Mastering Load Testing with Locust and Python: Unleash the Power of Scalability

In today’s digital world, handling heavy web traffic is vital for a smooth user experience. Where load testing is the key to success, in this blog, we’ll explore load testing with Locust, its popularity, and its features with code snippets.

Understanding Load Testing:

Before we dive into the specifics of Locust, let’s first understand what load testing is all about.

Load testing is the process of simulating real-world user activity on a system, application, or website to determine its performance under different levels of load.

This helps identify bottlenecks, performance issues, and scalability limitations, ensuring the application can handle the expected number of users without crashing or slowing down.

Introducing Locust (Load Testing with Locust):

Locust is an open-source load testing tool written in Python.

What sets Locust apart is its simplicity and scalability.

It allows you to define user behaviors as code, making it easy to create complex load testing scenarios. Additionally, Locust’s distributed architecture enables it to generate massive loads while maintaining control and accuracy.

Key Features of Locust
  1. User Behavior as Code: With Locust, you write your load testing scenarios using Python code. This makes it easy to define user behaviors, interactions, and data flows in a structured and flexible manner.
  2. Distributed Load Generation: Locust allows you to distribute load across multiple machines, enabling you to simulate large-scale traffic without overburdening a single device.
  3. Real-Time Monitoring: Locust provides a web-based user interface that displays real-time statistics about the current load, response times, failure rates, and more. This helps you monitor the health of your application during testing.
  4. Scalable and Extensible: Locust’s architecture is designed to handle heavy loads and can be extended through custom plugins to suit specific testing needs.
  5. Integration with Python Ecosystem: Since Locust is written in Python, it seamlessly integrates with the Python ecosystem, making it easy to use libraries, modules, and tools you’re already familiar with.

Getting Started with Locust

Let’s dive into some code snippets to understand how to get started with Locust.

Step 1: How to install Locust?

To begin, you need to install Locust using pip:

pip install locust
Step 2: Writing your first Locust test:

To create a simple “hello world” test using Locust, you can define a task that makes an HTTP request to a target URL. Here’s a basic example:

from locust import HttpUser, task, between

class HelloWorldUser(HttpUser):
    wait_time = between(1, 3)  # Add a random wait time between tasks

    def hello_world(self):
        response = self.client.get("/hello")  # Replace "/hello" with your target URL
        if response.status_code == 200:
            print("Hello, World!")

In this example:

  1. We import the necessary modules from Locust.
  2. We create a HelloWorldUser class that inherits from HttpUser, which represents a user that makes HTTP requests.
  3. We define the wait_time method to add a random wait time between tasks (1 to 3 seconds in this case).
  4. Inside the class, we define a task named hello_world, which sends an HTTP GET request to the “/hello” URL (you should replace this with the URL you want to test).
  5. We check the response status code, and if it’s 200 (OK), we print “Hello, World!”.

To run this Locust test, you can use the following command in your terminal:

Step 3: Running Locust Test:

Run Locust using the command:

locust -f

Locust will start a web interface http://localhost:8089 where you can configure the number of users, and hatch rate, and start the test.

Step 4: Monitoring and Analyzing Results

Access the Locust web interface in your browser and monitor real-time statistics about your test’s progress.

Increase the number of virtual users (VUs) or threads:

To increase the number of virtual users (VUs) or threads in a Locust test to put more load on an API.

You can modify the --users and --spawn-rate options when starting the Locust test from the command line.

locust -f --users 100 --spawn-rate 10

Here, --users specifies the total number of virtual users you want to simulate, and --spawn-rate defines how many users should be spawned per second.

In this example, it will start with 10 users per second and eventually reach a total of 100 users.

Generating test reports using Locust:

Locust doesn’t provide built-in HTML test report generation, but you can generate test reports with the help of third-party tools and libraries.

One commonly used approach is to use the --csv option in Locust to output test results in CSV format and then convert the CSV data into a more readable HTML report. Here’s how you can generate a test report using Locust:

Run Locust with CSV Output: When running Locust, specify the --csv option to save the test results in a CSV file. For example:

locust -f --csv=my_test_results

This will generate CSV files with test results named my_test_results_requests.csv and my_test_results_distribution.csv.


Locust provides logging capabilities to help you monitor and troubleshoot your load tests.

You need to configure and customize logging to capture various events and details during the test execution. Here’s how to use Locust logging effectively:

Import the Logging Module:

At the beginning of your Locust test script, import the logging module:

import logging
Configure Logging:

Locust uses the built-in Python logging module, which allows you to configure logging behavior. You can configure the logging settings according to your requirements. For example:


This sets the logging level to INFO, which means messages with severity INFO and above will be displayed.

You can change the logging level to DEBUG for more detailed logging.

Use Logging in Your Tasks:

Inside your Locust tasks (which represent user behaviors), you can use logging to capture specific events, errors, or custom messages. For example:

from locust import HttpUser, task, between
import logging

class MyUser(HttpUser):
    wait_time = between(1, 3)

    def my_task(self):
            # ... your test logic ...
  "Task completed successfully")
        except Exception as e:
            logging.error(f"Task failed: {e}")

In this example, and logging.error are used to log messages when a task is completed successfully or when an exception occurs.

View the Log Output:

When you run your Locust test, the log messages will be displayed in the terminal or console where you initiated the test.

You can also redirect the log output to a file for better organization and analysis. To redirect log output to a file, you can modify the logging configuration like this:

logging.basicConfig(filename='locust.log', level=logging.INFO)

This will save the log messages to a file named locust.log.

By using logging in your Locust tests, you can capture valuable information about the test execution, including success and failure events, exceptions, and custom messages.

This can be especially useful when you need to diagnose issues or analyze test results in more detail.

Load Testing of APIs using Locust:

API testing involves simulating a large number of virtual users making API requests to test the performance, scalability, and reliability of your API endpoints. Below is an example of how to perform API testing using Locust:

from locust import HttpUser, task, between

class APITestingUser(HttpUser):
    wait_time = between(1, 3)  # Add a random wait time between tasks

    def get_example_api(self):
        response = self.client.get("/example-api")  # Replace with your API endpoint URL
        if response.status_code == 200:
            print("API Request Successful")
            print(f"API Request Failed with Status Code: {response.status_code}")

    def post_example_api(self):
        payload = {"key1": "value1", "key2": "value2"}  # Replace with your POST request data
        headers = {"Content-Type": "application/json"}  # Replace with appropriate headers
        response ="/example-api", json=payload, headers=headers)  # Replace with your API endpoint URL
        if response.status_code == 201:  # Replace with the expected status code for a successful POST request
            print("API POST Request Successful")
            print(f"API POST Request Failed with Status Code: {response.status_code}")

In this example:

  1. We define a APITestingUser class that inherits from HttpUser, just like in the “hello world” example.
  2. We define two tasks: get_example_api and post_example_api, representing GET and POST requests to your API endpoints. Make sure to replace “/example-api” with the actual URL of your API.
  3. In the get_example_api task, we make a GET request to the API endpoint and check the response status code to determine if the request was successful.
  4. In the post_example_api task, we make a POST request to the API endpoint with sample payload data and headers. You should replace the payload and headers with your specific API requirements. We also check the response status code to verify the success of the POST request.

Can we use Locust for UI Testing?

Locust is primarily designed for load testing and API testing.

It may not be the best tool for UI testing, which typically involves interacting with web pages in a more complex manner, such as clicking buttons, filling out forms, and validating UI elements.

For UI testing, you might want to consider using specialized tools like Selenium, Playwright, or Cypress.

However, if you have specific use cases where you want to combine load testing with some basic UI interactions, you can use Locust in conjunction with Selenium, which allows you to automate browser interactions. Here’s a basic example of how you can perform simple UI interactions using Locust and Selenium:

from locust import HttpUser, task, between
from selenium import webdriver

class UITestingUser(HttpUser):
wait_time = between(1, 3) # Add a random wait time between tasks

def on_start(self):
    self.driver = webdriver.Chrome()  # Initialize a Selenium WebDriver (you need to install Selenium and ChromeDriver)

def on_stop(self):
    self.driver.quit()  # Close the browser when the user stops

def perform_ui_actions(self):
    self.driver.get("")  # Replace with your website URL

    # Simulate UI interactions (e.g., clicking buttons, filling forms) using Selenium
    element = self.driver.find_element_by_css_selector("button")

    # You can add more UI interactions as needed

    # Optionally, you can assert that certain UI elements are present or have specific properties
    assert "Example Domain" in self.driver.title

In this example:

  1. We import the necessary modules from Locust and Selenium.
  2. We define a UITestingUser class that inherits from HttpUser. We also define the on_start and on_stop methods to initialize and close the Selenium WebDriver.
  3. Inside the perform_ui_actions task, we use Selenium to perform UI interactions on a web page. Replace "" with the URL of the website you want to test.
  4. You can add more UI interactions within the perform_ui_actions task as needed.


Locust is a popular open-source load testing tool that comes with its own set of advantages and disadvantages. Here are the pros and cons of using Locust:

  1. Python-Based: Locust is written in Python, a widely used and accessible programming language. This makes it easy for developers and testers with Python experience to create and customize load tests.
  2. Distributed Testing: Locust supports distributed load testing, allowing you to scale your tests across multiple machines and simulate a large number of virtual users for more realistic testing scenarios.
  3. Flexible and Extensible: You can define complex test scenarios and behaviors using Python code, making it highly flexible and customizable. Locust also provides support for using external libraries and modules.
  4. Realistic Scenarios: Locust allows you to write test scenarios that closely resemble real-world user interactions, including handling user sessions, cookies, and HTTP sessions.
  5. Interactive Web UI: Locust comes with an interactive web user interface that provides real-time monitoring and reporting of test results, making it easy to analyze and identify performance bottlenecks.
  6. Active Community: There is an active and growing community around Locust, providing support, plugins, and extensions to enhance its functionality.
  7. Open Source: Being open-source, Locust is free to use and can be customized to suit your specific testing needs.
  1. Limited Protocols: Locust primarily focuses on HTTP-based load testing. While it can be used for other protocols, it may not be as feature-rich or specialized as tools dedicated to those protocols.
  2. Learning Curve: For users new to Python or load testing, there can be a learning curve when creating complex test scenarios.
  3. Less Out-of-the-Box Features: Locust is intentionally lightweight, which means it may lack some advanced features and integrations that other commercial load testing tools offer. You may need to write custom code or use external tools to supplement its functionality.
  4. Resource Intensive: Running a large number of virtual users or complex scenarios can be resource-intensive, which might require more powerful hardware or distributed testing setups.
  5. Reporting and Analysis: While Locust provides real-time reporting through its web interface, more extensive and customizable reporting and analysis might require additional tools or scripting.
  6. Limited Protocol Support: Although Locust can be extended to test various protocols, its primary focus is HTTP. Testing protocols other than HTTP might require additional effort and customization.

In summary, Locust is a powerful and flexible load testing tool, particularly well-suited for web applications and HTTP-based APIs. It offers the advantage of being open-source and highly customizable, but it may require more effort and expertise to set up and use effectively compared to some commercial load testing solutions. Your choice of whether to use Locust should depend on your specific testing needs, team expertise, and the nature of the applications you are testing.

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