Understanding Python WSGI with Examples
Coming from a strong PHP background, initially, exploring the web development landscape whilst delving into Python seemed rather confusing. As Python was not originally developed for the web, a specification called PEP 333 was accepted, which standardised the required interface between web servers and Python web frameworks/applications. Despite the additional complexity, the manner in which middleware applications can be integrated, along with the server choices, offers possibilities that I find hard to locate a comparable alternative for in PHP.
Basic Example
Simply put, a WSGI (Web Server Gateway Interface) compliant application must supply a callable (function or class) which accepts an ’environ’ dictionary and a start_response function.
For a familiar PHP comparison, you can think of the ’environ’ dictionary as a combination of $_SERVER, $_GET and $_POST, albeit with extra processing required.
This callable is expected to invoke the start_response function with the desired response code and header data, and then return an iterable of bytes containing the response body.
def app(environ, start_response):
start_response('200 OK', [('Content-Type', 'text/html')])
return [b'Hello, world!']
if __name__ == '__main__':
try:
from wsgiref.simple_server import make_server
httpd = make_server('', 8080, app)
print('Serving on port 8080...')
httpd.serve_forever()
except KeyboardInterrupt:
print('Goodbye.')
Using the single-threaded WSGI reference implementation provided with Python is a great choice for experimenting with these lower-level concepts. You will notice that as the example is written for Python 3, we must return an iterable (in this case a list) with declared ‘byte’ content inside.
Post Example
Now that we are familiar with the basic structure of a WSGI compliant application, we can experiment with a more practical example. Below, we provide the client with a simple form which posts a supplied ’name’ for the application to greet accordingly.
import cgi
form = b'''
<html>
<head>
<title>Hello User!</title>
</head>
<body>
<form method="post">
<label>Hello</label>
<input type="text" name="name">
<input type="submit" value="Go">
</form>
</body>
</html>
'''
def app(environ, start_response):
html = form
if environ['REQUEST_METHOD'] == 'POST':
post_env = environ.copy()
post_env['QUERY_STRING'] = ''
post = cgi.FieldStorage(
fp=environ['wsgi.input'],
environ=post_env,
keep_blank_values=True
)
html = b'Hello, ' + post['name'].value + '!'
start_response('200 OK', [('Content-Type', 'text/html')])
return [html]
if __name__ == '__main__':
try:
from wsgiref.simple_server import make_server
httpd = make_server('', 8080, app)
print('Serving on port 8080...')
httpd.serve_forever()
except KeyboardInterrupt:
print('Goodbye.')
Although somewhat verbose, we have been able to create a simple web application which handles supplied POST data using the CGI module’s FieldStorage class.
These are the very simplified building blocks used in popular frameworks such as Flask and Django.
