Working with Lists & Dictionaries in Python

Introduction

In this tutorial, we are covering two data structures in python programming. Python is one of the most popular and widely used programming languages in the world. It is used for data analytics, machine learning, and even design. It is used in a wide range of domains owing to its simple yet powerful nature. Python 3 is a high-level, general-purpose programming language. In this article, we will be discussing python dictionary and list datatypes.

Source: Google Images

Python also has advanced data structures as well, such as stacks or queues, which can be implemented with basic data structures. They are rarely used in data science and more commonly used in the software engineering and implementation of these complex algorithms so we won’t discuss them in this tutorial.

For Beginners, free Python tutorials and libraries like numpy and pandas, you can refer to Introduction to Python.
Learning Objectives

• In this tutorial, we will learn about lists and dictionaries.
• We will also learn about different ways to access lists and dictionaries.
• Lastly, we will learn how dictionaries are created.

This article was published as a part of the Data Science Blogathon.

Table of Contents

1. What Are Lists?
2. Accessing the List Elements
3. Index Range
4. Working With Operators in the List
5. Nested Lists
6. Updating the Python Lists
7. List Methods
8. Python Dictionary
9. Syntax of Creating Dictionary With Examples
10. Finding Length of Python Dictionary
11. Different Ways to Access List Elements
12. Printing Dictionary Object Values in Key, Value Pair
13. Converting a Python List Into a Python Dictionary Object
14. Copying a Python Dictionary Into a New Dictionary
15. Updating the Python Dictionary Object
16. How to Delete From Python Dictionary?
17. Looping Through a Python Dictionary Object
18. Conclusion

What Are Lists?

Lists are just like arrays. Python lists are mutable data types in Python. A list refers to the collection of index value pair-like arrays in C/C++/Java. Lists is a 0-based index datatype, meaning the index of the first element starts at 0. Lists are used to store multiple items in a single variable. Lists are one of the 4 data types present in Python, i.e., Lists, Dictionaries, Tuples & Sets. A list is created by placing elements in [ ] separated by commas ‘, ‘. We can use them to store more complex data structures other than integers, strings, or floats. A list of dictionaries is a great example.

Accessing the List Elements

The elements of lists can be done by placing index numbers in square brackets, as shown below:

Python Code:

Output

Ahmedabad
30

We also have a negative index number, as shown below:

print(city_list[-1])

Output

Coimbatore

A negative index references the list from the last value; in this case, it is Coimbatore.

Index Range

The index range represents the portion of the list that you want to extract, as shown in the example below:

print(city_list[2:5])

Output

['Ahmedabad', 'Surat', 'Lucknow']

If you want to find the index value of an item in lists provided the item is in the know, this can be done as shown below:

print(city_list.index('Chennai',0,3))

Output

1

Working With Operators in the List

The addition of two lists can be performed using the + operator. The + operator concatenates both the lists.

num_city_list= num_list+ city_list
print(num_city_list)

Output

[10, 20, 30, 40, 50, 60, 70, 80, 90, 'Agra', 'Chennai', 'Ahmedabad', 'Surat', 'Lucknow', 'Coimbatore']

The * operator is used to extend the list by copying the same values.

new_citylist= city_list*2
print(new_citylist)

Output

['Agra', 'Chennai', 'Ahmedabad', 'Surat', 'Lucknow', 'Coimbatore', 'Agra', 'Chennai', 'Ahmedabad', 'Surat', 'Lucknow', 'Coimbatore']

Nested Lists

A list is nested if another list is present inside it. There are two elements in the below list, and both are listed.

list_nest=[[10,20,30],[‘a’,’b’,’c’]]

list_nest

Output

[[10, 20, 30], ['a', 'b', 'c']]

The elements of a nested list can be accessed as shown below

print(list_nest[1][2])

Output

c

Updating the Python Lists

In updating the list, there is various function. One can append, extend, insert, and change the lists. We will see all of them one by one.

Append the Lists

Append is used when you want to add one item to the list.

city_list=['Agra','Chennai', 'Ahmedabad', 'Surat', 'Lucknow', 'Coimbatore']
city_list.append('Bangalore')
print(city_list)

Output

['Agra', 'Chennai', 'Ahmedabad', 'Surat', 'Lucknow', 'Coimbatore', 'Bangalore']

Extend the List

Extend is used when you want to add more than one item to the list

num_list=[10,20,30,40,50,60,70,80,90]
num_list.extend([100,110,120,130,140,150])
print(num_list)

Output

[10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150]

Change the Lists

Change is used when you want to update a specific item in a list.

num_list[7]= 200
print(num_list)

Output

[10, 20, 30, 40, 50, 60, 70, 200, 90, 100, 110, 120, 130, 140, 150]

Insert in the Lists

It is used to insert values at the provided index

city_list=['Agra','Chennai', 'Ahmedabad', 'Surat', 'Lucknow', 'Coimbatore']
city_list.insert(4,'Vadodara')
print(city_list)

Output

['Agra', 'Chennai', 'Ahmedabad', 'Surat', 'Vadodara', 'Lucknow', 'Coimbatore']

Deleting Items From a List

You can remove an item in a list by providing the item index or range that needs to be removed.

city_list=['Agra','Chennai', 'Ahmedabad', 'Surat', 'Lucknow', 'Coimbatore']
del city_list[4]
print(city_list)

Output

['Agra', 'Chennai', 'Ahmedabad', 'Surat', 'Coimbatore']

del city_list[4:6]
print(city_list)

Output

['Agra', 'Chennai', 'Ahmedabad', 'Surat']

You can also delete the complete list using the below command

del city_list

Emptying a List

This is very easy to do, as shown below:

city_list=['Agra','Chennai', 'Ahmedabad', 'Surat', 'Lucknow', 'Coimbatore']
city_list=[]
print(city_list)

Output

[]

List Methods

Length

Length is used to find how many elements are present inside the list.

city_list=['Agra','Chennai', 'Ahmedabad', 'Surat', 'Lucknow', 'Coimbatore']
print(len(city_list))

Output

6

Clear

Clear is used to delete elements in a list

num_list=[1,2,2,2,3,4,5,6,6,7,6,7]
print(num_list.clear())

Output

None

Reverse

The reverse function is used to reverse the elements of a list, as shown below:

city_list=['Agra','Chennai', 'Ahmedabad', 'Surat', 'Lucknow', 'Coimbatore']
city_list.reverse()
print(city_list)

Output

['Coimbatore', 'Lucknow', 'Surat', 'Ahmedabad', 'Chennai', 'Agra']

Sort

Sort is used to sort the elements in a list. By default, it sorts in ascending order. To sort in descending order, one parameter needs to be provided in the sort function, as shown below:

num_list=[1,2,2,2,3,4,5,6,6,7,6,7]
num_list.sort(reverse=True)
print(num_list)

Output

[7, 7, 6, 6, 6, 5, 4, 3, 2, 2, 2, 1]

Python Dictionary

Dictionaries are mutable data types,unordered in nature, meaning they can be updated after they are created.key:value pairs, and the keys must be unique. You can get a list of keys by calling a dictionary instance’s keys method. Syntactically they are written in a key, value pair format inside curly braces/curly brackets. We can update and delete the elements of the list of dictionaries. Dictionaries are implemented using hash tables.

{Key1:Value1,Key2:Value2,Key3:Value3,Key4:Value4}

Keys are always unique, and there cannot be any duplicates. There is no index in the dictionary, meaning they are not ordered. The key is the default iterator and is used to retrieve the value.

Source: trytoprogram

Syntax of Creating Dictionary With Examples

There are four ways in which a dictionary can be created.

Empty Python Dictionary

The syntax for an empty dictionary is :

dict_emp={}
print(dict_emp)

Output

{}

Keys as Integer Value

dict_city={1: 'Ahmedabad', 2: 'Chennai', 3: 'Coimbatore',4:'Surat',5:'Agra'}
print(dict_city)

Output

{1: 'Ahmedabad', 2: 'Chennai', 3: 'Coimbatore', 4: 'Surat', 5: 'Agra'}

Keys as String Values

dict_travel={'Country1':'USA', 'Country2': 'India', 'Country3':'Japan', 'Country4':'UK', 'Country5': 'Australia'}
print(dict_travel)

Output

{'Country1': 'USA', 'Country2': 'India', 'Country3': 'Japan', 'Country4': 'UK', 'Country5': 'Australia'}

Keys as String and Integer Values

mixed_dict= {'Country':'India', 1456:'Free','list':['city', 'road',12]}
print(mixed_dict)

Output

{'Country': 'India', 1456: 'Free', 'list': ['city', 'road', 12]}

Finding Length of Python Dictionary

print(len(dict_salesid))

Output

5

Different Ways to Access List Elements

Let’s create two dictionary

dict_salesid= {'SID1': Fiat
               'SID2': Mercedes
               'SID3': Maruti
               'SID4': Volkswagen
               'SID5': Kia}

dict_salesinfo= {'SID':Fiat,
                'Sales': 20000
                'LaunchDay':'Wed'
                'Cost': 500000}

Extracting Python Dictionary Elements Using Its Key

To access the value associated with any of them, we will use the following syntax:

sales_id='SID2'
if sales_id in dict_salesid:
    name= dict_salesid[sales_id]
    print('Sales ID is {}, Sales name is {}'. format(sales_id,name))
else:
    print('Sales ID {} not found'.format(sales_id))

Output

Sales ID is SID2, Sales name is Mercedes

Setting Python Dictionary Element Using Its Key

First, we set the dictionary element and then retrieve it using the get function.

dict_salesinfo['LaunchDay']='Thurs'
dict_salesinfo['Cost']=6000000
LaunchDay= dict_salesinfo.get('LaunchDay')
Cost=dict_salesinfo.get('Cost')
print('Launchday is {}, Cost is {}'. format(LaunchDay,Cost))

Output

Launchday is Thurs, Cost is 6000000

Printing Dictionary Object Values in Key, Value Pair

dict_values= dict_salesinfo.values()
print(dict_values)

Output

dict_values(['Fiat', 20000, 'Wed', 500000])

Using the items() Function

The item function converts a dictionary item into a tuple

dict_items= dict_salesinfo.items()
print(dict_items)
print(type(dict_items))

Output

dict_items([('SID', 'Fiat'), ('Sales', 20000), ('LaunchDay', 'Wed'), ('Cost', 500000)])
<class 'dict_items'>

Looping Through the items() Function

for key, value in dict_salesinfo.items():
    print(key +"-"+str(value))

Output

SID-Fiat
Sales-20000
LaunchDay-Wed
Cost-500000

Converting a Python List Into a Python Dictionary Object

A dictionary object contains key-value pairs, and the list must adhere to this format, or else it will throw an error.

sales_infolist=[['SID','Fiat'],['Sales','20000'],['LaunchDay','Wed'],['Cost','500000']]
print(type(sales_infolist))
sales_infolist_dict= dict(sales_infolist)
print(type(sales_infolist_dict))

Output

<class 'list'>
<class 'dict'>

Copying a Python Dictionary Into a New Dictionary

dict_salesinfo_new= dict_salesinfo.copy()
print(dict_salesinfo_new)

Output

{'SID': 'Fiat', 'Sales': 20000, 'LaunchDay': 'Wed', 'Cost': 500000}

Updating the Python Dictionary Object

The update method is used to update the dictionary object with a value

dict_salesinfo= {'SID':'Fiat','Sales': 20000,'LaunchDay':'Wed','Cost': 500000}
dict_salesinfo.update({'Profit':'50000'})
print(dict_salesinfo)

Output

{'SID': 'Fiat', 'Sales': 20000, 'LaunchDay': 'Wed', 'Cost': 500000, 'Profit': '50000'}

How to Delete From Python Dictionary?

A dictionary object can be deleted using the del statement

del dict_salesinfo

Output

The dictionary object is deleted

Deleting a Specific Item From Python Dictionary

To delete a specific item, pass on the dictionary key in the del statement as shown below:

del dict_salesinfo['SID']
print(dict_salesinfo)

Output

{'Sales': 20000, 'LaunchDay': 'Wed', 'Cost': 500000}

Another way to delete a specific item is by using the pop function.

print(dict_salesinfo.pop('SID'))

Output

Fiat

The pop function also returns the key value that is being deleted. In this case, it is Fiat.

The third method to delete a dictionary object is using the clear method.

print(dict_salesinfo.clear())

Output

None

Looping Through a Python Dictionary Object

We will use the same dictionary object dict_salesinfo. Using the keys function, first, save all the keys in a dict_key variable

dict_keys= dict_salesinfo.keys()

print(dict_keys)

print(type(dict_keys))

Output

dict_keys(['SID', 'Sales', 'LaunchDay', 'Cost'])
<class 'dict_keys'>

for var in dict_keys:

print(var + “:” + str(dict_salesinfo[var]))

Output

SID:Fiat
Sales:20000
LaunchDay:Wed
Cost:500000

Conclusion

In Python, a list is a class of data structures that can store one or more objects or values. A list can store multiple items in one variable and be created using square brackets. Dictionaries are used to store data values in key:value pairs. A dictionary is an ordered, changeable collection, and does not allow duplicates. Elements of the dictionary are accessed through the keys.

Key Takeaways

• A list is an ordered data structure with elements separated by a comma and enclosed within square brackets.
• A dictionary in Python is a collection of key-value pairs used to store data values like a map, unlike other data types, which hold only a single value as an element.

Frequently Asked Questions