Introduction to h11

The h11 library is a lightweight, pure-Python, and low-level HTTP/1.1 protocol library designed to be used as a building block for other libraries and frameworks. It is useful for anyone needing to interact with HTTP/1.1 without the overhead of more comprehensive libraries like Requests or urllib.

Key Features of h11

• Lightweight and minimal
• Low-level control over HTTP/1.1 communication
• Written in Python

APIs and Code Snippets

h11 offers several useful APIs. Here’s a brief overview of some of the most important ones, along with code examples.

Connection

The h11.Connection class manages the state of an HTTP/1.1 connection, including sending and receiving data. It operates in one of four states: ‘IDLE’, ‘SEND’, ‘RECV’, or ‘DONE’.

  import h11

  conn = h11.Connection(h11.CLIENT)
  print(conn.states)

Request and Response

To send a request and receive a response, you use the h11.Request and h11.Response message classes, respectively.

  import h11

  conn = h11.Connection(h11.CLIENT)
  request = h11.Request(
    method='GET',
    target='/',
    headers=[('Host', 'example.com'), ('User-Agent', 'h11-example')]
  )
  conn.send(request)

  import h11

  conn = h11.Connection(h11.SERVER)
  response = h11.Response(
    status_code=200,
    headers=[('Content-Type', 'text/plain')],
    reason=b'OK'
  )
  conn.send(response)

Data

The h11.Data class is used to send and receive the actual data payload. Here’s an example:

  import h11

  conn = h11.Connection(h11.CLIENT)
  data = h11.Data(data=b'Hello World')
  conn.send(data)

Event Handling

Handling incoming events is straightforward. Here is an example of handling different types of incoming events:

  import h11

  conn = h11.Connection(h11.SERVER)
  request = conn.next_event()
  if isinstance(request, h11.Request):
      print('Received request:', request)
  elif isinstance(request, h11.EndOfMessage):
      print('End of message')

Example Application

Below is a small example of an HTTP client built using h11:

  import h11
  import socket

  host = 'example.com'
  port = 80

  sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
  sock.connect((host, port))

  conn = h11.Connection(h11.CLIENT)
  request = h11.Request(
    method='GET',
    target='/',
    headers=[('Host', host), ('User-Agent', 'h11-example')]
  )
  sock.send(conn.send(request))
  sock.send(conn.send(h11.EndOfMessage()))

  while True:
      event = conn.next_event()
      if isinstance(event, h11.Response):
          print('Received response:', event.status_code)
      elif isinstance(event, h11.Data):
          print('Received data:', event.data.decode('utf-8'))
      elif isinstance(event, h11.EndOfMessage):
          break

This example demonstrates how to create a simple HTTP client that sends a GET request and processes the response using the h11 library.

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