, in which the items are stored in the order in which they are declared i.e. the insertion order is preserved.

Note : Only the immutable data types such as int, float, tuple, str can be used as a key, where value can be any Python data type.

• Important properties of a dictionary



• A dictionary is mutable i.e. existing items can be modified and we can append new items to it.
• We cannot store duplicate keys in a dictionary.
• There is no concept of index position in a dictionary, because every item is stored in the form of key:value pair, and we can access a value stored in a dictionary by using its key stored in a dictionary.







• Methods of the dictionary collection type


Now let us discuss some important methods of dictionary collection data type.



Methods	Description
get(key)	Returns value associated with key in dictionary, else returns None.
values()	Returns values in a dictionary.
update()	Updates an existing dictionary.
clear()	Clears/removes all the items in a dictionary.
copy()	Returns copy of a dictionary.
pop(key)	Removes a key and its value, if unsuccessful raises KeyError.
pop(key,value)	Removes a key and its value, if unsuccessful returns value.
popitem()	Removes an arbitrary key and its value from a dictonary, if unsuccessful raises KeyError.






Advertisement






• How to create a dictionary in Python?


In order to create a dictionary with items, we must declare its items within a pair of curly braces { }, where each item(represented by a key and a value) is separated by the comma, operator. Each key is followed by a colon: which is followed by its associated value. Let us show you a simple example of how to create a dictionary with items.



# Python - Dictionary Example


# Declaring a dictionary with key-value pairs
dict1 = {
    4:'Fourth',
    3:'Third',
    'two':'Second',
    'one':'First',
    5:'Fifth'
    }
print(dict1)


# Printing the keys
print('keys : ', dict1.keys())




Output




{4: 'Fourth', 3: 'Third', 'two': 'Second', 'one': 'First', 5: 'Fifth'}
keys :  dict_keys([4, 3, 'two', 'one', 5])


In the example above, we have created a dictionary named dict1, containing 5 items(in key-value pairs) separated by the comma, operator and all the items are enclosed within a pair of curly braces { }. The items of the dictionary are displayed in the order in which they were declared within a dictionary and we have printed the keys in the dictionary by using the keys() method.






• Creating a dictionary by calling the dict() function


Another way to create a dictionary of items is by calling the built-in dict() function. This function can be passed from zero to one argument, such as -
• If this function is passed zero arguments(no argument), it returns an empty dictionary.
• If this function is passed an existing dictionary as an argument, it returns its copy.
• If this function is passed a set of items in key:value pairs, it tries to convert it to a dictionary and returns it, otherwise reports an error.




#Python - Creating a dictionary with items using dict() function



# Creating an empty dictionary by calling dict() with no argument.
dict_ex = dict()


# Printing the first dictionary
print('First dictionary : ', dict_ex)




# Creating a dictionary
dict2 = {
    4:'Fourth',
    3:'Third',
    'two':'Second',
    'one':'First',
    5:'Fifth'
    }


# Creating a dictionary by calling dict() with an existing dictionary as its argument
dict_ex2 = dict(dict2)



# Printing the second dictionary
print('Seoond dictionary : ', dict_ex2)




# Creating a dictionary by calling dict() with key:value pairs i.e. a dictionary as its arguments  
dict_ex3 = dict({1:'One',
                 2:'Two',
                 3:'Three'
                 })


# Printing the third dictionary
print('Third dictionary : ', dict_ex3)




Output




First dictionary :  {}
Seoond dictionary :  {4: 'Fourth', 3: 'Third', 'two': 'Second', 'one': 'First', 5: 'Fifth'}
Third dictionary :  {1: 'One', 2: 'Two', 3: 'Three'}








• Accessing each item of a dictionary


Each item in a dictionary is stored as a key:value pair where a unique key is used to store its associated value. Using the value of a key, we can access its associated value stored in a dictionary. Let us see how we do this.



# Python - Dictionary Example

dict1 = {
    4:'Fourth',
    3:'Third',
    'two':'Second',
    'one':'First',
    5:'Fifth'
    }
print(dict1)

# Accessing the value of key 3
print('The value of key 3 : ', dict1[3])

# Accessing the value of key 'two'
print('The value of key two : ',dict1['two'])

# Accessing the value of key 5
print('The value of key 5 : ', dict1[5])




Output




{4: 'Fourth', 3: 'Third', 'two': 'Second', 'one': 'First', 5: 'Fifth'}
The value of key 3 :  Third
The value of key two :  Second
The value of key 5 :  Fifth


In the example above, we have created a dictionary named dic1, containing 5 items(in the pair of key:value) and we have accessed a specific value stored in this dictionary by accessing its key.





• A dictionary is mutable


Once we have declared a dictionary with its items, it is mutable i.e. its existing items can be modified and we can insert or append new items to it. Let us first see how we can modify a dictionary once it is created.



# Python - Dictionary Example

dict1 = {
    4:'Fourth',
    3:'Third',
    'two':'Second',
    'one':'First',
    5:'Fifth'
    }


print('Original dictionary : ', dict1)

# Updating value assciated with key 3
dict1[3] = 'Think'

# Updating value assciated with key 'two'
dict1['two'] = 'Positive'

# Updating value assciated with key 5
dict1[5] = 'And smile'

print('Modified dictionary : ', dict1)




Output




Original dictionary :  {4: 'Fourth', 3: 'Third', 'two': 'Second', 'one': 'First', 5: 'Fifth'}
Modified dictionary :  {4: 'Fourth', 3: 'Think', 'two': 'Positive', 'one': 'First', 5: 'And smile'}








• Duplicate keys are not stored in a dictionary


We cannot store duplicate key values in a dictionary, trying to do it will not throw any error but the duplicate keys and their associated values will be automatically removed from the dictionary.



# Python - Dictionary Example

dict1 = {
    4:'Fourth',
    3:'Third',
    'two':'Second',
    'one':'First',
    5:'Fifth',
    4:'Four',
    'two':2
    }


print('Dictionary : ', dict1)




Output




Dictionary :  {4: 'Four', 3: 'Third', 'two': 2, 'one': 'First', 5: 'Fifth'}








• Iterating over items in a dictionary using for loop


With the for loop, we can use use the membership operator in, which allows us to iterate over keys in a dictionary and by using the key of each item, we can access its associated value. Let us see how we do this.



# Python - Iterating over elements of a Dictionary  

dict1 = {
    4:'Fourth',
    3:'Third',
    'two':'Second',
    'one':'First',
    5:'Fifth'
    }

print('Key : Value')

# Using for loop to iterate over keys
for key in dict1:
    value = dict1[key]
    print(key, ':', value)




Output




Key : Value
4 : Fourth
3 : Third
two : Second
one : First
5 : Fifth


As we know that items are stored in a dictionary in a pair of key and value. By using a for loop with in operator, we can only iterate over keys in a dictionary and not values.






• To find the total number of items in a dictionary


We can use the built-in len() function to find the total number of items in a dictionary. This function takes a dictionary as an argument and returns the total number of items it contains.



# Python - Finding total length of a dictionary  


#Creating a dictionary
dict1 = {
    4:'Fourth',
    3:'Third',
    'two':'Second',
    'one':'First',
    5:'Fifth'
    }

# Printing items of dictionary
print(dict1)

# Printing total length of dictionary
print('Total length of dictionary : ', len(dict1))




Output




{4: 'Fourth', 3: 'Third', 'two': 'Second', 'one': 'First', 5: 'Fifth'}
Total length of dictionary :  5








• Modifying a dictionary using update() method


We could update an existing dictionary by adding another dictionary to it, adding single or multiple items(key:value pairs, which are also a dictionary), and by modifying the existing items by calling the update() method of dictionary collection data type. The update() method is called by using a dictionary object, having a form of key:value pairs.

The existing keys and their associated values in a dictionary are replaced by the ones passed to the update() method, otherwise new key:value pairs are added to the dictionary.



# Python - Modifying a dictionary using update() method.



# First dictionary
dict1 = {
    4:'Fourth',
    3:'Third',
    'two':'Second',
    'one':'First',
    5:'Fifth'
    }


# Second dictionary 
dict2 = {
    40:'Forty',
    30:'Thirty'
    }

	
	
# Original dictionary 
print('Original dictionary : ', dict1)


# Calling update() method  to add  a dictionary to another  dictionary
dict1.update(dict2)


# Calling update() method to add a key:value pair to a dictionary
dict1.update({'X': 'Dead End'})


# Calling update() method to update existing values in a dictionary
dict1.update({3:'Three', 5:'Five'})


# Updated dictionary 
print('Updated dictionary : ', dict1)




Output




Original dictionary :  {4: 'Fourth', 3: 'Third', 'two': 'Second', 'one': 'First', 5: 'Fifth'}
Updated dictionary :  {4: 'Fourth', 3: 'Three', 'two': 'Second', 'one': 'First', 5: 'Five', 40: 'Forty', 30: 'Thirty', 'X': 'Dead End'}


In the last program, we have called update() method to modify an existing dictionary by :
• Adding another dictionary to it.
• Adding new key:value pairs to it(enclosed in curly braces i.e. also a dictionary).
• Updating the associated values of existing keys with their new values.



Note : Unlike in the case of collection data types like tuple, list, we cannot use arithmetic operator + to concatenate two dictionaries, therefore we have to use the update() method of dictionary collection data type.






• Example with methods of dictionary

# Python - Example with methods of dictionary



# First dictionary
dict1 = {
    4:'Fourth',
    3:'Third',
    'two':'Second',
    'one':'First',
    5:'Fifth'
    }


# Second dictionary 
dict2 = {
    40:'Forty',
    30:'Thirty'
    }

	
	
# Original dictionary 
print('Original dictionary : ', dict1)


# Calling pop(key) method to remove a key and its associated value
dict1.pop(4)


# Updated dictionary 
print('Updated dictionary : ', dict1)



# Calling pop(key,value) method to remove a key and its associated value
dict1.pop('two', '2nd')


# Updated dictionary 
print('Updated dictionary : ', dict1)



# Calling popitem() method to pop an arbitrary item
dict1.popitem()


# Updated dictionary 
print('Updated dictionary : ', dict1)



# Printing the values in a dictionary
print('Values in dictionary : ', dict1.values())

Output

Original dictionary :  {4: 'Fourth', 3: 'Third', 'two': 'Second', 'one': 'First', 5: 'Fifth'}
Updated dictionary :  {3: 'Third', 'two': 'Second', 'one': 'First', 5: 'Fifth'}
Updated dictionary :  {3: 'Third', 'one': 'First', 5: 'Fifth'}
Updated dictionary :  {3: 'Third', 'one': 'First'}
Values in dictionary :  dict_values(['Third', 'First'])

• The method call pop(4) has removed the key 4 and its associated value Fourth.
• The method call pop('two', '2nd') has removed the key two and its associated value second, even though we have passed a wrong value 2nd.
• The method call popitem() has removed an arbitrary key and its associated value from the dictionary.
• The method call values() has returned us the values stored in the dictionary.

Some important built-in functions to work with dictionary

Functions	Description
max()	Returns the maximum value of keys in a dictionary.
min()	Returns the minimum value of keys in a dictionary.
len()	Returs the total length of a dictionary.
sum()	Returns sum of all keys in a dictionary.
sorted()	Returns a sorted copy of a dictionary.
any()	Returns true if any key in a dictionary is true, else false.
all()	Returns true if all key in a dictionary are true, else false.

Example of using built-in functions with dictionary

#Python - Built-in functions to work with dictionary



# First dictionary
dict1 = {
    4:'Fourth',
    3:'Third',
    2:'Second',
    1:'First',
    5:'Fifth'
    }


# Second dictionary 
dict2 = {
    40:'Forty',
    True:'Yes',
    False:'No'
    }

# 1 is boolean true, 0  for boolean false
# True(with capital T) is boolean true, False(with capital F) is boolean false

 
#Printing dictionarys
print('Dictionary1 : ', dict1)
print('Dictionary2 : ', dict2)



#Using max() function with a dictionary.
print('Maximum key in dict1 : ', max(dict1))


#Using min() function with a dictionary.
print('Minimum key in dict1 : ', min(dict1))


#Using sum() function with a dictionary.
print('Sum of all keys in dict1 : ', sum(dict1))


#Using sorted() function with a dictionary.
print('Sorted copy of dict1 : ', sorted(dict1))


#Using all() function with a dictionary.
print('all() boolean true keys in dict2 : ', all(dict2))


#Using any() function with a dictionary.
print('any boolean true key in dict2 : ', any(dict2))

Output

Dictionary1 :  {4: 'Fourth', 3: 'Third', 2: 'Second', 1: 'First', 5: 'Fifth'}
Dictionary2 :  {40: 'Forty', True: 'Yes', False: 'No'}
Maximum key in dict1 :  5
Minimum key in dict1 :  1
Sum of all keys in dict1 :  15
Sorted copy of dict1 :  [1, 2, 3, 4, 5]
all() boolean true keys in dict2 :  False
any boolean true key in dict2 :  True

Please share this article -

< Prev

Next >

< Python Set

Python - String >

Advertisement