Integrating Python with web servers was much improved by the creation of PEP 333, which defines the Python Web Server Gateway Interface (WSGI): http://legacy.python.org/dev/peps/pep-0333/.
WSGI introduced a single calling convention that every web server could implement, thereby making that web server instantly compatible with all of the Python web applications and web frameworks that also support WSGI.
At the Python library you can get more information about the wsgiref module.This module provides a variety of utility functions for working with WSGI environments.The wsgiref package, whose simple_server we will use in the example, also contains several utilities for
working with WSGI. It includes functions for examining, further unpacking, and modifying the environ
object; a prebuilt iterator for streaming large files back to the server; and even a validate sub-module
whose routines can check a WSGI application to see whether it complies with the specification when
presented with a series of representative requests.
Developers generally avoid writing raw. WSGI applications because the conveniences of even a
simple web framework make code so much easier to write and maintain. But, for the sake of illustration,
wsgi_app.py shows a small WSGI application whose front page asks the user to type a string. Submitting
the string takes the user to a second web page, where he can see its base64 encoding. From there, a link
will take him back to the first page to repeat the process.
import cgi, base64
from wsgiref.simple_server import make_server
def page(content, *args):
yield '<html><head><title>wsgi_app.py</title></head><body>'
yield content % args
yield '</body>'
def simple_app(environ, start_response):
gohome = '<br><a href="/">Return to the home page</a>'
q = cgi.parse_qs(environ['QUERY_STRING'])
if environ['PATH_INFO'] == '/':
if environ['REQUEST_METHOD'] != 'GET' or environ['QUERY_STRING']:
start_response('400 Bad Request', [('Content-Type', 'text/plain')])
return ['Error: the front page is not a form']
start_response('200 OK', [('Content-Type', 'text/html')])
return page('Welcome! Enter a string: <form action="encode">'
'<input name="mystring"><input type="submit"></form>')
elif environ['PATH_INFO'] == '/encode':
if environ['REQUEST_METHOD'] != 'GET':
start_response('400 Bad Request', [('Content-Type', 'text/plain')])
return ['Error: this form does not support POST parameters']
if 'mystring' not in q or not q['mystring'][0]:
start_response('400 Bad Request', [('Content-Type', 'text/plain')])
return ['Error: this form requires a "mystring" parameter']
my = q['mystring'][0]
start_response('200 OK', [('Content-Type', 'text/html')])
return page('<tt>%s</tt> base64 encoded is: <tt>%s</tt>' + gohome,
cgi.escape(repr(my)), cgi.escape(base64.b64encode(my)))
else:
start_response('404 Not Found', [('Content-Type', 'text/plain')])
return ['That URL is not valid']
print 'Listening on localhost:8000'
make_server('localhost', 8000, simple_app).serve_forever()
import cgi, base64 from wsgiref.simple_server import make_server
def page(content, *args): yield '
def simple_app(environ, start_response):
gohome = '
Return to the home page'
q = cgi.parse_qs(environ['QUERY_STRING'])
if environ['PATH_INFO'] == '/':
if environ['REQUEST_METHOD'] != 'GET' or environ['QUERY_STRING']:
start_response('400 Bad Request', [('Content-Type', 'text/plain')])
return ['Error: the front page is not a form']
start_response('200 OK', [('Content-Type', 'text/html')])
return page('Welcome! Enter a string: <form action="encode">'
'<input name="mystring"><input type="submit"></form>')
elif environ['PATH_INFO'] == '/encode':
if environ['REQUEST_METHOD'] != 'GET':
start_response('400 Bad Request', [('Content-Type', 'text/plain')])
return ['Error: this form does not support POST parameters']
if 'mystring' not in q or not q['mystring'][0]:
start_response('400 Bad Request', [('Content-Type', 'text/plain')])
return ['Error: this form requires a "mystring" parameter']
my = q['mystring'][0]
start_response('200 OK', [('Content-Type', 'text/html')])
return page('<tt>%s</tt> base64 encoded is: <tt>%s</tt>' + gohome,
cgi.escape(repr(my)), cgi.escape(base64.b64encode(my)))
else:
start_response('404 Not Found', [('Content-Type', 'text/plain')])
return ['That URL is not valid']
The first thing to note in this code listing is that two very different objects are being created: a WSGI server that knows how to use HTTP to talk to a web browser and an application written to respond correctly when invoked per the WSGI calling convention. Note that these two pieces—the client and server—could easily be swapped out. This code example should make the calling convention clear enough:
start_response().start_response(). Its headers will be
combined with any headers that the WSGI server might already provide to the
client.