Asyncore Example Basic HTTP Client

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Today we’re going to take a closer look into one of my favorite topics: Asyncore. If you haven’t heard about it before, let me give you a quick rundown Asyncore is an old-school library for handling network I/O in Python that allows us to write non-blocking servers and clients using the event loop pattern.

Now, if you’re like me, you might be thinking “Whoa there, buddy! Non-blocking servers? That sounds fancy!” And you’d be right it is pretty cool. But don’t worry, we won’t get too deep into the technical details today. Instead, let’s take a look at an example that will hopefully make things clearer:

# Import the asyncore module
import asyncore

# Create a class for the HTTP client, inheriting from the asyncore.dispatcher class
class HTTPClient(asyncore.dispatcher):
    # Initialize the class with the host and path parameters
    def __init__(self, host, path):
        # Call the __init__ method of the parent class
        super().__init__()
        # Create a socket for the client
        self.create_socket()
        # Connect to the specified host and port 80
        self.connect((host, 80))
        # Create a buffer with the HTTP request, using the host and path parameters
        self.buffer = bytes('GET {} HTTP/1.0\r\nHost: {}\r\n\r\n'.format(path, host), 'ascii')

    # Define a method to handle the connection
    def handle_connect(self):
        # Do nothing for now
        pass

    # Define a method to handle the closing of the connection
    def handle_close(self):
        # Close the connection
        self.close()

    # Define a method to handle reading data from the server
    def handle_read(self):
        # Print the received data
        print(self.recv(8192))

    # Define a method to check if the client is ready to write data
    def writable(self):
        # Return True if the buffer is not empty
        return len(self.buffer) > 0

    # Define a method to handle writing data to the server
    def handle_write(self):
        # Send the data in the buffer to the server
        sent = self.send(self.buffer)
        # Update the buffer to remove the sent data
        self.buffer = self.buffer[sent:]

# Create an instance of the HTTPClient class, specifying the host and path
client = HTTPClient('www.python.org', '/')
# Start the asyncore loop to handle the client's events
asyncore.loop()

This is a basic HTTP client that uses Asyncore to handle the network I/O. Let’s break it down:

1. We import `asyncore`.
2. We define our custom class, `HTTPClient`, which inherits from `asyncore.dispatcher`. This allows us to use the event loop pattern and handle events like connect, read, write, etc.
3. In the constructor of `HTTPClient`, we initialize the socket using `self.create_socket()` and then call `connect((host, 80))` to establish a connection with our server (in this case, www.python.org). We also set up our buffer for sending data later on.
4. The `handle_connect` method is called when the socket has been successfully connected. In this example, we don’t do anything special here just pass it along to the parent class.
5. Similarly, in `handle_close`, we call the superclass method and then close our own socket using `self.close()`.
6. The `handle_read` method is called when data has been received from the server. In this case, we just print it out to the console.
7. The `writable` method checks if there’s any data in our buffer that needs to be sent. If so, we call `send(self.buffer)`, which sends the data and returns the number of bytes that were actually sent (which is stored in `sent`). We then update our buffer by slicing it from the beginning up until the end of what was just sent using `self.buffer = self.buffer[sent:]`.
8. Finally, we create an instance of our custom class and call `asyncore.loop()`, which starts the event loop and allows us to handle events like connect, read, write, etc.

And that’s it! With this basic example, you should be able to see how Asyncore can help us write non-blocking servers and clients using the event loop pattern in Python.

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