Assigning a Method in WSGI/Python - wsgi

Question

Hi I'm going through a Werkzeug tutorial and I'm a little confused about the statement: app.wsgi_app = SharedDataMiddleware(app.wsgi_app, {
'/static': os.path.join(os.path.dirname(__file__), 'static')
})
Contained inside:
class Shortly(object):
def __init__(self, config):
self.redis = redis.Redis(config['redis_host'], config['redis_port'])
def dispatch_request(self, request):
return Response('Hello World!')
def wsgi_app(self, environ, start_response):
request = Request(environ)
response = self.dispatch_request(request)
return response(environ, start_response)
def __call__(self, environ, start_response):
return self.wsgi_app(environ, start_response)
def create_app(redis_host='localhost', redis_port=6379, with_static=True):
app = Shortly({
'redis_host': redis_host,
'redis_port': redis_port
})
if with_static:
app.wsgi_app = SharedDataMiddleware(app.wsgi_app, {
'/static': os.path.join(os.path.dirname(__file__), 'static')
})
return app
What's going on there? Why is app.wsgi_app being assigned to whatever SharedDataMiddleware is returning? I mean, isn't app.wsgi_app just a method name?
Any insight will be appreciated :) Thanks!

Answer 1

TL;DR In this example an extra functionality is added to the Shortly app by wrapping its entry point with a SharedDataMiddleware object.
Long explanation
app.wsgi_app is not a method name. It's rather a reference to the method. And since Python is pretty dynamic language, one can replace any object's methods after this object was created.
Let's consider a simple example:
class App:
def run(self, param):
print(param)
app = App()
app.run('foo') # prints 'foo'
original_run = app.run # save a reference to the original run method
def run_ex(param):
print('run_ex') # add our extra functionality
original_run(param) # call the original one
app.run = run_ex
app.run('foo') # prints 'run_ex' and then 'foo'
So, in such way one can easy replace app's method run with a regular function.
Now, let's go back to Werkzeug. As you know from the WSGI specification, an application must be a callable with a specific signature. And Shortly's wsgi_app method is the desired callable. However, somebody wants to add an extra functionality to the Shortly app via SharedDataMiddleware middleware. If you look closer to it, you'll see, that it's a regular class with a __call__ method. Hence, all objects of SharedDataMiddleware class can be called.
middleware = SharedDataMiddleware(...) # create an object
middleware(...) # call the object. It can be slightly confusing at a first glance
So, for now you can see that any objects of SharedDataMiddleware is a WSGI applications by itself. And since you already know how to replace app's main method wsgi_app, the only one thing you have to do - is to store a reference to the original method Shortly.wsgi_app to be able to call it later. It's done by passing it to the SharedDataMiddleware constructor.