Creating an app

From scratch

Apps can be created using the dashboard or directly from the filesystem. Here, we are going to do it manually, as the Dashboard is already described in its own chapter.

Keep in mind that an app is a Python module; therefore it needs only a folder and a file in that folder.


An empty file is not strictly needed since Python 3.3, but it will be useful later on.

Open a command prompt and go to your main py4web folder. Enter the following simple commands in order to create a new empty myapp app:

mkdir apps/myapp
echo '' > apps/myapp/


for Windows, you must use backslashes (i.e. \) instead of slashes.

If you now restart py4web or press the “Reload Apps” in the Dashboard, py4web will find this module, import it, and recognize it as an app, simply because of its location. By default py4web runs in lazy watch mode (see the run command option) for automatic reloading of the apps whenever it changes, which is very useful in a development environment. In production or debugging environment, it’s better to run py4web with a command like this:

py4web run apps --watch off

A py4web app is not required to do anything. It could just be a container for static files or arbitrary code that other apps may want to import and access. Yet typically most apps are designed to expose static or dynamic web pages.

Static web pages

To expose static web pages you simply need to create a static subfolder, and any file in there will be automatically published:

mkdir apps/myapp/static
echo 'Hello World' > apps/myapp/static/hello.txt

The newly created file will be accessible at


Notice that static is a special path for py4web and only files under the static folder are served.

Important: internally py4web uses the ombott (One More BOTTle) <>`__, It supports streaming, partial content, range requests, and if-modified-since. This is all handled automatically based on the HTTP request headers.

Dynamic Web Pages

To create a dynamic page, you must create a function that returns the page content. For example edit the myapp/ as follows:

import datetime
from py4web import action

def page():
    return "hello, now is %s" %

Reload the app, and this page will be accessible at




(notice that index is optional)

Unlike other frameworks, we do not import or start the webserver within the myapp code. This is because py4web is already running, and it may be serving multiple apps. py4web imports our code and exposes functions decorated with @action(). Also notice that py4web prepends /myapp (i.e. the name of the app) to the url path declared in the action. This is because there are multiple apps, and they may define conflicting routes. Prepending the name of the app removes the ambiguity. But there is one exception: if you call your app _default, or if you create a symlink from _default to myapp, then py4web will not prepend any prefix to the routes defined inside the app.

On return values

py4web actions should return a string or a dictionary. If they return a dictionary you must tell py4web what to do with it. By default py4web will serialize it into json. For example edit again and add at the end

def colors():
    return {'colors': ['red', 'blue', 'green']}

This page will be visible at


and returns a JSON object {"colors": ["red", "blue", "green"]}. Notice we chose to name the function the same as the route. This is not required, but it is a convention that we will often follow.

You can use any template language to turn your data into a string. PY4WEB comes with yatl, a full chapter will be dedicated later and we will provide an example shortly.


It is possible to map patterns in the URL into arguments of the function. For example:

def color(name):
    if name in ['red', 'blue', 'green']:
        return 'You picked color %s' % name
    return 'Unknown color %s' % name

This page will be visible at


The syntax of the patterns is the same as the Bottle routes. A route wildcard can be defined as

  • <name> or

  • <name:filter> or

  • <name:filter:config>

And these are possible filters (only :re has a config):

  • :int matches (signed) digits and converts the value to integer.

  • :float similar to :int but for decimal numbers.

  • :path matches all characters including the slash character in a non-greedy way, and may be used to match more than one path segment.

  • :re[:exp] allows you to specify a custom regular expression in the config field. The matched value is not modified.

The pattern matching the wildcard is passed to the function under the specified variable name.

Also, the action decorator takes an optional method argument that can be an HTTP method or a list of methods:

@action('index', method=['GET','POST','DELETE'])

You can use multiple decorators to expose the same function under multiple routes.

The request object

From py4web you can import request

from py4web import request

def paint():
    if 'color' in request.query:
       return 'Painting in %s' % request.query.get('color')
    return 'You did not specify a color'

This action can be accessed at:


Notice that the request object is equivalent to a Bottle request object. with one additional attribute:


Which you can use the code to identify the name and the folder used for the app.


In order to use a yatl template you must declare it. For example create a file apps/myapp/templates/paint.html that contains:

      body {background:[[=color]]}
    <h1>Color [[=color]]</h1>

then modify the paint action to use the template and default to green.

def paint():
    return dict(color = request.query.get('color', 'green'))

The page will now display the color name on a background of the corresponding color.

The key ingredient here is the decorator @action.uses(...). The arguments of action.uses are called fixtures. You can specify multiple fixtures in one decorator or you can have multiple decorators. Fixtures are objects that modify the behavior of the action, that may need to be initialized per request, that may filter input and output of the action, and that may depend on each-other (they are similar in scope to Bottle plugins but they are declared per-action, and they have a dependency tree which will be explained later).

The simplest type of fixture is a template. You specify it by simply giving the name of the file to be used as template. That file must follow the yatl syntax and must be located in the templates folder of the app. The object returned by the action will be processed by the template and turned into a string.

You can easily define fixtures for other template languages. This is described later.

Some built-in fixtures are:

  • the DAL object (which tells py4web to obtain a database connection from the pool at every request, and commit on success or rollback on failure)

  • the Session object (which tells py4web to parse the cookie and retrieve a session at every request, and to save it if changed)

  • the Translator object (which tells py4web to process the accept-language header and determine optimal internationalization/pluralization rules)

  • the Auth object (which tells py4web that the app needs access to the user info)

They may depend on each other. For example, the Session may need the DAL (database connection), and Auth may need both. Dependencies are handled automatically.

The _scaffold app

Most of the times, you do not want to start writing code from scratch. You also want to follow some sane conventions outlined here, like not putting all your code into PY4WEB provides a Scaffolding (_scaffold) app, where files are organized properly and many useful objects are pre-defined. Also, it shows you how to manage users and their registration. Just like a real scaffolding in a building construction site, scaffolding could give you some kind of a fast and simplified structure for your project, on which you can rely to build your real project.


You will normally find the scaffold app under apps, but you can easily create a new clone of it manually or using the Dashboard.

Here is the tree structure of the _scaffold app:


The scaffold app contains an example of a more complex action:

from py4web import action, request, response, abort, redirect, URL
from yatl.helpers import A
from . common import db, session, T, cache, auth

@action('welcome', method='GET')
@action.uses('generic.html', session, db, T, auth.user)
def index():
    user = auth.get_user()
    message = T('Hello {first_name}'.format(**user))
    return dict(message=message, user=user)

Notice the following:

  • request, response, abort are defined by which is a fast bottlepy spin-off.

  • redirect and URL are similar to their web2py counterparts

  • helpers (A, DIV, SPAN, IMG, etc) must be imported from yatl.helpers . They work pretty much as in web2py

  • db, session, T, cache, auth are Fixtures. They must be defined in

  • @action.uses(auth.user) indicates that this action expects a valid logged-in user retrievable by auth.get_user(). If that is not the case, this action redirects to the login page (defined also in and using the Vue.js auth.html component).

When you start from scaffold, you may want to edit, templates, and but probably you don’t need to change anything in

In your html, you can use any JS library that you want because py4web is agnostic to your choice of JS and CSS, but with some exceptions. The auth.html which handles registration/login/etc. uses a vue.js component. Hence if you want to use that, you should not remove it.

Copying the _scaffold app

The scaffold app is really useful, and you will surely use it a lot as a starting point for testing and even developing full features new apps.

It’s better not to work directly on it: always create new apps copying it. You can do it in two ways:

  • using the command line: copy the whole apps/_scaffold folder to another one (apps/my_app for example). Then reload py4web and it will be automatically loaded.

  • using the Dashboard: select the button Create/Upload App under the “Installed Applications” upper section. Just give the new app a name and check that “Scaffold” is selected as the source. Finally press the Create button and the dashboard will be automatically reloaded, along with the new app.


Watch for files change

As described in the run command option, Py4web facilitates a development server’s setup by automatically reloads an app when its Python source files change (by default). But in fact any other files inside an app can be watched by setting a handler function using the @app_watch_handler decorator.

Two examples of this usage are reported now. Do not worry if you don’t fully understand them: the key point here is that even non-python code could be reloaded automatically if you explicit it with the @app_watch_handler decorator.

Watch SASS files and compile them when edited:

from py4web.core import app_watch_handler
import sass #

def sass_compile(changed_files):
    print(changed_files) # for info, files that changed, from a list of watched files above
    ## ...
    compiled_css = sass.compile(filename=filep, include_paths=includes, output_style="compressed")
    dest = os.path.join(app, "static/css/all.css")
    with open(dest, "w") as file:

Validate javascript syntax when edited:

import esprima # Python implementation of Esprima from Node.js

def validate_js(changed_files):
    for cf in changed_files:
        print("JS syntax validation: ", cf)
        with open(os.path.abspath(cf)) as code:

Filepaths passed to @app_watch_handler decorator must be relative to an app. Python files (i.e. “*.py”) in a list passed to the decorator are ignored since they are watched by default. Handler function’s parameter is a list of filepaths that were changed. All exceptions inside handlers are printed in terminal.