code.delx.au - learning/blob - lesson2_dynamic-web.md

   1 # Lesson 2 the dynamic web
   2
   3 ## References:
   4 - https://docs.python.org/3
   5 - https://docs.python.org/3/library/http.html
   6 - https://github.com/python/cpython/blob/3.12/Lib/http/server.py
   7 - https://docs.python.org/3/tutorial
   8 - https://docs.python.org/3/tutorial/inputoutput.html#formatted-string-literals
   9 - https://developer.mozilla.org/en-US/docs/Glossary/Favicon
  10
  11 ## Overview
  12
  13 In the mid to late 1990s the web started becoming dynamic. Web servers might return different content to different users who requested the same URL. A common example of this was a hit counter which would increment each time any user visited the website, or a guest book to allow users to leave messages.
  14
  15 ## Learning objective
  16
  17 The goal is to build a simple dynamic web server.
  18 - Understand how the HTTP method and path map to a Python function call.
  19 - Build a web server that returns dynamically generated content.
  20
  21 ## Exercises
  22
  23 ### Build a simple HTTP server in Python
  24
  25 Create a file, `ex2.py` and paste the following into it:
  26 ```
  27 import http.server
  28
  29 def main():
  30     listen_address = ('localhost', 8000)
  31     request_handler = http.server.SimpleHTTPRequestHandler
  32     server = http.server.HTTPServer(listen_address, request_handler)
  33     server.serve_forever()
  34
  35 if __name__ == '__main__':
  36     main()
  37 ```
  38
  39 Run it with `python3 ex2.py`. It should work exactly the same as `python3 -mhttp.server` from the previous exercise.
  40
  41
  42 ### Return HTML from a function instead of a file
  43
  44 ```
  45 import http.server
  46
  47 def main():
  48     listen_address = ('localhost', 8000)
  49     request_handler = MyRequestHandler
  50     server = http.server.HTTPServer(listen_address, request_handler)
  51     server.serve_forever()
  52
  53 class MyRequestHandler(http.server.BaseHTTPRequestHandler):
  54     def write(self, text):
  55         self.wfile.write(text.encode('utf-8'))
  56
  57     def do_GET(self):
  58         self.send_response(200)
  59         self.send_header('Content-type', 'text/html')
  60         self.end_headers()
  61         self.write('<html>')
  62         self.write('<head><title>My web server!</title></head>')
  63         self.write('<body>Hi there!</body>')
  64         self.write('</html>')
  65
  66 if __name__ == '__main__':
  67     main()
  68 ```
  69
  70
  71 ### Make the HTML dynamic!
  72
  73 Add `COUNTER = 42` to the top of the file.
  74
  75 Then modify your `GET` handler to return some dynamic HTML! Something like this...
  76
  77 ```
  78 global COUNTER
  79 COUNTER = COUNTER + 1
  80 self.write(f'We have had <b>{COUNTER}</b> visitors today')
  81 ```
  82
  83 You can try adding other information to the response too:
  84 ```
  85 self.write('You requested: ' + self.path + '<br>')
  86 self.write('You are using this client: ' + self.headers.get('user-agent') + '<br>')
  87 ```
  88
  89
  90 ### A few things to note
  91
  92 Remember doing raw HTTP requests in the previous exercise? With the code above if a client does `GET /file.txt` then Python's `http.server` library parses the HTTP request and does something like the following:
  93 - Creates a new instance of the `MyRequestHandler` class.
  94 - Sets the HTTP path as: `self.path = '/file.txt'` for this new instance.
  95 - Calls the `do_GET()` function on this new instance, because the HTTP method was `GET`.
  96
  97 ### Fix the double-counting bug
  98
  99 Notice that if you press `ctrl-shift-R` to reload your counter is going up by two at a time? If you look at the log you can see this is because Firefox is requesting `/favicon.ico`. This is the little icon next to the URL in the address bar. Our site isn't fancy enough for this, so we should modify `do_GET()` to return a `404 not found` for these requests.
 100
 101 Something like this:
 102 ```
 103 if not self.path.endswith('.html'):
 104     self.send_response(404)
 105     self.end_headers()
 106     self.write('File not found')
 107     return
 108 ```
 109
 110 Now try visiting some URL that doesn't end with `.html` and you'll see your 'not found' message.