Build a Site Connectivity Checker in Python

Set Up the Development Environment

Before you start coding a new project, you should do some preparation. In Python, you typically start by creating a virtual environment for the project. A virtual environment provides an isolated Python interpreter and a space to install your project’s dependencies.

To kick things off, go ahead and create the project’s root directory, called rpchecker_project/. Then move to this directory and run the following commands on your system’s command line or terminal:

• Windows
• Linux + macOS

PS> python -m venv venv
PS> venv\Scripts\activate
(venv) PS>

$ python -m venv venv
$ source venv/bin/activate
(venv) $

The first command creates a fully functional Python virtual environment called venv inside the project’s root directory, while the second command activates the environment. Now run the following command to install the project’s dependencies with pip, the standard Python package manager:

(venv) $ python -m pip install aiohttp

With this command, you install aiohttp into your virtual environment. You’ll use this third-party library along with Python’s async features to handle asynchronous HTTP requests in your site connectivity checker app.

Cool! You have a working Python virtual environment with all the dependencies that you’ll need to start building your project. Now you can create the project’s layout to organize your code following Python best practices.

Organize Your Site Connectivity Checker Project

Python is surprisingly flexible when it comes to structuring applications, so you may find pretty different structures from project to project. However, small installable Python projects typically have a single package, which is often named after the project itself.

Following this practice, you can organize your site connectivity checker app using the following directory structure:

rpchecker_project/
│
├── rpchecker/
│   ├── __init__.py
│   ├── __main__.py
│   ├── checker.py
│   └── cli.py
│
├── README.md
└── requirements.txt

You can use any name for this project and its main package. In this tutorial, the project will be named rpchecker as a combination of Real Python (rp) and checker, which points out the app’s main functionality.

The README.md file will contain the project’s description and instructions for installing and running the application. Adding a README.md file to your projects is a best practice in programming, especially if you’re planning to release the project as an open source solution. To learn more about writing good README.md files, check out How to write a great README for your GitHub project.

The requirements.txt file will hold the list of your project’s external dependencies. In this case, you only need the aiohttp library, because the rest of the tools and modules that you’ll use are available out of the box in the Python standard library. You can use this file to automatically reproduce the appropriate Python virtual environment for your app using pip, the standard package manager.

Inside the rpchecker/ directory, you’ll have the following files:

• __init__.py enables rpchecker/ as a Python package.
• __main__.py works as an entry-point script for the app.
• checker.py provides the application’s core functionalities.
• cli.py contains the command-line interface for the application.

Now go ahead and create all these files as empty files. You can do this by using your favorite code editor or IDE. Once you finish creating the project’s layout, then you can start coding the app’s main functionality: checking if a website is online or not.

Implement a Connectivity Checker Function

There are several Python tools and libraries that you can use to check if a website is online at a given time. For example, a popular option is the requests third-party library, which allows you to perform HTTP requests using a human-readable API.

However, using requests has the drawback of installing an external library just to use a minimal part of its functionality. It’d be more efficient to find an appropriate tool in the Python standard library.

With a quick look at the standard library, you’ll find the urllib package, which provides several modules for handling HTTP requests. For example, to check if a website is online, you can use the urlopen() function from the urllib.request module:

>>> from urllib.request import urlopen

>>> response = urlopen("https://python.org")
>>> response.read()
b'<!doctype html>\n<!--[if lt IE 7]>
    ...

The urlopen() function takes a URL and opens it, returning its content as a string or a Request object. But you just need to check if the website is online, so downloading the entire page would be wasteful. You need something more efficient.

What about a tool that gives you lower-level control over your HTTP request? That’s where the http.client module comes in. This module provides the HTTPConnection class, representing a connection to a given HTTP server.

HTTPConnection has a .request() method that allows you to perform HTTP requests using the different HTTP methods. For this project, you can use the HEAD HTTP method to ask for a response containing only the headers of the target website. This option will reduce the amount of data to download, making your connectivity checker app more efficient.

At this point, you have a clear idea of the tool to use. Now you can do some quick tests. Go ahead and run the following code in a Python interactive session:

>>> from http.client import HTTPConnection

>>> connection = HTTPConnection("pypi.org", port=80, timeout=10)
>>> connection.request("HEAD", "/")

>>> response = connection.getresponse()
>>> response.getheaders()
[('Server', 'Varnish'), ..., ('X-Permitted-Cross-Domain-Policies', 'none')]

In this example, you first create an HTTPConnection instance targeting the pypi.org website. The connection uses port 80, which is the default HTTP port. Finally, the timeout argument provides the number of seconds to wait before timing out connection attempts.

Then you perform a HEAD request on the site’s root path, "/", using .request(). To get the actual response from the server, you call .getresponse() on the connection object. Finally, you inspect the response’s headers by calling .getheaders().

Your website connectivity checker just needs to create a connection and make a HEAD request. If the request is successful, then the target website is online. Otherwise, the site is offline. In the latter case, it’d be appropriate to display an error message to the user.

Now go ahead and open the checker.py file in your code editor. Then add the following code to it:

 1# checker.py
 2
 3from http.client import HTTPConnection
 4from urllib.parse import urlparse
 5
 6def site_is_online(url, timeout=2):
 7    """Return True if the target URL is online.
 8
 9    Raise an exception otherwise.
10    """
11    error = Exception("unknown error")
12    parser = urlparse(url)
13    host = parser.netloc or parser.path.split("/")[0]
14    for port in (80, 443):
15        connection = HTTPConnection(host=host, port=port, timeout=timeout)
16        try:
17            connection.request("HEAD", "/")
18            return True
19        except Exception as e:
20            error = e
21        finally:
22            connection.close()
23    raise error

Here’s a breakdown of what this code does line by line:

• Line 3 imports HTTPConnection from http.client. You’ll use this class to establish a connection with the target website and handle HTTP requests.

• Line 4 imports urlparse() from urllib.parse. This function will help you parse the target URLs.

• Line 6 defines site_is_online(), which takes a url and a timeout argument. The url argument will hold a string representing a website’s URL. Meanwhile, timeout will hold the number of seconds to wait before timing out connection attempts.

• Line 11 defines a generic Exception instance as a placeholder.

• Line 12 defines a parser variable containing the result of parsing the target URL using urlparse().

• Line 13 uses the or operator to extract the hostname from the target URL.

• Line 14 starts a for loop over the HTTP and HTTPS ports. This way, you can check if the website is available on either port.

• Line 15 creates an HTTPConnection instance using host, port, and timeout as arguments.

• Lines 16 to 22 define a try … except … finally statement. The try block attempts to make a HEAD request to the target website by calling .request(). If the request succeeds, then the function returns True. If an exception occurs, then the except block keeps a reference to that exception in error. The finally block closes the connection to free the acquired resources.

• Line 23 raises the exception stored in error if the loop finishes without a successful request.

Your site_is_online() function returns True if the target website is available online. Otherwise, it raises an exception pointing out the problem it encountered. This latter behavior is convenient because you need to show an informative error message when the site isn’t online. Now it’s time to try out your new function.

Run Your First Connectivity Checks

To try out your site_is_online() function, go ahead and get back to your interactive session. Then run the following code:

>>> from rpchecker.checker import site_is_online

>>> site_is_online("python.org")
True

>>> site_is_online("non-existing-site.org")
Traceback (most recent call last):
    ...
socket.gaierror: [Errno -2] Name or service not known

In this code snippet, you first import site_is_online() from the checker module. Then you call the function with "python.org" as an argument. Because the function returns True, you know that the target site is online.

In the final example, you call site_is_online() with a non-existing website as a target URL. In this case, the function raises an exception that you can later catch and process to display an error message to the user.

Great! You’ve implemented the application’s main functionality of checking a website’s connectivity. Now you can continue with your project by setting up its CLI.

Parse Website URLs at the Command Line

To build the application’s CLI with argparse, you need to create an ArgumentParser instance so that you can parse arguments provided at the command line. Once you have an argument parser, then you can start adding arguments and options to your app’s CLI.

Now go ahead and open the cli.py file in your code editor. Then add the following code:

# cli.py

import argparse

def read_user_cli_args():
    """Handle the CLI arguments and options."""
    parser = argparse.ArgumentParser(
        prog="rpchecker", description="check the availability of websites"
    )
    parser.add_argument(
        "-u",
        "--urls",
        metavar="URLs",
        nargs="+",
        type=str,
        default=[],
        help="enter one or more website URLs",
    )
    return parser.parse_args()

In this code snippet, you create read_user_cli_args() to keep the functionality related to the argument parser in a single place. To build the parser object, you use two arguments:

• prog defines the program’s name.
• description provides a suitable description for the application. This description will be displayed when you call the app with the --help option.

After creating the argument parser, you add a first command-line argument using .add_argument(). This argument will allow the user to enter one or more URLs at the command line. It’ll use the -u and --urls switches.

The rest of the arguments to .add_argument() work as follows:

• metavar sets a name for the argument in usage or help messages.
• nargs tells argparse to accept a list of command-line arguments after the -u or --urls switch.
• type sets the data type of the command-line arguments, which is str in this argument.
• default sets the command-line argument to an empty list by default.
• help provides a help message for the user.

Finally, your function returns the result of calling .parse_args() on the parser object. This method returns a Namespace object containing the parsed arguments.

Load Website URLs From a File

Another valuable option to implement in your site connectivity checker is the ability to load a list of URLs from a text file on your local machine. To do this, you can add a second command-line argument with the -f and --input-file flags.

Go ahead and update read_user_cli_args() with the following code:

# cli.py
# ...

def read_user_cli_args():
    # ...
    parser.add_argument(
        "-f",
        "--input-file",
        metavar="FILE",
        type=str,
        default="",
        help="read URLs from a file",
    )
    return parser.parse_args()

To create this new command-line argument, you use .add_argument() with almost the same arguments as in the section above. In this case, you aren’t using the nargs argument, because you want the application to accept only one input file at the command line.

Display the Check Results

An essential component of every application that interacts with the user through the command line is the application’s output. Your application needs to show the result of its operations to the user. This feature is vital for ensuring a pleasant user experience.

Your site connectivity checker doesn’t need a very complex output. It just needs to inform the user about the current status of the checked websites. To implement this functionality, you’ll code a function called display_check_result().

Now get back to the cli.py file and add the function at the end:

# cli.py
# ...

def display_check_result(result, url, error=""):
    """Display the result of a connectivity check."""
    print(f'The status of "{url}" is:', end=" ")
    if result:
        print('"Online!" 👍')
    else:
        print(f'"Offline?" 👎 \n  Error: "{error}"')

This function takes the connectivity check result, the checked URL, and an optional error message. The conditional statement tests to see if result is true, in which case an "Online!" message is printed to the screen. If result is false, then the else clause prints "Offline?" along with an error report about the actual problem that has just occurred.

That’s it! Your website connectivity checker has a command-line interface to allow the user to interact with the application. Now it’s time to put everything together in the application’s entry-point script.

Create the Application’s Entry-Point Script

The next step in building your website connectivity checker app is to define the entry-point script with a suitable main() function. To do this, you’ll use the __main__.py file that lives in the rpchecker package. Including a __main__.py file in a Python package enables you to run the package as an executable program using the command python -m <package_name>.

To start populating __main__.py with code, go ahead and open the file in your code editor. Then add the following:

 1# __main__.py
 2
 3import sys
 4
 5from rpchecker.cli import read_user_cli_args
 6
 7def main():
 8    """Run RP Checker."""
 9    user_args = read_user_cli_args()
10    urls = _get_websites_urls(user_args)
11    if not urls:
12        print("Error: no URLs to check", file=sys.stderr)
13        sys.exit(1)
14    _synchronous_check(urls)

After importing read_user_cli_args() from the cli module, you define the app’s main() function. Inside main(), you’ll find a few lines of code that don’t work yet. Here’s what this code should do after you provide the missing functionality:

• Line 9 calls read_user_cli_args() to parse the command-line arguments. The resulting Namespace object is then stored in the user_args local variable.

• Line 10 puts together a list of target URLs by calling a helper function named _get_websites_urls(). You’ll be coding this function in a moment.

• Line 11 defines an if statement to check if the list of URLs is empty. If that’s the case, then the if block prints an error message to the user and exits the application.

• Line 14 invokes a function called _synchronous_check(), which takes the list of target URLs as an argument and runs the connectivity check over each URL. As the name points out, this function will run the connectivity checks synchronously, or one after the other. Again, you’ll be coding this function in a moment.

With main() in place, you can start coding the missing pieces to make it work correctly. In the following sections, you implement _get_websites_urls() and _synchronous_check(). Once they’re ready to go, you’ll be able to run your website connectivity checker app for the first time.

Build the List of Target Website URLs

Your site connectivity checker app will be able to check multiple URLs in every execution. Users will feed URLs into the app by listing them at the command line, providing them in a text file, or both. To create the internal list of target URLs, the app will first process URLs provided at the command line. Then it’ll add additional URLs from a file, if any.

Here’s the code that accomplishes these tasks and returns a list of target URLs that combines both sources, the command line and an optional text file:

 1# __main__.py
 2
 3import pathlib
 4import sys
 5
 6from rpchecker.cli import read_user_cli_args
 7
 8def main():
 9    # ...
10
11def _get_websites_urls(user_args):
12    urls = user_args.urls
13    if user_args.input_file:
14        urls += _read_urls_from_file(user_args.input_file)
15    return urls
16
17def _read_urls_from_file(file):
18    file_path = pathlib.Path(file)
19    if file_path.is_file():
20        with file_path.open() as urls_file:
21            urls = [url.strip() for url in urls_file]
22            if urls:
23                return urls
24            print(f"Error: empty input file, {file}", file=sys.stderr)
25    else:
26        print("Error: input file not found", file=sys.stderr)
27    return []

The first update in this code snippet is to import pathlib to manage the path to the optional URLs file. The second update is to add the _get_websites_urls() helper function, which does the following:

• Line 12 defines urls, which initially stores the list of URLs provided at the command line. Note that if the user doesn’t supply any URLs, then urls will store an empty list.

• Line 13 defines a conditional that checks if the user has provided a URLs file. If so, then the if block augments the list of target URLs resulting from calling _read_urls_from_file() with the file provided in the user_args.input_file command-line argument.

• Line 15 returns the resulting list of URLs.

At the same time, _read_urls_from_file() runs the following actions:

• Line 18 turns the file argument into a pathlib.Path object to facilitate further processing.

• Line 19 defines a conditional statement that checks if the current file is an actual file in the local file system. To perform this check, the conditional calls .is_file() on the Path object. Then the if block opens the file and reads its content using a list comprehension. This comprehension strips any possible leading and ending whitespace from every line in the file to prevent processing errors later.

• Line 22 defines a nested conditional to check if any URL has been gathered. If so, then line 23 returns the resulting list of URLs. Otherwise, line 24 prints an error message to inform the reader that the input file is empty.

The else clause on lines 25 to 26 prints an error message to point out that the input file doesn’t exit. If the function runs without returning a valid list of URLs, it returns an empty list.

Wow! That was a lot, but you made it to the end! Now you can continue with the final part of __main__.py. In other words, you can implement the _synchronous_check() function so that the app can perform connectivity checks on multiple websites.

Check the Connectivity of Multiple Websites

To run connectivity checks over multiple websites, you need to iterate through the list of target URLs, do the checks, and display the corresponding results. That’s what the _synchronous_check() function below does:

 1# __main__.py
 2
 3import pathlib
 4import sys
 5
 6from rpchecker.checker import site_is_online
 7from rpchecker.cli import display_check_result, read_user_cli_args
 8
 9# ...
10
11def _synchronous_check(urls):
12    for url in urls:
13        error = ""
14        try:
15            result = site_is_online(url)
16        except Exception as e:
17            result = False
18            error = str(e)
19        display_check_result(result, url, error)
20
21if __name__ == "__main__":
22    main()

In this piece of code, you first update your imports by adding site_is_online() and display_check_result(). Then you define _synchronous_check(), which takes a list of URLs as arguments. The function’s body works like this:

• Line 12 starts a for loop that iterates over the target URLs.

• Line 13 defines and initializes error which will hold the message that will be displayed if the app doesn’t get a response from the target website.

• Lines 14 to 18 define a try … except statement that catches any exception that may occur during the connectivity checks. These checks run on line 15, which calls site_is_online() with the target URL as an argument. Then lines 17 and 18 update the result and error variables if a connection problem happens.

• Line 19 finally calls display_check_result() with appropriate arguments to display the connectivity check result to the screen.

To wrap up the __main__.py file, you add the typical Python if __name__ == "__main__": boilerplate code. This snippet calls main() when the module is run as a script or executable program. With these updates, your application is now ready for a test flight!

Run Connectivity Checks From the Command Line

You’ve written a ton of code without having the chance to see it in action. You’ve coded the site connectivity checker’s CLI and its entry-point script. Now it’s time to give your application a try. Before doing that, make sure you’ve downloaded the bonus material mentioned at the beginning of this step, especially the sample-urls.txt file.

Now get back to your command line and execute the following commands:

$ python -m rpchecker -h
python -m rpchecker -h
usage: rpchecker [-h] [-u URLs [URLs ...]] [-f FILE] [-a]

check the availability of web sites

options:
  -h, --help            show this help message and exit
  -u URLs [URLs ...], --urls URLs [URLs ...]
                        enter one or more website URLs
  -f FILE, --input-file FILE
                        read URLs from a file

$ python -m rpchecker -u python.org pypi.org peps.python.org
The status of "python.org" is: "Online!" 👍
The status of "pypi.org" is: "Online!" 👍
The status of "peps.python.org" is: "Online!" 👍

$ python -m rpchecker --urls non-existing-site.org
The status of "non-existing-site.org" is: "Offline?" 👎
  Error: "[Errno -2] Name or service not known"

$ cat sample-urls.txt
python.org
pypi.org
docs.python.org
peps.python.org

$ python -m rpchecker -f sample-urls.txt
The status of "python.org" is: "Online!" 👍
The status of "pypi.org" is: "Online!" 👍
The status of "docs.python.org" is: "Online!" 👍
The status of "peps.python.org" is: "Online!" 👍

Your website connectivity checker works great! When you run rpchecker with the -h or --help option, you get a usage message that explains how to use the app.

The application can take several URLs at the command line or from a text file and check them for connectivity. If an error occurs during the check, then you get a message on the screen with information about what’s causing the error.

Go ahead a try some other URLs and features. For example, try to combine URLs at the command line with URLs from a file using the -u and -f switches. Additionally, check what happens when you provide a URLs file that’s empty or nonexistent.

Cool! Your site connectivity checker app works nicely and smoothly, doesn’t it? However, it has a hidden issue. The execution time can be overwhelming when you run the application with a long list of target URLs, because all the connectivity checks run synchronously.

To work around this issue and improve the application’s performance, you can implement asynchronous connectivity checks. That’s what you’ll do in the following section.

Implement an Asynchronous Connectivity Checker Function

The first step in the process of making your website connectivity checker work concurrently is to write an async function that allows you to perform a single connectivity check on a given website. This will be the asynchronous equivalent to your site_is_online() function.

Get back to the checker.py file and add the following code:

 1# checker.py
 2
 3import asyncio
 4from http.client import HTTPConnection
 5from urllib.parse import urlparse
 6
 7import aiohttp
 8# ...
 9
10async def site_is_online_async(url, timeout=2):
11    """Return True if the target URL is online.
12
13    Raise an exception otherwise.
14    """
15    error = Exception("unknown error")
16    parser = urlparse(url)
17    host = parser.netloc or parser.path.split("/")[0]
18    for scheme in ("http", "https"):
19        target_url = scheme + "://" + host
20        async with aiohttp.ClientSession() as session:
21            try:
22                await session.head(target_url, timeout=timeout)
23                return True
24            except asyncio.exceptions.TimeoutError:
25                error = Exception("timed out")
26            except Exception as e:
27                error = e
28    raise error

In this update, you first add the required imports, asyncio and aiohttp. Then you define site_is_online_async() on line 10. It’s an async function that takes two arguments: the URL to check and the number of seconds before the requests time out. The function’s body does the following:

• Line 15 defines a generic Exception instance as a placeholder.

• Line 16 defines a parser variable containing the result of parsing the target URL using urlparse().

• Line 17 uses the or operator to extract the hostname from the target URL.

• Line 18 defines a for loop over the HTTP and HTTPS schemes. This will allow you to check if the website is available on either one.

• Line 19 builds a URL using the current scheme and the hostname.

• Line 20 defines an async with statement to handle an aiohttp.ClientSession instance. This class is the recommended interface for making HTTP requests with aiohttp.

• Lines 21 to 27 define a try … except statement. The try block performs and awaits a HEAD request to the target website by calling .head() on the session object. If the request succeeds, then the function returns True. The first except clause catches TimeoutError exceptions and sets error to a new Exception instance. The second except clause catches any other exceptions and updates the error variable accordingly.

• Line 28 raises the exception stored in error if the loop finishes without a successful request.

This implementation of site_is_online_async() is similar to the implementation of site_is_online(). It returns True if the target website is online. Otherwise, it raises an exception pointing out the encountered problem.

The main difference between these functions is that site_is_online_async() performs the HTTP requests asynchronously using the aiohttp third-party library. This distiction can help you optimize your app’s performance when you have a long list of websites to check.

With this function in place, you can proceed to update your application’s CLI with a new option that allows you to run the connectivity checks asynchronously.

Add an Asynchronous Option to the Application’s CLI

Now you need to add an option to your site connectivity checker app’s CLI. This new option will tell the app to run the checks asynchronously. The option can just be a Boolean flag. To implement this type of option, you can use the action argument of .add_argument().

Now go ahead and update read_user_cli_args() on the cli.py file with the following code:

# cli.py
# ...

def read_user_cli_args():
    """Handles the CLI user interactions."""
    # ...
    parser.add_argument(
        "-a",
        "--asynchronous",
        action="store_true",
        help="run the connectivity check asynchronously",
    )
    return parser.parse_args()

# ...

This call to .add_argument() on the parser object adds a new -a or --asynchronous option to the application’s CLI. The action argument is set to "store_true", which tells argparse that -a and --asynchronous are Boolean flags that will store True when provided at the command line.

With this new option in place, it’s time to write the logic for checking the connectivity of multiple websites asynchronously.

Check the Connectivity of Multiple Websites Asynchronously

To check the connectivity of multiple websites asynchronously, you’ll write an async function that calls and awaits site_is_online_async() from the checker module. Get back to the __main__.py file and add the following code to it:

 1# __main__.py
 2import asyncio
 3import pathlib
 4import sys
 5
 6from rpchecker.checker import site_is_online, site_is_online_async
 7from rpchecker.cli import display_check_result, read_user_cli_args
 8# ...
 9
10async def _asynchronous_check(urls):
11    async def _check(url):
12        error = ""
13        try:
14            result = await site_is_online_async(url)
15        except Exception as e:
16            result = False
17            error = str(e)
18        display_check_result(result, url, error)
19
20    await asyncio.gather(*(_check(url) for url in urls))
21
22def _synchronous_check(urls):
23    # ...

In this piece of code, you first update your imports to access site_is_online_async(). Then you define _asynchronous_check() on line 10 as an asynchronous function using the async keyword. This function takes a list of URLs and checks their connectivity asynchronously. Here’s how it does that:

• Line 11 defines an inner async function called _check(). This function allows you to reuse the code that checks a single URL for connectivity.

• Line 12 defines and initializes a placeholder error variable, which will be used in the call to display_check_result() later.

• Lines 13 to 17 define a try … except statement to wrap the connectivity check. The try block calls and awaits site_is_online_async() with the target URL as an argument. If the call succeeds, then result ends up being True. If the call raises an exception, then result will be False, and error will hold the resulting error message.

• Line 18 calls display_check_result() using result, url, and error as arguments. This call displays information about the website’s availability.

• Line 20 calls and awaits the gather() function from the asyncio module. This function runs a list of awaitable objects concurrently and returns an aggregated list of resulting values if all the awaitable objects complete successfully. To provide the list of awaitable objects, you use a generator expression that calls _check() for each target URL.

Okay! You’re almost ready to try out the asynchronous capabilities of your site connectivity checker app. Before doing that, you need to take care of a final detail: updating the main() function to integrate this new feature.

Add Asynchronous Checks to the App’s Main Code

To add the asynchronous functionality to your application’s main() function, you’ll use a conditional statement to check if the user provided the -a or --asynchronous flag at the command line. This conditional will allow you to run the connectivity checks with the right tool according to the user’s input.

Go ahead and open the __main__.py file again. Then update main() like in the following code snippet:

 1# __main__.py
 2# ...
 3
 4def main():
 5    """Run RP Checker."""
 6    user_args = read_user_cli_args()
 7    urls = _get_websites_urls(user_args)
 8    if not urls:
 9        print("Error: no URLs to check", file=sys.stderr)
10        sys.exit(1)
11
12    if user_args.asynchronous:
13        asyncio.run(_asynchronous_check(urls))
14    else:
15        _synchronous_check(urls)
16
17# ...

The conditional statement on lines 12 to 15 checks if the user has provided the -a or --asynchronous flag at the command line. If that’s the case, main() runs the connectivity checks asynchronously using asyncio.run(). Otherwise, it runs the checks synchronously using _synchronous_check().

That’s it! You can now test this new feature of your website connectivity checker in practice. Get back to your command line and run the following:

$ python -m rpchecker -h
usage: rpchecker [-h] [-u URLs [URLs ...]] [-f FILE] [-a]

check the availability of web sites

options:
  -h, --help            show this help message and exit
  -u URLs [URLs ...], --urls URLs [URLs ...]
                        enter one or more website URLs
  -f FILE, --input-file FILE
                        read URLs from a file
  -a, --asynchronous    run the connectivity check asynchronously

$ # Synchronous execution
$ python -m rpchecker -u python.org pypi.org docs.python.org
The status of "python.org" is: "Online!" 👍
The status of "pypi.org" is: "Online!" 👍
The status of "docs.python.org" is: "Online!" 👍

$ # Asynchronous execution
$ python -m rpchecker -u python.org pypi.org docs.python.org -a
The status of "pypi.org" is: "Online!" 👍
The status of "docs.python.org" is: "Online!" 👍
The status of "python.org" is: "Online!" 👍

The first command shows that your application now has a new -a or --asynchronous option that will run the connectivity checks asynchronously.

The second command makes rpchecker run the connectivity checks synchronously, just like you did in the previous section. This is because you don’t provide the -a or --asynchronous flags. Note that the URLs are checked in the same order that they’re entered at the command line.

Finally, in the third command, you use the -a flag at the end of the line. This flag makes rpchecker run the connectivity checks concurrently. Now the check results aren’t displayed in the same order as the URLs are entered but in the order in which the responses come from the target websites.

As an exercise, you can try to run your site connectivity checker application with a long list of target URLs and compare the execution time when the app runs the checks synchronously and asynchronously.