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# DjanJinja v0.8

DjanJinja: the sound you make when you've got peanut butter stuck to the roof of your mouth. Incidentally, it also happens to be the name of a new re-usable Django app. This one, in fact.

DjanJinja exists to help you leverage the power of [Jinja2](http://jinja.pocoo.org/2/) templates in your Django projects. It's simple to get started.

## Installing and Using DjanJinja

### Installing DjanJinja

  1. Install DjanJinja using easy_install djanjinja, pip install djanjinja, or by grabbing a copy of the Mercurial repo and running python setup.py install.
  2. Add 'djanjinja' to your INSTALLED_APPS list.
  3. (Optionally) add 'djanjinja.middleware.RequestContextMiddleware' to your MIDDLEWARE_CLASSES list.

It’s assumed that you have fairly recent versions of Jinja2 and Django installed. The author recommends the most recent stable versions of each, which should be installable via PyPI (i.e. a simple easy_install or pip install). A lot of people have their own ways of installing things, so the author hasn’t put any explicit requirements in the setup.py file. DjanJinja just expects to find jinja2 and django on the import path.

### Using DjanJinja

  • Instead of using django.shortcuts.render_to_response, use one of the Jinja2-based functions provided.
  • Instead of using the Django loaders to load templates, get them from the Jinja2 environment created for you by DjanJinja.
  • Instead of using Django’s provided generic views, use those contained within djanjinja.generic (at the moment the only one is direct_to_template()).

## Shortcut Functions

DjanJinja provides you with two shortcut functions for rendering templates, render_to_response and render_to_string. These are very similar to those provided by Django in the django.shortcuts module, except they use Jinja2 instead of the Django templating system. To use them from your views, just do from djanjinja.views import render_to_response, render_to_string at the top of your views module.

## Bundles

A Jinja2 environment can contain additional filters, tests and global variables which will be available in all templates rendered through that environment. Since individual Django apps will have their own set of things to add to the environment, DjanJinja adds the concept of ‘bundles’; small objects containing some global variables, filters and tests. Each app may define any number of these bundles which can then be loaded as required.

### Defining Bundles

It’s relatively easy to define a bundle; an example is shown below:

from djanjinja.loader import Bundle

foo = Bundle() foo.globals['myvar'] = 12345

@foo.envfilter def myenvfilter(environment, value):

pass # do something here...

@foo.ctxfunction def mycontextfunction(context, value):

pass # do something here...

Here we define a bundle called foo, with a global variable of myvar containing the value 12345, an environment filter and a context function (for more information on each of these please consult the Jinja2 documentation). The Bundle class also supplies these handy decorators (the full list can be found as djanjinja.loader.Bundle.TYPES) to define various components.

DjanJinja expects to find bundles in a bundles submodule of your Django app. You can lay things out in one of two ways:

  • Add a file called bundles.py to your app, and within this define multiple Bundle instances.
  • Add a package called bundles to your app (i.e. a bundles directory containing an empty file called __init__.py), and within this define submodules for each of your bundles. Each submodule should have a top-level bundle variable which is an instance of the Bundle class.

You can actually mix and match these; you could add some bundle instances to the bundles/__init__.py file with different names, in addition to having the submodules. These are loaded lazily, so DjanJinja sees no real difference. It doesn’t scour the bundles module for definitions, it just loads what you ask it to.

### Addressing Bundles

In order to use the functions, filters and tests defined in a bundle, you first have to load it into the environment. Bundles are specified in two parts: the ‘app label’ and the ‘bundle name’. The app label is simply the name of the app which contains it. For example, it may be django.contrib.auth, or simply auth, since you may just give the last part of the full name and DjanJinja will figure it out from looking at the INSTALLED_APPS setting.

If a bundle is defined within a bundles.py or a bundles/__init__.py file, then the bundle name will be the name in the module with which it was defined. For example:

# in the file myapp/bundles.py foo = Bundle() foo.globals['myvar'] = 12345

In this case, the app label will be myapp, and the bundle name will be foo. If the bundles are defined in submodules, then the bundle name will be the name of the submodule.

### Loading Bundles

In order to load any bundles into the global Jinja2 environment, you need to specify a DJANJINJA_BUNDLES setting in your settings.py file. This is a list or tuple of bundle specifiers in an 'app_label.bundle_name' format. For example:

DJANJINJA_BUNDLES = (
'djanjinja.cache', 'djanjinja.humanize', 'djanjinja.site',

)

You can also add bundles to the environment programmatically. This is useful when:

  • Your app needs to do some initial setup before a bundle is loaded.
  • Your app relies on a particular bundle being present in the global environment anyway, and you don’t want the user to have to add the bundle to DJANJINJA_BUNDLES manually.
  • Your app needs to load bundles dynamically.
  • Your app wants to use a bundle locally, not globally.

You can load a bundle into an environment like this:

import djanjinja env = djanjinja.get_env() env.load('app_label', 'bundle_name', reload=False)

This will load the bundle into the environment, passing through if it’s already loaded. If you specify reload=True, you can make it reload a bundle even if it’s been loaded.

You should put this code somewhere where it will get executed when you want it to. If you want it to be executed immediately, as Django starts up, put it in myapp/__init__.py.

You can also use a bundle within only one app, by using a local environment copied from the global environment. Here’s how you might do this:

import djanjinja global_env = djanjinja.get_env() local_env = global_env.copy() local_env.load('app_label', 'bundle_name')

You'd then use that local environment later on in your code. For example, the above code might be in myapp/__init__.py; so your views might look like this:

from django.http import HttpResponse from myapp import local_env

def view(request):
template = local_env.get_template('template_name.html') data = template.render({'key1': 'value1', 'key2': 'value2'}) return HttpResponse(content=data)

Of course, you’re probably going to want to use the usual shortcuts (i.e. render_to_response() and render_to_string()). You can build these easily using djanjinja.views.shortcuts_for_environment():

from djanjinja.views import shortcuts_for_environment

render_to_response, render_to_string = shortcuts_for_environment(local_env)

Do this at the top of your views.py file, and then you can use the generated functions throughout all of your views.

### Caveats and Limitations

Jinja2 does not yet support scoped filters and tests; as a result of this, the contents of bundles specified in DJANJINJA_BUNDLES will be loaded into the global environment. It is important to make sure that definitions in your bundle do not override those in another bundle. This is especially important with threaded web applications, as multiple bundles overriding one another could cause unpredictable behavior in the templates.

### Included Bundles

DjanJinja provides three bundles already which either replace Django counterparts or add some useful functionality to your Jinja2 templates:

  • djanjinja.cache: Loading this bundle will add a global cache object to the environment; this is the Django cache, and allows you to carry out caching operations from within your templates (such as cache.get(key), et cetera).
  • djanjinja.humanize: This will add all of the filters contained within the django.contrib.humanize app; consult the official Django docs for more information on the filters provided.
  • djanjinja.site: This will add two functions to the global environment: url, and setting. The former acts like Django’s template tag, by reversing URLconf names and views into URLs, but because Jinja2 supports a richer syntax, it can be used via {{ url(name, *args, **kwargs) }} instead. setting attempts to resolve a setting name into a value, returning an optional default instead (i.e. setting('MEDIA_URL', '/media')).

## Extensions

Jinja2 supports the concept of environment extensions; these are non-trivial plugins which enhance the Jinja2 templating engine itself. By default, the environment is configured with the do statement and the loop controls (i.e. break and continue), but if you want to add extensions to the environment then you can do so with the JINJA_EXTENSIONS setting. Just add this to your settings.py file:

JINJA_EXTENSIONS = (
'jinja2.ext.i18n', # i18n Extension ...

)

For all the extensions you wish to load. This will be passed in directly to the jinja2.Environment constructor.

If you have set USE_I18N = True in your settings file, then DjanJinja will automatically initialize the i18n machinery for the Jinja2 environment, loading your Django translations during the bootstrapping process. For more information on how to use the Jinja2 i18n extension, please consult the Jinja2 documentation.

### Autoescaping

To enable autoescaping, just add JINJA_AUTOESCAPE = True to your settings file. This will add the extension and set up the Jinja2 environment correctly.

### Cache

DjanJinja also provides an extension for fragment caching using the Django cache system. The code for this borrows heavily from the example in the Jinja2 documentation, but with a few extras thrown in. You can use the extension like this:

{% cache (parameter1, param2, param3), timeout %}
...

{% endcache %}

The tuple of parameters is used to generate the cache key for this fragment. You can place any object here, so long as it is suitable for serialization by the standard library marshal module. The cache key for the fragment is generated by marshaling the parameters, hashing them and then using the digest with a prefix as the key. This allows you to specify cached fragments which vary depending on multiple variables. The timeout is optional, and should be given in seconds.

## 404 and 500 Handlers

Your project’s URLconf must specify two variables—handler404 and handler500—which give the name of a Django view to be processed in the event of a 404 "Not Found" and a 500 "Server Error" response respectively. These are set to a default which uses the Django templating system to render a response from templates called 404.html and 500.html. If you were to use the Jinja2 templating system instead, you will be able to define richer error pages, and your error pages will be able to inherit from and extend other Jinja2 master templates on the template path.

It’s relatively simple to set Django up to do this. Simply override the handler variables like so from your urls.py file:

handler404 = 'djanjinja.handlers.page_not_found' handler500 = 'djanjinja.handlers.server_error'

## RequestContext

One of Django's most useful features is the RequestContext class, which allows you to specify several context processors which each add some information to the context before templates are rendered. Luckily, this feature is template-agnostic, and is therefore fully compatible with DjanJinja.

However, DjanJinja also provides you with some very helpful shortcuts for using request contexts. Usually, without DjanJinja, you would use them like this:

from django.shortcuts import render_to_response from django.template import RequestContext

def myview(request):

context = {'foo': bar, 'spam': eggs} return render_to_response('template_name.html',

context, context_instance=RequestContext())

To be honest,this doesn't look very much like a 'shortcut' at all. For this reason, DjanJinja contains a subclass of RequestContext specialised for Jinja2, which is used like this:

from djanjinja.views import RequestContext

def myview(request):
context = RequestContext(request, {'foo': bar, 'spam': eggs}) return context.render_response('template_name.html')

This code is much more concise, but loses none of the flexibility of the previous example. The main changes made are the addition of render_response and render_string methods to the context object itself. This is highly specialised to rendering Jinja2 templates, so it may not be a very reusable approach (indeed, other code which does not use Jinja2 will need to use the full Django syntax), but it works for the problem domain it was designed for.

## Middleware

One important thing to note from before is that each time a RequestContext instance is constructed, it is necessary to explicitly pass the request. In object-oriented programming, and Python expecially, when we have functions to which we must always pass an object of a certain type, it makes sense to make that function a method of the type. When that function is not, in fact, a function, but a constructor, this seems more difficult. However, thanks to a feature of Python known as metaprogramming, we can do this very easily. Because it's not exactly obvious how to do so, DjanJinja includes a special piece of middleware which can help make your code a lot shorter yet still retain all the functionality and flexibility of the previous two examples.

To use this middleware, simply add 'djanjinja.middleware.RequestContextMiddleware' to your MIDDLEWARE_CLASSES list in the settings module of your project. Then, you can write view code like this:

def myview(request):
return request.Context({'foo': bar, 'spam': eggs}).render_response(
'template_name.html')

As you can see, we've greatly reduced the verbosity of the previous code, but it's still obvious what this code does. The middleware attaches a Context attribute to each request object. This attribute is in fact a fully-fledged Python class, which may itself be subclassed and modified later on. When constructed, it behaves almost exactly the same as the usual RequestContext, only it uses the request object to which it has been attached, so you don't have to pass it in to the constructor every time.

## Template Loading

DjanJinja hooks directly into the Django template loader machinery to load templates. This means you can mix Jinja2 templates freely with Django templates, in your TEMPLATE_DIRS and your applications, and render each type independently and seamlessly. If you want more information on how it actually works, please consult the djanjinja/environment.py file.

## (Un)license

This is free and unencumbered software released into the public domain.

Anyone is free to copy, modify, publish, use, compile, sell, or distribute this software, either in source code form or as a compiled binary, for any purpose, commercial or non-commercial, and by any means.

In jurisdictions that recognize copyright laws, the author or authors of this software dedicate any and all copyright interest in the software to the public domain. We make this dedication for the benefit of the public at large and to the detriment of our heirs and successors. We intend this dedication to be an overt act of relinquishment in perpetuity of all present and future rights to this software under copyright law.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

For more information, please refer to <http://unlicense.org/>