Creating a simple web server

What You’ll Build in This Chapter

In this chapter, you will:

• Start a very simple web server with Python and Flask.
• Understand how a Python script listens for web requests.
• See how a URL in the browser is connected to a function in your code.
• Run and test your web server on your own computer.

You’ll need Flask installed (covered earlier in this chapter’s parent section). All examples assume you have a file named app.py and can run it with Python.

The Basic Structure of a Flask Web Server

A minimal Flask web server has three main parts:

1. Import Flask
2. Create the app object
3. Define routes (URLs) and what they return
4. Start the server

Here is the smallest useful Flask server:

from flask import Flask
app = Flask(__name__)  # 1. create the app object
@app.route("/")        # 2. connect URL "/" to this function
def home():
    return "Hello from my first web server!"  # 3. response
if __name__ == "__main__":  # 4. start the server when script is run
    app.run()

Line by line, what’s happening that is specific to web servers:

• Flask(__name__): creates a web application object. This object:
• Knows how to listen for web requests.
• Knows how to send back responses.
• @app.route("/"): tells Flask:
• “When someone visits the root URL (/), run the home() function.”
• return "...": whatever this function returns becomes the HTTP response body (what the browser shows).
• app.run(): starts a development web server on your computer.

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Running the Server and Visiting It in the Browser

Save the previous code as app.py, then in your terminal:

python app.py

You should see output similar to:

 * Serving Flask app 'app'
 * Debug mode: off
 * Running on http://127.0.0.1:5000

Key points:

• http://127.0.0.1:5000 is the address of your web server:
• 127.0.0.1 or localhost means “this computer”.
• :5000 is the port number Flask uses by default.
• Open a browser and go to:
• http://127.0.0.1:5000/
  or
• http://localhost:5000/

You should see:

• A blank page with the text:
  Hello from my first web server!

Behind the scenes:

1. Your browser sends a request to localhost:5000/.
2. Flask receives it and sees that "/" matches the @app.route("/") decorator.
3. Flask calls the home() function.
4. The string it returns is sent back to the browser.

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Adding More Pages (Routes)

A web server can respond to different URLs. Each URL usually has its own function.

You can add more routes like this:

from flask import Flask
app = Flask(__name__)
@app.route("/")
def home():
    return "Welcome to the home page."
@app.route("/about")
def about():
    return "This is the about page."
@app.route("/contact")
def contact():
    return "Contact us at: example@example.com"
if __name__ == "__main__":
    app.run()

Now you can visit:

• http://localhost:5000/ → home page
• http://localhost:5000/about → about page
• http://localhost:5000/contact → contact page

Important details:

• URLs usually do not end with .py or .html here. They are just paths like /about.
• Each @app.route("...") decorator connects a URL path to a Python function.

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Returning Simple HTML from Your Server

So far we are returning plain text, but web pages are usually HTML.

You can return small HTML snippets directly as strings:

from flask import Flask
app = Flask(__name__)
@app.route("/")
def home():
    return """
    <h1>My Simple Web Server</h1>
    <p>This is a basic page served by Flask.</p>
    """
@app.route("/about")
def about():
    return """
    <h1>About</h1>
    <p>This page is also generated by Python!</p>
    """
if __name__ == "__main__":
    app.run()

Notes:

• Triple quotes """ ... """ let you write multi-line strings.
• The browser interprets the string as HTML, not as plain code.

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Changing the Host and Port (Optional but Useful)

By default, app.run():

• Uses host 127.0.0.1 (only your computer).
• Uses port 5000.

You can change these:

if __name__ == "__main__":
    app.run(host="0.0.0.0", port=8000)

• host="0.0.0.0": listen on all network interfaces (useful if you want other devices on your local network to access it).
• port=8000: change the port number. Then you would visit:
  http://localhost:8000/

For practice, just using the default app.run() is enough.

Using Debug Mode While Developing

While learning and testing, it is common to use debug mode:

if __name__ == "__main__":
    app.run(debug=True)

Debug mode:

• Automatically restarts the server when you change your code.
• Shows a more detailed error page in the browser when something goes wrong.

Use debug mode only on your own computer, not on a real public server.

A Tiny Multi-Page Example

Here is a slightly more complete example of a simple web server with multiple pages:

from flask import Flask
app = Flask(__name__)
@app.route("/")
def home():
    return """
    <h1>Welcome!</h1>
    <p>This is the home page of my simple Flask website.</p>
    <p>Try visiting <a href="/hello">/hello</a> or <a href="/about">/about</a>.</p>
    """
@app.route("/hello")
def hello():
    return "<h1>Hello there!</h1><p>This page says hello.</p>"
@app.route("/about")
def about():
    return """
    <h1>About This Site</h1>
    <p>This site is powered by a tiny Python web server using Flask.</p>
    """
if __name__ == "__main__":
    app.run(debug=True)

Steps:

1. Run python app.py.
2. Open your browser at http://localhost:5000/.
3. Click the links to visit the other routes.

You now have a working multi-page web server written in just a few lines of Python.

Common Issues When Starting Your First Server

Here are some typical problems and quick checks:

1. “ModuleNotFoundError: No module named 'flask'”
• Flask is not installed in your Python environment.
• Install it with pip install flask (as covered earlier).
2. Browser says: “This site can’t be reached”
• Is your server running in the terminal?
• Did you close the terminal or stop the server with Ctrl + C?
• Are you visiting the right URL (http://localhost:5000/)?
3. Port already in use
• Another process might be using port 5000.
• Run the app on a different port:

     app.run(port=5001)

4. Code changes not showing up
• Did you restart the server after editing app.py?
• Or did you enable debug mode: app.run(debug=True)?

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Summary

In this chapter, you:

• Created a basic Flask web server in a file like app.py.
• Started the server with app.run() and visited it in the browser.
• Mapped URLs to Python functions using @app.route.
• Returned text and simple HTML from your routes.
• Added multiple pages to your simple web server.

You now have the foundation needed to connect this simple server to templates and user input (forms) in the next sections.