“I drink my WSGI clear”A barebones introduction to Python Web Programming.

3/14/2011

Who are we?

• Doug Morgan (@dougzor)

• Alex Conrad (@alexconrad)

• Whit Morriss (@whitmo)

What is WSGI?

WSGI

• Web Server Gateway Interface, pronounced ‘whizky’ (duh)

• PEP 333

• A standardized specification to define communication between a web server and your Python application.

What do you need?

• An HTTP server with WSGI support aka a Host (we’ll get into these later).

• A Python file (your app).

How does it work?

• 1) An HTTP request comes in to your server.• 2) The HTTP server calls your Python app with two

arguments: The environment and the “start response” function.

• 3) Your application returns a Response body which is sent back to the client.

• 4) …• 5) Profit!

Example

def application(environ, start_response):# Let’s setup some data to send back to the clientbody = “WSGI is awesome and tastes delicious!”headers = [(“Content-Type”, “text/plain”),

(“Content-Length, str(len(body)))]

# Send response headers back to the clientstart_response(“200 OK”, headers)

# Send the body back to the client to complete the Response

return body

Example

def application(environ, start_response):# Let’s setup some data to send back to the clientbody = “WSGI is awesome and tastes delicious!”headers = [(“Content-Type”, “text/plain”),

(“Content-Length, str(len(body)))]

# Send response headers back to the clientstart_response(“200 OK”, headers)

# Send the body back to the client to complete the Response

return body

Example

def application(environ, start_response):# Let’s setup some data to send back to the clientbody = “WSGI is awesome and tastes delicious!”headers = [(“Content-Type”, “text/plain”),

(“Content-Length, str(len(body)))]

# Send response headers back to the clientstart_response(“200 OK”, headers)

# Send the body back to the client to complete the Response

return body

Example

def application(environ, start_response):# Let’s setup some data to send back to the clientbody = “WSGI is awesome and tastes delicious!”headers = [(“Content-Type”, “text/plain”),

(“Content-Length, str(len(body)))]

# Send response headers back to the clientstart_response(“200 OK”, headers)

# Send the body back to the client to complete the Response

return body

Arguments & Return Value

• environ: A dictionary (or Map if you’re from the Java land) of data about the incoming request, i.e. HTTP Headers, HTTP Host, etc.

{"HTTP_HOST": "surveymonkey.com", “HTTP_PATH”: “/surveys” "REQUEST_METHOD": "GET", …}

• start_response: A Python function supplied by the server that your app calls to send headers back to the client.

start_response("200 OK", headers)

• What your app returns will be the response body.return "<b>WSGI rules</b>"

WSGI Hosts

• Python comes with a built-in one that you can use, wsgiref, batteries included.

• Other WSGI Hosts:Apache mod_wsgiGoogle App EnginePaste (Python)uWsgi (Nginx, Cherokee, Apache)

WSGIRef Example

docs.python.org/library/wsgiref.html

run the following as “python server.py”

from wsgiref.simple_server import make_server

def application(environ, start_response):…. (from previous example)

# Server application on port 8000httpd = make_server(‘’, 8000, application)httpd.serve_forever() # run into process is killed

That’s it!

WSGI on the rocks

WSGI on the Rocks

Libraries providing basic abstraction over raw WSGI providing request and response classes:

• Webob (apt-get install python-webob or easy_install webob)

• Werkzeug

WebOb Example

from webob import Responsefrom webob.dec import wsgify

@wsgifydef hello_world_app(request):

response = Response(“Hello World”)return response

WebOb Example

from webob import Responsefrom webob.dec import wsgify

@wsgifydef hello_world_app(request):

response = Response(“Hello World”)return response

And now for somethingmore useful

WebOb URL Routing

@wsgifydef main_app(request):

path = request.environ[‘PATH_INFO’]if path in app_paths:

return app_paths[path](request)return response.status = 404

def foo_app(request):return Response(“Got into /foo”)

app_paths = {‘/foo’: foo_app, ‘/bar’: bar_app, …}

WebOb URL Routing

@wsgifydef main_app(request):

path = request.environ[‘PATH_INFO’]if path in app_paths:

return app_paths[path](request)return response.status = 404

def foo_app(request):return Response(“Got into /foo”)

app_paths = {‘/foo’: foo_app, ‘/bar’: bar_app, …}

WebOb URL Routing

@wsgifydef main_app(request):

path = request.environ[‘PATH_INFO’]if path in app_paths:

return app_paths[path](request)return response.status = 404

def foo_app(request):return Response(“Got into /foo”)

app_paths = {‘/foo’: foo_app, ‘/bar’: bar_app, …}

WebOb URL Routing

@wsgifydef main_app(request):

path = request.environ[‘PATH_INFO’] # ‘/foo’if path in app_paths:

return app_paths[path](request)return response.status = 404

def foo_app(request):return Response(“Got into /foo”)

app_paths = {‘/foo’: foo_app, ‘/bar’: bar_app, …}

WebOb URL Routing

@wsgifydef main_app(request):

path = request.environ[‘PATH_INFO’]if path in app_paths:

return app_paths[path](request)return response.status = 404

def foo_app(request):return Response(“Got into /foo”)

app_paths = {‘/foo’: foo_app, ‘/bar’: bar_app, …}

WebOb URL Routing

@wsgifydef main_app(request):

path = request.environ[‘PATH_INFO’]if path in app_paths:

return app_paths[path](request)return response.status = 404

def foo_app(request):return Response(“Got into /foo”)

app_paths = {‘/foo’: foo_app, ‘/bar’: bar_app, …}

Middleware

• What we just built, a routing interface, is a ‘middleware’.

• A middleware is a WSGI application that sits between the web server the your main application which can do pre and post processing on the request and response.

• Takes as input, the request object, and the next application to call.

Uppercase Middleware Example

def app(req): response = Response("The quick brown fox jumped over buzz.") return response

@wsgify.middlewaredef upper_middleware(request, app):

response = app(request)response.body = response.body.upper()

return response

wrapper_app = upper_middleware(app)httpd = make_server("", 8000, wrapper_app)

Uppercase Middleware Example

def app(req): response = Response("The quick brown fox jumped over buzz.") return response

@wsgify.middlewaredef upper_middleware(request, app):

response = app(request)response.body = response.body.upper()

return response

wrapper_app = upper_middleware(app)httpd = make_server("", 8000, wrapper_app)

Uppercase Middleware Example

def app(req): response = Response("The quick brown fox jumped over buzz.") return response

@wsgify.middlewaredef upper_middleware(request, app):

response = app(request)response.body = response.body.upper()

return response

wrapper_app = upper_middleware(app)httpd = make_server("", 8000, wrapper_app)

Gzip Middleware Example

def gzip_middleware(request, app):# Don’t want to do any pre-processing so just call the

appresponse = app(request)

# Compress the body and set headerresponse.body = gzip(request.body)response.headers[‘Content-Encoding’] = ‘gzip’

return response

Middleware Considerations

• Two things:• Middlewares are not aware of anything before or after them in

the ‘pipeline.’• Middlewares should be reusable.

• The middleware you write should act more like a filter as opposed to having a lot of business logic. Rule of thumb: if the code is specific to your application (not reusable), it should probably be a library and not a middleware.

That was so easy...

• A cavema... Err anyone can do it!

• There are dozens of web frameworks that are available (most are open-source) for your use which do a lot of the leg work for you.Examples:• Pylons/Pyramid (What we use at SurveyMonkey)• Django• TurboGears• Repoze.bfg• Bottle• Flask

Questions?