User Guide

Wheezy Web is a lightweight WSGI framework that aims take most benefits out of standard python library and serves a sort of glue with other libraries. It can be run from python 2.4 up to the most cutting edge python 3. The framework aims to alleviate the overhead associated with common activities performed in Web application development.

Wheezy Web framework follows the model–view–controller (MVC) architectural pattern to separate the data model from the user interface. This is considered a good practice as it modularizes code, promotes code reuse.

Wheezy Web framework follows a push-based architecture. Handlers do some processing, and then “push” the data to the template layer to render the results.

Web Handlers

Handler is any callable that accepts an instance of HTTPRequest and returns HTTPResponse:

def handler(request):
    return response

Wheezy Web comes with the following handlers:

  • MethodHandler - represents the most generic handler. It serves dispatcher purpose for HTTP request method (GET, POST, etc). Base class for all handlers.
  • BaseHandler - provides methods that integrates such features as: routing, i18n, model binding, template rendering, authentication, xsrf/resubmission protection.
  • RedirectRouteHandler - redirects to given route name.
  • FileHandler - serves static files out of some directory.
  • TemplateHandler - serves templates that don’t require up front data processing.

MethodHandler

Wheezy Web routes incoming web request to handler per url mapping (it uses wheezy.routing for this purpose):

all_urls = [
    url('', WelcomeHandler, name='default'),
    url('welcome', welcome, name='welcome')
]

You subclass from MethodHandler or BaseHandler and define methods get() or post() that handle HTTP request methods GET or POST.

class WelcomeHandler(BaseHandler):

    def get(self):
        response = HTTPResponse()
        response.write('Hello World!')
        return response

This method must return an HTTPResponse object.

MethodHandler has a number of useful attributes:

  • options - a dictionary of application configuration options.
  • request - an instance of wheezy.http.HTTPRequest.
  • route_args - a dictionary of arguments matched in url routing.
  • cookies - a list of cookies that extend HTTPResponse.

Please note that this handler automatically responds with HTTP status code 405 (method not allowed) in case the requested HTTP method is not overridden in your handler, e.g. there is incoming POST request but your handler does not provide an implementation.

BaseHandler

BaseHandler provides methods that integrates such features as:

  1. routing
  2. AJAX
  3. i18n
  4. model binding
  5. JSON
  6. template rendering
  7. authentication
  8. authorization
  9. xsrf/resubmission protection
  10. context sharing

You need to inherit from this class and define get() and/or post() to be able respond to HTTP requests. This class inherits from MethodHandler, so everything mentioned for MethodHandler applies to BaseHandler as well.

Routing

Routing feature is provided via integration with wheezy.routing package. There are the following methods:

  • path_for(name, **kwargs) - returns url path by route name. Any missing parameters are obtained from that current route.
  • absolute_url_for(name, **kwargs) - returns absolute url for the given route name by combining current request with route information.
  • redirect_for(name, **kwargs) - returns redirect found response (HTTP status code 302) by route name.
  • see_other_for(name, **kwargs) - returns see other redirect response (HTTP status code 303) by route name.

All these methods support the following arguments:

  • name - a name of the route.
  • kwargs - extra arguments necessary for routing.

Please refer to wheezy.routing documentation for more information.

AJAX

Both redirects redirect_for and see_other_for understands AJAX requests and change HTTP status code to 207 while preserving HTTP header Location.

Browsers incorrectly handle redirect response to ajax request, so there is used HTTP status code 207 that javascript is capable to receive and process browser redirect. Here is an example for jQuery (see file core.js):

$.ajax({
    // ...
    success: function(data, textStatus, jqXHR) {
        if (jqXHR.status == 207) {
            window.location.replace(
                jqXHR.getResponseHeader('Location'));
        } else {
            // ...
        }
    }
});

If AJAX response status code is 207, browser navigates to URL specified in HTTP response header Location.

Please refer to wheezy.http documentation for more information.

Internationalization

Internationalization feature is provided via integration with wheezy.core package (module i18n). There are the following attributes:

  • locale - default implementation return a value resolved from route arguments, particularly to name locale.
  • translations - returns TranslationsManager (wheezy.core feature) for the current locale.
  • translation - returns translations for the current handler. Default implementation return NullTranslations object. Your application handler must override this attribute to provide valid gettext translations.

Here is example from template demo application:

class SignInHandler(BaseHandler):

    @attribute
    def translation(self):
        return self.translations['membership']

This code loads membership translations from i18n directory. In order to function properly the following configuration options must be defined:

from wheezy.core.i18n import TranslationsManager

options = {}
options['translations_manager'] = TranslationsManager(
            directories=['i18n'],
            default_lang='en')

See example in public demo application config.py.

Model Binding

Once the html form is submitted, you need a way to bind these values to some domain model, validate, report errors, etc. This is where integration with wheezy.validation package happens.

There are the following attributes and methods:

  • errors - a dictionary where each key corresponds to attribute being validated and value to a list of errors reported.
  • try_update_model(model, values=None) - tries update domain model with values. If values is not specified it is the same as using self.request.form. You can pass here self.request.query or self.route_args.
  • ValidationMixin::validate(model, validator) - shortcut for domain model validation per validator.
  • ValidationMixin::error(message) - adds a general error (this error is added with key __ERROR__).

Here is an example from the template demo application (see file membership/web/views.py):

class SignInHandler(BaseHandler):

    def get(self, credential=None):
        if self.principal:
            return self.redirect_for('default')
        credential = credential or Credential()
        return self.render_response('membership/signin.html',
                self.widgets(credential=credential))

    def post(self):
        credential = Credential()
        if (not self.try_update_model(credential)
            or not self.validate(credential, credential_validator)):
            return self.get(credential)
        return self.redirect_for('default')

On POST this handler updates credential with values from the html form submitted. In case try_update_model or valida fails. we re-display the sign-in page with errors reported.

Here is an example from the template demo application that demonstrates how to use the general error (see file membership/web/views.py):

class SignUpHandler(BaseHandler):

    def post(self):
        if not self.validate_resubmission():
            self.error('Your registration request has been queued. '
                    'Please wait while your request will be processed. '
                    'If your request fails please try again.')
            return self.get()
        ...

Read more about model binding and validation in wheezy.validation package.

JSON

There is integration with wheezy.http package in JSON object encoding.

  • json_response() - returns HTTPResponse with JSON content.

Here is an example:

class SignInHandler(BaseHandler):
    ...
    def post(self):
        ...
        credential = Credential()
        if (not self.try_update_model(credential)
                ...):
            if self.request.ajax:
                return self.json_response({'errors': self.errors})
            return self.get(credential)
        ...
        return self.see_other_for('default')

In case of error in ajax requests, the handler returns JSON object with any errors reported, otherwise it renders response the template. This way you are able to serve both: browsers with javascript enabled or disabled.

See file core.js for an example of how errors are processed by browser.

Templates

Wheezy Web is not tied to some specific template engine, instead it provides you a convinient contract to add one you prefer (see file config.py). Template contract is any callable of the following form:

def render_template(self, template_name, **kwargs):
    return string

There are the following attributes and methods:

  • helpers - a dictionary of context objects to be passed to render_template implementation (you need to override this method in case you need more specific context information in template).
    • _ - gettext translations support.
    • errors - a dictionary with errors reported during validation. Key corresponds to attribute validated and value to a list of errors.
    • handler - an instance of currently executing handler.
    • route_args, absolute_url_for, path_for - relates to routing related methods.
    • principal - an instance of wheezy.security.Principal for the authenticated request or None.
    • resubmission - resubmission HTML form widget.
    • xsrf - XSRF protection HTML form widget.
  • render_template(template_name, widgets=None, **kwargs) - renders template with name template_name and pass it context information in **kwargs.
  • render_response(template_name, widgets=None, **kwargs) - writes result of render_template into wheezy.http.HTTPResponse and return it.

widgets argument in render_template and render_response is used to explicitly wrap HTML widgets (see wheezy.html package). Note, if you are using template engine that comes with widgets preprocessing you do not need to explicitly initialize this argument.

Widgets

Widgets are coming from wheezy.html package. You have to be explicit when you need widgets. Here is SignUpHandler from demo:

class SignUpHandler(BaseHandler):

    ...

    @handler_cache(profile=none_cache_profile)
    def get(self, registration=None):
        registration = registration or Registration()
        return self.render_response('membership/signup.html',
                self.widgets( # ====== Wrapping widgets ======
                    registration=registration,
                    credential=registration.credential,
                    account=registration.account,
                    model=self.model),
                questions=sorted(
                    self.factory.membership.password_questions.items(),
                    key=itemgetter(1)),
                account_types=sorted(
                    self.factory.membership.account_types.items(),
                    key=itemgetter(1)))

In the example above four models (registration, credential, accout and model) are wrapped by self.widgets and two (questions and account_types) are passed as is.

The benefit of using widgets is a syntax sugar in html template.

Mako example:

<p>
    ${account.email.label('Email:')}
    ${account.email.textbox(autocomplete='off')}
    ${account.email.error()}
</p>

Wheezy Template example:

<p>
    @account.account_type.label('Account Type:')
    @account.account_type.radio(choices=account_types)
    @account.account_type.error()
</p>

Please note that wheezy.html package provides optimization of widgets per template engine used. That optimization is provided through use of template specific constructs. Preprocessor for Mako / Jinja2 / Tenjin / Wheezy.Template templates translates widgets to template engine specific operations offering optimal performance.

If you are using template engine with preprocessing you can eliminate need to wrap widgets explicitely in your handler (since all calls to widgets in template are replaced with appropriate template native constructs).

Read more about available widgets in wheezy.html package.

Authentication

Authentication is a process of confirming the truth of security principal. In a web application it usually relates to creating an encrypted cookie value, which can not easily be compromised by attacker. This is where integration with wheezy.security happens.

The process of creating authentication cookie is as simple as assiging instance of wheezy.security.Principal to attribute principal. Let’s demonstrate this by example:

from wheezy.security import Principal

class SignInHandler(BaseHandler):

    def post(self):
        ...
        self.principal = Principal(
            id=credential.username,
            alias=credential.username)
        ...

Once we confirmed user has entered valid username and password we create an instance of Principal and assign it to principal attribute. In setprincipal implementation authentication cookie is created with a dump of Principal object and it value is protected by wheezy.security.crypto.Ticket (read more in wheezy.security).

Here are authentication configuration options (see file config.py):

# wheezy.security.crypto.Ticket
options = {}
options.update({
        'CRYPTO_ENCRYPTION_KEY': '4oqiKhW3qzP2EiattMt7',
        'CRYPTO_VALIDATION_KEY': 'A7GfjxIBCBA3vNqvafWf'
})

options.update({
        'ticket': Ticket(
            max_age=1200,
            salt='JNbCog95cDTo1NRb7inP',
            options=options),

        'AUTH_COOKIE': '_a',
        'AUTH_COOKIE_DOMAIN': None,
        'AUTH_COOKIE_PATH': '',
        'AUTH_COOKIE_SECURE': False,
})

You can obtain current security Principal by requesting principal attribute. The example below redirects user to default route in case he or she is already authenticated:

class SignInHandler(BaseHandler):

    def get(self, credential=None):
        if self.principal:
            return self.redirect_for('default')
        ...

Sign out is even simpler, just delete principal attribute:

class SignOutHandler(BaseHandler):

    def get(self):
        del self.principal
        return self.redirect_for('default')

Authorization

Authorization specify access rights to resources and provide access control in particular to your application.

You are able to request authorization by decorating your handler method with authorize():

from wheezy.web import authorize

class MembersOnlyHandler(BaseHandler):

    @authorize
    def get(self, registration=None):
        return response

There is also a way to demand specific role:

class BusinessOnlyHandler(BaseHandler):

    @authorize(roles=('business',))
    def get(self, registration=None):
        return response

In case there are multiple roles specified in authorize() decorator than first match grant access. That means user is required to be at least in one role to pass this guard.

authorize() decorator return http status code 401 (Unauthorized). It is recommended to use HTTPErrorMiddleware to route 401 status code to signin page. Read more in HTTPErrorMiddleware section.

@secure

Decorator secure accepts only secure requests (those that are communication via SSL) and if incoming request is not secure, issue permanent redirect to HTTPS location:

class MyHandler(BaseHandler):
    @secure
    def get(self):
        ...
        return response

The behavior can be controlled via enabled (in case it is False no checks performed, defaults to True).

XSRF/Resubmission

Cross-site request forgery (CSRF or XSRF), also known as a one-click attack is a type of malicious exploit of a website whereby unauthorized commands are transmitted from a user that the website trusts. Logging out of sites and avoiding their “remember me” features can mitigate CSRF risk.

Forms that can be accidentally, or maliciously submitted multiple times can cause undesired behavior and/or result in your application. Resubmits can happen for many reasons, mainly through page refresh, browser back button and incident multiple button clicks.

Regardless a source of issue you need to be aware it happening.

Wheezy Web has built-in XSRF and resubmission protection. Configuration options let you customize name used:

options = {}
options.update({
        'XSRF_NAME': '_x',
        'RESUBMISSION_NAME': '_c'
})

You need include XSRF and/or resubmission widget into your form. Each template has context functions xsrf() and resubmission() for this purpose:

<form method="post">
    ${xsrf()}
    ...
</form>

Validation happens in handler, here is how it implemented in membership/web/views.py:

class SignInHandler(BaseHandler):

    def get(self, credential=None):
        if not self.validate_xsrf_token():
            return self.redirect_for(self.route_args.route_name)
        ...

If XSRF token is invalid we redisplay the same page. Or we can show user an error message, here is use case for resubmission check:

class SignUpHandler(BaseHandler):

    def post(self):
        if not self.validate_resubmission():
            self.error('Your registration request has been queued. '
                    'Please wait while your request will be processed. '
                    'If your request fails please try again.')
            return self.get()
        ...

Since there is no simple rule of thumb when to use which protection and how to react in case it happening, it still strongly recommended take into account such situations during application development and provide unified application wide behavior.

Context

BaseHandler holds a number of useful features that other application layers (e.g. service layer, business logic) can benefit from.

There context attribute is available for this purpose. It is a dictionary that extends options with the following information: errors, locale, principal and translations.

Here is example from the template demo application (see membership/web/views.py):

class SignInHandler(BaseHandler):

    @attribute
    def factory(self):
        return Factory(self.context)

Context is passed to service factory.

Redirect Handler

RedirectRouteHandler redirects to a given route name (HTTP status code 302). You can use redirect_handler() in url mapping declaration:

all_urls = [
    url('', redirect_handler('welcome'), name='default'),
    ...
]

The example above always performs a redirect match for route default to route welcome. It asks browser to redirect it request to another page.

Permanent Redirect

PermanentRedirectRouteHandler performs a permanent redirect (HTTP status code 301) to the given route name. You can use permanent_redirect_handler() in the url mapping declaration:

all_urls = [
    url('', permanent_redirect_handler('welcome'), name='default'),
    ...
]

The example above results in a permanent redirect for route default to route welcome.

FileHandler

FileHandler serves static files out of some directory. You can use file_handler() in url mapping declaration:

all_urls = [
    url('static/{path:any}', file_handler(
            root='content/static/',
            age=timedelta(hours=1)), name='static'),
    ...
]

file_handler() accepts the following arguments:

  • root - a root path of directory that holds static files, e.g. .css, .js, .jpg, etc. It is recommended that this directory be isolated from any other part of the application.
  • age - controls http browser cache policy period.

Request Headers

FileHandler handles both GET and HEAD browser requests, provides Last-Modified and ETag HTTP response headers, as well as understands If-Modified-Since and If-None-Match request headers, as sent by browser for static content.

GZip and Caching

It is recommended to use file_handler() together with gzip_transform and response_cache (requries HTTP cache middleware).

Here is example from template demo application:

from wheezy.http import response_cache
from wheezy.http.transforms import gzip_transform
from wheezy.http.transforms import response_transforms
from wheezy.web.handlers import file_handler


static_files = response_cache(static_cache_profile)(
    response_transforms(gzip_transform(compress_level=6))(
        file_handler(
            root='content/static/',
            age=timedelta(hours=1))))

all_urls = [
    url('static/{path:any}', static_files, name='static'),
    ...
]

Templates

Path for static files is provided by standard wheezy.routing path_for(name, **kwargs) function:

path_for('static', path='core.js')

TemplateHandler

TemplateHandler serves templates that do not require up front data processing. This mostly relates to some static pages, e.g. about, help, error, etc.

You can use template_handler() in the url mapping declaration:

from wheezy.web.handlers import template_handler

public_urls = [
    url('about', template_handler('public/about.html'), name='about'),
]

template_handler() supports the following arguments:

  • template_name - template name used to render response.
  • status_code - HTTP status code to set in response. Defaults to 200.

Middleware

Wheezy Web extends middleware provided by wheezy.http by adding the following:

  • bootstrap defaults
  • path routing middleware
  • http error middleware

Bootstrap Defaults

bootstrap_defaults() middleware factory does not provide any middleware, instead it is used to check application options and provide defaults.

The following options are checked:

  • path_router - if it is not defined already an instance of wheezy.routing.PathRouter is created. Argument url_mapping is passed to PathRouter.add_routes method.
  • render_template - defaults to an instance of wheezy.web.templates.MakoTemplate.
  • translations_manager - defaults to an instance of wheezy.core.i18n.TranslationsManager.
  • ticket - defaults to an instance of wheezy.security.crypto.Ticket.

PathRoutingMiddleware

PathRoutingMiddleware provides integration with wheezy.routing package. It is added to WSGIApplication via path_routing_middleware_factory().

options = {}
main = WSGIApplication(
    middleware=[
        bootstrap_defaults(url_mapping=all_urls),
        path_routing_middleware_factory
    ],

This factory requires path_router to be available in application options.

HTTPErrorMiddleware

HTTPErrorMiddleware provides a custom error page in case http status code is above 400 (HTTP status codes from 400 and up relates to client error, 500 and up - server error). This middleware is initialized with error_mapping dictionary, where key corresponds to HTTP status code and value to route name. In case of a status code match it redirects incoming request to route per error_mapping.

HTTPErrorMiddleware can be added to WSGIApplication via http_error_middleware_factory():

main = WSGIApplication(
    middleware=[
        bootstrap_defaults(url_mapping=all_urls),
        http_cache_middleware_factory,
        http_error_middleware_factory,
        path_routing_middleware_factory
    ],
    options=options)

The following configuration options available:

from wheezy.core.collections import defaultdict

options = {}
options['http_errors'] = defaultdict(lambda: 'http500', {
            # HTTP status code: route name
            400: 'http400',
            401: 'signin',
            403: 'http403',
            404: 'http404',
            500: 'http500',
        }),
})

defaultdict is used to provide default route name if there is no match in http_errors dictionary. All routes defined in http_errors must exist. These checks occur in http_error_middleware_factory().

Transforms

Transforms are a way to manipulate handler response accordingly to some algorithm. Wheezy Web provides decorator handler_transforms() to adapt transforms available in wheezy.http to web handlers sub-classed from BaseHandler:

from wheezy.http.transforms import gzip_transform
from wheezy.web.handlers import BaseHandler
from wheezy.web.transforms import handler_transforms

class MyHandler(BaseHandler):

    @handler_transforms(gzip_transform(compress_level=9))
    def get(self):
        return response

Please refer to wheezy.http documentation for more information.

Templates

Wheezy Web does not provide its own implementation for template rendering instead it offers integration with the following packages:

Contract

Template contract is any callable of the following form:

def render_template(self, template_name, **kwargs):
    return 'unicode string'

Jinja2 Templates

Here is the configuration option to define that Jinja2 templates are rendered within the application (see config.py for details):

from jinja2 import Environment
from jinja2 import FileSystemLoader
from wheezy.html.ext.jinja2 import WidgetExtension
from wheezy.html.ext.jinja2 import WhitespaceExtension
from wheezy.html.utils import format_value
from wheezy.web.templates import Jinja2Template

env = Environment(
    loader=FileSystemLoader('content/templates'),
    auto_reload=False,
    extensions=[
        WidgetExtension,
        WhitespaceExtension
    ])
env.globals.update({
    'format_value': format_value,
})
render_template = Jinja2Template(env)

The arguments passed to Environment are specific to Jinja2 templates and not explained here. Please refer to Jinja2 documentation.

Mako Templates

Here is the configuration option to define that Mako templates are rendered within application (see config.py for details):

from wheezy.html.ext.mako import whitespace_preprocessor
from wheezy.html.ext.mako import widget_preprocessor
from wheezy.web.templates import MakoTemplate


render_template = MakoTemplate(
        module_directory='/tmp/mako_modules',
        filesystem_checks=False,
        directories=['content/templates'],
        cache_factory=cache_factory,
        preprocessor=[
            widget_preprocessor,
            whitespace_preprocessor,
        ])

The arguments passed to MakoTemplate are specific to Mako templates and not explained here. Please refer to Mako documentation.

Tenjin Templates

Here is configuration option to define that Tenjin templates are rendered within application (see config.py for details):

from wheezy.html.ext.tenjin import whitespace_preprocessor
from wheezy.html.ext.tenjin import widget_preprocessor
from wheezy.html.utils import format_value
from wheezy.web.templates import TenjinTemplate

render_template = TenjinTemplate(
        path=['content/templates'],
        pp=[
            widget_preprocessor,
            whitespace_preprocessor,
        ],
        helpers={
            'format_value': format_value
        })

The arguments passed to TenjinTemplate are specific to Tenjin templates and not explained here. Please refer to Tenjin documentation.

Wheezy Template

Here is configuration option to define that Wheezy.Template templates are rendered within application (see config.py for details):

from wheezy.html.ext.template import WhitespaceExtension
from wheezy.html.ext.template import WidgetExtension
from wheezy.html.utils import format_value
from wheezy.html.utils import html_escape
from wheezy.template.engine import Engine
from wheezy.template.ext.core import CoreExtension
from wheezy.template.loader import FileLoader
from wheezy.web.templates import WheezyTemplate

searchpath = ['content/templates-wheezy']
engine = Engine(
        loader=FileLoader(searchpath),
        extensions=[
            CoreExtension(),
            WhitespaceExtension(),
            WidgetExtension(),
])
engine.global_vars.update({
    'format_value': format_value,
    'h': html_escape,
})
render_template = WheezyTemplate(engine)

The arguments passed to Engine are specific to Wheezy.Template and not explained here. Please refer to Wheezy.Template documentation.

Caching

Wheezy Web provides decorator handler_cache() to adapt cache interface available in wheezy.http to web handlers sub-classed from BaseHandler:

from wheezy.http import CacheProfile
from wheezy.web.handlers import BaseHandler
from wheezy.web.caching import handler_cache

none_cache_profile = CacheProfile(
        'none',
        no_store=True,
        enabled=True)

class MyHandler(BaseHandler):

    @handler_cache(profile=none_cache_profile)
    def get(self, credential=None):
        return response

Please refer to wheezy.http documentation for more information. All features available in wheezy.http caching are applicable.

Content caching plus cache dependency is the most advanced boost of your application performance. Regardless of template engine this can give up to 8-10 times better performance.