Skip to content
geeksforgeeks
  • Tutorials
    • Python
    • Java
    • Data Structures & Algorithms
    • ML & Data Science
    • Interview Corner
    • Programming Languages
    • Web Development
    • CS Subjects
    • DevOps And Linux
    • Software and Tools
    • School Learning
    • Practice Coding Problems
  • Go Premium
  • Python Tutorial
  • Interview Questions
  • Python Quiz
  • Python Glossary
  • Python Projects
  • Practice Python
  • Data Science With Python
  • Python Web Dev
  • DSA with Python
  • Python OOPs
Open In App
Next Article:
Prototype Method Design Pattern in Python
Next article icon

Observer method - Python Design Patterns

Last Updated : 12 Jul, 2025
Comments
Improve
Suggest changes
Like Article
Like
Report

The observer method is a Behavioral design Pattern which allows you to define or create a subscription mechanism to send the notification to the multiple objects about any new event that happens to the object that they are observing. The subject is basically observed by multiple objects. The subject needs to be monitored and whenever there is a change in the subject, the observers are being notified about the change. This pattern defines one to Many dependencies between objects so that one object changes state, all of its dependents are notified and updated automatically.
 

Problem Without using Observer Method


Imagine you want to create a calculator application that has different features such as addition, subtraction, changing base of the numbers to hexadecimal, decimal, and many other features. But one of your friends is interested in changing the base of his favorite number to Octal base number and you are still developing the application. So, what could be the solution to it? Should your friend check the application daily just to get to know about the status? But don't you think it would result in a lot of unnecessary visits to the application which were definitely not required. Or you may think about that each time you add the new feature and send the notification to each user. Is it OK? Sometimes yes but not every time. Might be some users get offended by a lot of unnecessary notifications which they really don't want.
 

Solution using Observer Method


Let's discuss the solution to the above-described problem. Here comes the object Subject into the limelight. But it also notifies the other objects also that's why we generally call it Publisher. All the objects that want to track changes in the publisher's state are called subscribers.
 

Observer-method-solution-diagram
Observer-method-solution-diagram


 

Python3
class Subject:

    """Represents what is being observed"""

    def __init__(self):

        """create an empty observer list"""

        self._observers = []

    def notify(self, modifier = None):

        """Alert the observers"""

        for observer in self._observers:
            if modifier != observer:
                observer.update(self)

    def attach(self, observer):

        """If the observer is not in the list,
        append it into the list"""

        if observer not in self._observers:
            self._observers.append(observer)

    def detach(self, observer):

        """Remove the observer from the observer list"""

        try:
            self._observers.remove(observer)
        except ValueError:
            pass



class Data(Subject):

    """monitor the object"""

    def __init__(self, name =''):
        Subject.__init__(self)
        self.name = name
        self._data = 0

    @property
    def data(self):
        return self._data

    @data.setter
    def data(self, value):
        self._data = value
        self.notify()


class HexViewer:

    """updates the Hexviewer"""

    def update(self, subject):
        print('HexViewer: Subject {} has data 0x{:x}'.format(subject.name, subject.data))

class OctalViewer:

    """updates the Octal viewer"""

    def update(self, subject):
        print('OctalViewer: Subject' + str(subject.name) + 'has data '+str(oct(subject.data)))


class DecimalViewer:

    """updates the Decimal viewer"""

    def update(self, subject):
        print('DecimalViewer: Subject % s has data % d' % (subject.name, subject.data))

"""main function"""

if __name__ == "__main__":

    """provide the data"""

    obj1 = Data('Data 1')
    obj2 = Data('Data 2')

    view1 = DecimalViewer()
    view2 = HexViewer()
    view3 = OctalViewer()

    obj1.attach(view1)
    obj1.attach(view2)
    obj1.attach(view3)

    obj2.attach(view1)
    obj2.attach(view2)
    obj2.attach(view3)

    obj1.data = 10
    obj2.data = 15

Class Diagram

Following is the class diagram for the Observer Method
 

Class-diagram-Observer-method
Class-diagram-Observer-method


 

Output


 

DecimalViewer: Subject Data 1 has data 10
HexViewer: Subject Data 1 has data 0xa
OctalViewer: SubjectData 1has data 0o12
DecimalViewer: Subject Data 2 has data 15
HexViewer: Subject Data 2 has data 0xf
OctalViewer: SubjectData 2has data 0o17


 

Advantages


 

  • Open/Closed Principle: Introducing subscriber classes is much easier in Observer method as compared to others without making changes in the client's code.
  • Establishes Relationships: Its really easy to establishes the relationships at the runtime between the objects.
  • Description: It carefully describes about the coupling present between the objects and the observer. Hence, there is no need to modify Subject to add or remove observers.


 

Disadvantages


 

  • Memory Leakage: Memory leaks caused by Lapsed Listener Problem because of explicit register and unregistering of observers.
  • Random Notifications: All the subscribers present gets notification in the random order.
  • Risky Implementations: If the pattern is not implemented carefully, there are huge chances that you will end up with large complexity code.


 

Applicability


 

  • Multi-Dependency: We should use this pattern when multiple objects are dependent on the state of one object as it provides a neat and well tested design for the same.
  • Getting Notifications: It is used in social media, RSS feeds, email subscription in which you have the option to follow or subscribe and you receive latest notification.
  • Reflections of Object: When we do not coupled the objects tightly, then the change of a state in one object must be reflected in another object.


Further Read - Observer Method in Java
 


Next Article
Prototype Method Design Pattern in Python

C

chaudhary_19
Improve
Article Tags :
  • Python
  • python-design-pattern
Practice Tags :
  • python

Similar Reads

    Python Design Patterns Tutorial
    Design patterns in Python are communicating objects and classes that are customized to solve a general design problem in a particular context. Software design patterns are general, reusable solutions to common problems that arise during the design and development of software. They represent best pra
    7 min read

    Creational Software Design Patterns in Python

    Factory Method - Python Design Patterns
    Factory Method is a Creational Design Pattern that allows an interface or a class to create an object, but lets subclasses decide which class or object to instantiate. Using the Factory method, we have the best ways to create an object. Here, objects are created without exposing the logic to the cli
    4 min read
    Abstract Factory Method - Python Design Patterns
    Abstract Factory Method is a Creational Design pattern that allows you to produce the families of related objects without specifying their concrete classes. Using the abstract factory method, we have the easiest ways to produce a similar type of many objects. It provides a way to encapsulate a group
    4 min read
    Builder Method - Python Design Patterns
    Builder Method is a Creation Design Pattern which aims to "Separate the construction of a complex object from its representation so that the same construction process can create different representations." It allows you to construct complex objects step by step. Here using the same construction code
    5 min read
    Prototype Method Design Pattern in Python
    The Prototype Method Design Pattern in Python enables the creation of new objects by cloning existing ones, promoting efficient object creation and reducing overhead. This pattern is particularly useful when the cost of creating a new object is high and when an object's initial state or configuratio
    6 min read
    Singleton Method - Python Design Patterns
    Prerequisite: Singleton Design pattern | IntroductionWhat is Singleton Method in PythonSingleton Method is a type of Creational Design pattern and is one of the simplest design patterns available to us. It is a way to provide one and only one object of a particular type. It involves only one class t
    5 min read

    Structural Software Design Patterns in Python

    Adapter Method - Python Design Patterns
    Adapter method is a Structural Design Pattern which helps us in making the incompatible objects adaptable to each other. The Adapter method is one of the easiest methods to understand because we have a lot of real-life examples that show the analogy with it. The main purpose of this method is to cre
    4 min read
    Bridge Method - Python Design Patterns
    The bridge method is a Structural Design Pattern that allows us to separate the Implementation Specific Abstractions and Implementation Independent Abstractions from each other and can be developed considering as single entities.The bridge Method is always considered as one of the best methods to or
    5 min read
`; $(commentSectionTemplate).insertBefore(".article--recommended"); } loadComments(); }); }); function loadComments() { if ($("iframe[id*='discuss-iframe']").length top_of_element && top_of_screen articleRecommendedTop && top_of_screen articleRecommendedBottom)) { if (!isfollowingApiCall) { isfollowingApiCall = true; setTimeout(function(){ if (loginData && loginData.isLoggedIn) { if (loginData.userName !== $('#followAuthor').val()) { is_following(); } else { $('.profileCard-profile-picture').css('background-color', '#E7E7E7'); } } else { $('.follow-btn').removeClass('hideIt'); } }, 3000); } } }); } $(".accordion-header").click(function() { var arrowIcon = $(this).find('.bottom-arrow-icon'); arrowIcon.toggleClass('rotate180'); }); }); window.isReportArticle = false; function report_article(){ if (!loginData || !loginData.isLoggedIn) { const loginModalButton = $('.login-modal-btn') if (loginModalButton.length) { loginModalButton.click(); } return; } if(!window.isReportArticle){ //to add loader $('.report-loader').addClass('spinner'); jQuery('#report_modal_content').load(gfgSiteUrl+'wp-content/themes/iconic-one/report-modal.php', { PRACTICE_API_URL: practiceAPIURL, PRACTICE_URL:practiceURL },function(responseTxt, statusTxt, xhr){ if(statusTxt == "error"){ alert("Error: " + xhr.status + ": " + xhr.statusText); } }); }else{ window.scrollTo({ top: 0, behavior: 'smooth' }); $("#report_modal_content").show(); } } function closeShareModal() { const shareOption = document.querySelector('[data-gfg-action="share-article"]'); shareOption.classList.remove("hover_share_menu"); let shareModal = document.querySelector(".hover__share-modal-container"); shareModal && shareModal.remove(); } function openShareModal() { closeShareModal(); // Remove existing modal if any let shareModal = document.querySelector(".three_dot_dropdown_share"); shareModal.appendChild(Object.assign(document.createElement("div"), { className: "hover__share-modal-container" })); document.querySelector(".hover__share-modal-container").append( Object.assign(document.createElement('div'), { className: "share__modal" }), ); document.querySelector(".share__modal").append(Object.assign(document.createElement('h1'), { className: "share__modal-heading" }, { textContent: "Share to" })); const socialOptions = ["LinkedIn", "WhatsApp","Twitter", "Copy Link"]; socialOptions.forEach((socialOption) => { const socialContainer = Object.assign(document.createElement('div'), { className: "social__container" }); const icon = Object.assign(document.createElement("div"), { className: `share__icon share__${socialOption.split(" ").join("")}-icon` }); const socialText = Object.assign(document.createElement("span"), { className: "share__option-text" }, { textContent: `${socialOption}` }); const shareLink = (socialOption === "Copy Link") ? Object.assign(document.createElement('div'), { role: "button", className: "link-container CopyLink" }) : Object.assign(document.createElement('a'), { className: "link-container" }); if (socialOption === "LinkedIn") { shareLink.setAttribute('href', `https://www.linkedin.com/sharing/share-offsite/?url=${window.location.href}`); shareLink.setAttribute('target', '_blank'); } if (socialOption === "WhatsApp") { shareLink.setAttribute('href', `https://api.whatsapp.com/send?text=${window.location.href}`); shareLink.setAttribute('target', "_blank"); } if (socialOption === "Twitter") { shareLink.setAttribute('href', `https://twitter.com/intent/tweet?url=${window.location.href}`); shareLink.setAttribute('target', "_blank"); } shareLink.append(icon, socialText); socialContainer.append(shareLink); document.querySelector(".share__modal").appendChild(socialContainer); //adding copy url functionality if(socialOption === "Copy Link") { shareLink.addEventListener("click", function() { var tempInput = document.createElement("input"); tempInput.value = window.location.href; document.body.appendChild(tempInput); tempInput.select(); tempInput.setSelectionRange(0, 99999); // For mobile devices document.execCommand('copy'); document.body.removeChild(tempInput); this.querySelector(".share__option-text").textContent = "Copied" }) } }); // document.querySelector(".hover__share-modal-container").addEventListener("mouseover", () => document.querySelector('[data-gfg-action="share-article"]').classList.add("hover_share_menu")); } function toggleLikeElementVisibility(selector, show) { document.querySelector(`.${selector}`).style.display = show ? "block" : "none"; } function closeKebabMenu(){ document.getElementById("myDropdown").classList.toggle("show"); }
geeksforgeeks-footer-logo
Corporate & Communications Address:
A-143, 7th Floor, Sovereign Corporate Tower, Sector- 136, Noida, Uttar Pradesh (201305)
Registered Address:
K 061, Tower K, Gulshan Vivante Apartment, Sector 137, Noida, Gautam Buddh Nagar, Uttar Pradesh, 201305
GFG App on Play Store GFG App on App Store
Advertise with us
  • Company
  • About Us
  • Legal
  • Privacy Policy
  • In Media
  • Contact Us
  • Advertise with us
  • GFG Corporate Solution
  • Placement Training Program
  • Languages
  • Python
  • Java
  • C++
  • PHP
  • GoLang
  • SQL
  • R Language
  • Android Tutorial
  • Tutorials Archive
  • DSA
  • Data Structures
  • Algorithms
  • DSA for Beginners
  • Basic DSA Problems
  • DSA Roadmap
  • Top 100 DSA Interview Problems
  • DSA Roadmap by Sandeep Jain
  • All Cheat Sheets
  • Data Science & ML
  • Data Science With Python
  • Data Science For Beginner
  • Machine Learning
  • ML Maths
  • Data Visualisation
  • Pandas
  • NumPy
  • NLP
  • Deep Learning
  • Web Technologies
  • HTML
  • CSS
  • JavaScript
  • TypeScript
  • ReactJS
  • NextJS
  • Bootstrap
  • Web Design
  • Python Tutorial
  • Python Programming Examples
  • Python Projects
  • Python Tkinter
  • Python Web Scraping
  • OpenCV Tutorial
  • Python Interview Question
  • Django
  • Computer Science
  • Operating Systems
  • Computer Network
  • Database Management System
  • Software Engineering
  • Digital Logic Design
  • Engineering Maths
  • Software Development
  • Software Testing
  • DevOps
  • Git
  • Linux
  • AWS
  • Docker
  • Kubernetes
  • Azure
  • GCP
  • DevOps Roadmap
  • System Design
  • High Level Design
  • Low Level Design
  • UML Diagrams
  • Interview Guide
  • Design Patterns
  • OOAD
  • System Design Bootcamp
  • Interview Questions
  • Inteview Preparation
  • Competitive Programming
  • Top DS or Algo for CP
  • Company-Wise Recruitment Process
  • Company-Wise Preparation
  • Aptitude Preparation
  • Puzzles
  • School Subjects
  • Mathematics
  • Physics
  • Chemistry
  • Biology
  • Social Science
  • English Grammar
  • Commerce
  • World GK
  • GeeksforGeeks Videos
  • DSA
  • Python
  • Java
  • C++
  • Web Development
  • Data Science
  • CS Subjects
@GeeksforGeeks, Sanchhaya Education Private Limited, All rights reserved
We use cookies to ensure you have the best browsing experience on our website. By using our site, you acknowledge that you have read and understood our Cookie Policy & Privacy Policy
Lightbox
Improvement
Suggest Changes
Help us improve. Share your suggestions to enhance the article. Contribute your expertise and make a difference in the GeeksforGeeks portal.
geeksforgeeks-suggest-icon
Create Improvement
Enhance the article with your expertise. Contribute to the GeeksforGeeks community and help create better learning resources for all.
geeksforgeeks-improvement-icon
Suggest Changes
min 4 words, max Words Limit:1000

Thank You!

Your suggestions are valuable to us.

What kind of Experience do you want to share?

Interview Experiences
Admission Experiences
Career Journeys
Work Experiences
Campus Experiences
Competitive Exam Experiences