Handling requests

weppy provides several instruments to help you dealing with requests in your application. Let's see them.

The request object

When a request comes from a client, weppy binds useful informations about it within the request object, which can be accessed just with an import:

from weppy import request

It contains useful information about the current processing request, in particular:

attribute description
scheme could be http or https
method the request HTTP method
now a Python datetime object created with request
client the IP Address of the client doing the request (if available)
cookies the cookies passed with the request
env contains environment variables like HTTP headers and WSGI parameters
isajax boolean which states if the request was made in AJAX (check for xmlhttprequest presence in headers)

Please keep in mind that the now attribute uses the UTC timezone, by default. You can use the local machine's timezone instead:

app.now_reference = "local"

If you need to access both the local time of the request and the UTC time, you can directly access these values from a request using:

# request datetime in UTC timezone
# request datetime in local machine timezione

Now, let's see how to deal with request variables.

Request variables

weppy's request object also provides three important attributes about the active request:

attribute description
query_params contains the URL query parameters
body_params contains parameters passed into the request body
params contains both the query parameters and the body parameters

All three attributes work in the same way, and an example may help you understand their dynamic:

from weppy import App, request

app = App(__name__)

def post(id):
    editor = request.params.editor
    if editor == "markdown":
        # code
    elif editor == "html":
        # code

Now, when a client calls the URL /post/123?editor=markdown, the editor parameter will be mapped into request.params and we can access its value simply calling the parameter name as an attribute.

When the URL doesn't contain the query parameter you're trying to look at, this will be None, so it's completely safe to call it. It wont raise an exception.

Now, what happens when the client does a POST request with the following body on the URL /post/123?editor=markdown?

    "text": "this is an example post",
    "date": "2014-10-15"

Simple: the three request attributes will look like this:

>>> request.params
<sdict {'date': '2014-10-15', 'text': 'this is a sample post', 'editor': 'markdown'}>
>>> request.query_params
<sdict {'editor': 'markdown'}>
>>> request.body_params
<sdict {'date': '2014-10-15', 'text': 'this is a sample post'}>

You can always access the variables you need.

Handlers and Helpers

Quite often, your application will need to perform operations before and after the request is actually processed by weppy using your exposed function.

weppy helps you do this with the Handlers:

from weppy import Handler

class MyHandler(Handler):
    def on_start(self):
        # code
    def on_success(self):
        # code
    def on_failure(self):
        # code
    def on_end(self):
        # code

As you can see, Handler provides methods to run your code before the request is processed by your function (with the on_start method) and after your function were executed. weppy provides different methods for you to use, which will be called based on what happened during your function call. If an exception occurred, weppy will call the on_failure method; otherwise, on_success is called. The on_end method is always called after every request has been processed, after the response has been created and before sending it to the client.

To better understand the usage of all these methods, let's assume we are writing a database handler that will connect to the database when a request arrives, will do a commit or a rollback depending on what happened during the request, and will close the connection after completion:

class DBHandler(Handler):
    def on_start(self):
        # connect to the db
    def on_success(self):
        # commit to the db
    def on_failure(self):
        # rollback the operations
    def on_end(self):
        # close the connection

Now, to register your handler to a function, you just need to write:

@app.route("/url", handlers=[MyHandler()])
def f():

If you need to register your handler to all your application functions, you can omit the handler from the route() decorator, writing instead:

app.common_handlers = [MyHandler()]

A peculiar Handler: the Helper class

Another common scenario you may encounter while building your application is when you need to add the same contents to your exposed functions' outputs, to make them available for the templates.

For example, let's say you have a function that makes your datetimes objects prettier:

>>> prettydate(datetime.now()-timedelta(days=1))
'One day ago'

And you want to use it in your templates:

{{for post in posts:}}
<div class="post">
    <div class="post-date">{{=prettydate(post.date)}}</div>
    <div class="post-content">{{=post.text}}</div>

Instead of adding prettydate to every exposed function, you can do this:

from weppy import Helper

class MyHelper(Helper):
    def prettydate(d):
        # your prettydate code

app.common_helpers = [MyHelper()]

and you can access your prettydate function in every template.

So, basically, the Helper class of weppy adds everything you define inside it (functions and attributes) into your exposed functions' returning dict.

Errors and redirects

Speaking of handling requests, you would like to perform specific actions on errors.

If we look at the given example for the request.params again, what happens when the user calls the URL without passing the editor query parameter?

Maybe you want to redirect the client with a default parameter:

from weppy import redirect, url

def post(id):
    editor = request.params.editor
    if editor == "markdown":
        # code
    elif editor == "html":
        # code
        redirect(url('post', id, params={'editor': 'markdown'}))

which means that, when the editor var is missing, we force the user to markdown.

The redirect function of weppy accepts a string for the URL, and acts like an exception, interrupting the execution of your code.

Maybe, you prefer to show your 404 page:

from weppy import abort

def not_found():
    return app.render_template("404.html")

def post(id):
    editor = request.params.editor
    if editor == "markdown":
        # code
    elif editor == "html":
        # code

That's all it takes.

So you've just learned three handy aspects of weppy:

  • redirect and abort allow you to stop the execution of your code;
  • you can set specific actions for your application to perform when it encounters a particular HTTP error code with app.on_error();
  • you can use app.render_template() to render a specific template without the presence of an exposed function or a specific context.