Signals - Part 2: Return to sender

This week we return with a follow up to one of the most popular topics on this blog - signals. In my first post on the topic (Signals: Letting People Know When You've Got Something To Say) I covered the basics of how to register a function to listen to signals as well as how to create your own signals and use them to send messages. Since that post there has been a new decorator added to help register your listeners which I will talk about more shortly. The other topics I will be covering are some of the other options that can be passed in when registering a signal and the mechanism provided for obtaining responses from listeners when a signal is sent.

The Receiver Decorator

This decorator was added in django 1.3 and provides a shortcut to having to call connect for your listener functions. It's very simple to use and takes the same parameters that connect does (behind the scenes it passes these straight through to the connect function for you) as shown in my first example (modified from example 1 of the first signals post.

# File: test_signal/models.py from django.db import models from django.contrib.auth.models import User from django.db.models.signals import post_save from django.dispatch import receiver class UserProfile(models.Model): user = models.ForeignKey(User, unique=True) allow_our_emails = models.BooleanField(default=False) allow_other_emails = models.BooleanField(default=False) @receiver(post_save, sender=User) def ensure_profile_exists(sender, **kwargs): if kwargs.get('created', False): UserProfile.objects.create(user=kwargs.get('instance')) post_save.connect(ensure_profile_exists, sender=User)

The second example shows a change from Django 1.5 to the receiver decorator which allows you to assign a function to listen to multiple signals in one go.

# File: test_signal/models.py from django.db import models from django.contrib.auth.models import User from django.db.models.signals import post_save, pre_delete from django.dispatch import receiver class UserProfile(models.Model): user = models.ForeignKey(User, unique=True) allow_our_emails = models.BooleanField(default=False) allow_other_emails = models.BooleanField(default=False) @receiver([post_save,pre_delete,], sender=User) def ensure_profile_exists(sender, **kwargs): if kwargs.get('created', False): UserProfile.objects.create(user=kwargs.get('instance')) signal = kwargs.get('signal', None) if signal == post_save: # After saving ensure that a UserProfile exists. if kwargs.get('created', False): UserProfile.objects.create(user=kwargs.get('instance')) elif signal == pre_delete: # Before deleting remove the UserProfile UserProfile.objects.filter(user=kwargs.get('instance')).delete()

Other Connection Arguments

So far I've only mentioned the sender argument that can be provided to connect or receiver there are a couple of other arguments that may be useful from time to time.

dispatch_uid

This allows you to pass in a unique identifier for your listener. If the signal you're trying to connect to already has a listener with that id then the new one won't be added. You can use this for situations where the code connecting receiver may end up running more than once, or for situations where different parts of the code may try to add a listener for a specific purpose but you only need one of them to actually receive the signals.

weak

This is True by default and refers to the way in which the signal stores the listener. If the value is True then a weak reference to the listening function is stored so if the function drops out of scope then it will be forgotten about and won't receive signals any more. If the value is False however you can pass in local functions without worrying that they'll be garbage collected.

Example 3 shows a situation where these parameters may be useful

# File: test_signal/models.py from django.db import models from django.db.models.signals import post_save from django.dispatch import receiver class LoggableModel1(models.Model): value = models.TextField() class LoggableModel2(models.Model): val = models.TextField() def add_logging_signals(models): for model_inst in models: uid = "%s-logger" % str(model_inst) if hasattr(model_inst, 'val') @receiver(post_save, sender=model_inst, weak=False, dispatch_uid=uid) def _listener(sender, kwargs**): print kwargs['instance'].val elif hasattr(model_inst, 'value') @receiver(post_save, sender=model_inst, weak=False, dispatch_uid=uid) def _listener(sender, kwargs**): print kwargs['instance'].value add_logging_signals([LoggableModel1, LoggableModel2, LoggableModel1,])

In this example because the listener functions are being created on the fly when add_logging_signals is called we specify weak=False so that the signal keeps hold of the function even when it drops out of scope.

We also make use of dispatch_uid so that even if we accidentally pass a model into the signal creator twice (which we then did) it will only add the one listener and so only output the logging code once.

Responding to a signal

The final topic for this week is on responding to signals. Signals don't have to be just a one-way mechanism!

The next example shows this in action

# File: signals/signals.py from django.dispatch import Signal pizza_deliverer = Signal(providing_args=["topping","size",])
# File: signals/models.py from django.db import models from django.dispatch import receiver from django.db.models.signals import post_save, class_prepared from signals import pizza_deliverer # List of toppings that our pizzas can have: TOPPING_PLAIN = 0 TOPPING_PEPPERONI = 1 TOPPING_VEGETARIAN = 2 TOPPINGS = ((TOPPING_PLAIN, 'Plain',), (TOPPING_PEPPERONI, 'Pepperoni',), (TOPPING_VEGETARIAN, 'Vegetarian',), ) # List of sizes that the pizzas could be: SIZE_SMALL = 8 SIZE_MEDIUM = 10 SIZE_LARGE = 12 SIZES = ((SIZE_SMALL, 'Small',), (SIZE_MEDIUM, 'Medium',), (SIZE_LARGE, 'Large',), ) class Pizza(models.Model): topping = models.IntegerField(choices=TOPPINGS) size = models.IntegerField(choices=SIZES) customer = models.ForeignKey('Customer', null=True, blank=True, default=None) @property def uid(self): return "pizza-%s" % self.id def serve_pizza(self, customer): # We have a customer so stop listening for new ones. self.customer = customer self.save() customer.receive_pizza() pizza_deliverer.disconnect(dispatch_uid=self.uid, sender=Customer, weak=False) class Customer(models.Model): topping_wanted = models.IntegerField(choices=TOPPINGS) size_wanted = models.IntegerField(choices=SIZES) @property def uid(self): return "customer-%s" % self.id def receive_pizza(self): # We have our pizza so stop listening for new ones. pizza_deliverer.disconnect(dispatch_uid=self.uid, sender=Pizza, weak=False) def _process_pizza(pizza): hungry_customers = pizza_deliverer.send(sender=Pizza, topping=pizza.topping, size=pizza.size) for _receiver, customer in hungry_customers: if customer is not None: # Pizza goes to the first customer that wants it. pizza.serve_pizza(customer) break else: # Nobody wants this pizza yet so listen for someone that does. @receiver(pizza_deliverer, sender=Customer, weak=False, dispatch_uid=pizza.uid) def _wait_for_customer(sender, **kwargs): topping = kwargs.get('topping') size = kwargs.get('size') if topping == pizza.topping and size == pizza.size: return pizza @receiver(post_save, sender=Pizza) def new_pizza(sender, **kwargs): if kwargs.get('created', False): pizza = kwargs.get('instance') _process_pizza(pizza) def _process_customer(customer): prepared_pizzas = pizza_deliverer.send(sender=Customer, topping=customer.topping_wanted, size=customer.size_wanted) for _receiver, pizza in prepared_pizzas: if pizza is not None: # Customer takes the first pizza offered that matches their criteria. pizza.serve_pizza(customer) break else: # The pizza this customer wants isn't ready yet so wait for it. @receiver(pizza_deliverer, sender=Pizza, weak=False, dispatch_uid=customer.uid) def _wait_for_pizza(sender, **kwargs): topping = kwargs.get('topping') size = kwargs.get('size') if customer.topping_wanted == topping and customer.size_wanted == size: return customer @receiver(post_save, sender=Customer) def new_customer(sender, **kwargs): if kwargs.get('created', False): customer = kwargs.get('instance') _process_customer(customer) # Ensure listeners are added for any unallocated pizzas or customers. for pizza in Pizza.objects.filter(customer=None): _process_pizza(pizza) for customer in Customer.objects.filter(pizza=None): _process_customer(customer)

Now this rather convoluted example simulates a potential (inneficient) pizzaria and makes use of all the features of signals discussed so far.

The signal is created as before and is set up to take two parameters a topping and a size. This signal represents the person behind the counter who will look for a waiting customer when a new pizza arrives or look for a waiting pizza when there's a new customer.

When a new pizza is added, a call is sent out to all the listening customers with the topping and size. Received responses are then iterated through until a positive reply is found, at which point the pizza is allocated to that customer.

If no suitable customers are listening then the pizza registers a listener function with our deliverer signal so it can wait for an appropriate customer to arrive.

A similar setup is then created for the customers - new ones send out a signal looking for matching pizzas and if there's none they sit and wait.

The listening functions _wait_for_customer and _wait_for_pizza both require a strong reference as they're created on the fly as well as both utilising the dispatch_uid to ensure that there's only one listener per pizza or customer.

This code also make use of the disconnect method on a signal to ensure that once a pizza and customer are matched up neither of them listens out for any further messages.

The final mention on signals is for the send_robust this would be used the same way that we used send in example 4 however it will catch any exceptions thrown by the listening functions and add that as the response from that listener.

Therefore to use it in our example we would also need to change the check when we iterate through the responses to ensure that the response was not None and that the response was also not of type Exception

This concludes the post on advanced signalling in Django. Until next time - keep up the communicating!

It's alive!

After a long respite this blog is finally back up and running!

I've got lots of ideas for topics ready to go but in order to try and maintain things this time I'm going to try and limit things to one post per week.

Take a look in a couple of minutes at the first post of the return where I'll be revisiting signals and trying to explain some of the more advanced features of them.

Middleware: Like An Onion But With Less Crying

This week will be the first part of a look into the middleware system that Django provides. This is an extremely powerful system that allows you to create pieces of code to run on each and every request that comes in.

If you've done even a small project in Django you'll no doubt have come across middleware, possibly without even realising it. The main starting point is located in the settings.py for a project. If you inspect this file you should find an entry similar to:

#...
MIDDLEWARE_CLASSES = (
'django.middleware.common.CommonMiddleware',
'django.contrib.sessions.middleware.SessionMiddleware',
'django.contrib.auth.middleware.AuthenticationMiddleware',
)
#...

This tuple of classes is what Django will refer to when it loads up its middleware and not only defines the items that will be loaded but also the order in which they will be executed. The items in the example given here represent a very simple set and are all that is needed for a very basic site. CommonMiddleware provides some core properties that are used by the other classes, SessionMiddleware is responsible for attaching the session object to a request (allowing you to store information from one request to another) and also for ensuring that the session is saved back to the database once the request has completed being processed. Finally AuthenticationMiddleware is responsible for adding things like the user object to the request - not a necessity as such but still extremely handy none the less.

There are four separate phases that middleware can act on. Any individual item of middleware can act on any number of these phases and executes each middleware class in a specific order which are:

1. The process_request phase - middleware is executed in the order specified;


2. The process_view phase - middleware is executed in the order specified;


3. The process_response phase - middleware is executed in the reverse of the order specified;


4. The process_exception phase - middleware is executed in the reverse of the order specified;

The process_request phase is the earliest available point that middleware can be called. After Django receives a request and generates a request object then it calls all the middleware classes that implement the process_request function passing them the newly created request object. This phase is generally the best place to run any essential checks or add things to the session that would cause the system to break if they were missing.

The process_view phase is the second phase to run. It occurs after the request has been processed and the view that will be run has been determined. At this point all the middleware classes that implement the process_response function are called and given access not only to the request object but also to the view object that is about to be executed along with any arguments that are going to be passed to the view object. This phase is a good place to put checks where you may want to redirect the user to a page asking them to log in, redirecting the user to somewhere requesting them to complete filling in profile details or initialising a log entry (as you know at this point more information about what the user is doing).

The process_response phase runs at the point after a view has been executed and Django has been given a response object to display back to the user. All classes that implement the process_response function will be provided with the original request object as well as the response object that was generated. This phase is good for things like tidying up anything that should be stored between requests or completing and writing out log entries.

The process_exception phase is a slightly special case that runs whenever an exception cuts in and interrupts the execution of a request. All classes that implement the process_exception function will be provided with the original request and the exception that has been thrown. This phase is good for logging any problems that have occurred and for trying to either send the user to a friendly error page or to try and process something that wont cause an error instead.

Any of the functions that are implemented in the class have two choices of what to return. If they return None then that processing will continue with the next item of middleware in order otherwise they can return a response object which will then be used straight away, if the phase returning the new response is the request, view or exception phase then things will jump straight to the response phase and only middleware in that phase will be run subsequently. If the phase returning the new response is the response phase then no more middleware will be run.

A quick note on the ordering - be aware that during the response and exception phase middleware is run in a reverse order. This means that for things like the session middleware. Anything listed after it in the settings.py file can add things to the session at any point and it will still be saved since the session will be set up early on (as the request and view phase run in the normal order) and it wont be saved until the latest possible moment.

That concludes this weeks look at what goes into the middleware. Next week I will be providing some examples of some interesting things that you can do with middleware.

Pagination: Splitting Your Data Into Bite-Size Chunks For Easier Digestion

This week it's time for something nice and simple, Django has some very useful tools for handling pagination and that is what will be covered.

If you have a site that is constantly having to return long lists of results/data, then sooner or later you are probably going to want to look for a way to easily split that data over several pages so that the user can view it in easier to read chunks. Django's pagination framework is one quick and easy answer to such a problem. Take the example given below:

#File: views.py
from django.shortcuts import render_to_response
from django.template import RequestContext

def pagination_test(request, word_string):
word_chars = list(word_string)
anagrams = []
for anagram in _generate_anagrams(word_chars):
anag = "".join(anagram)
if anag not in anagrams:
anagrams.append(anag)
response_dict = {'anagrams':anagrams,
'original':word_string,
}
context_instance = RequestContext(request, response_dict)
return render_to_response('pagination_test.html',
context_instance=context_instance)

def _generate_anagrams(word_chars):
# Warning - This function is horrendously inefficient and should
# not really be used for words longer than 6 characters.
char_count = len(word_chars)
if char_count == 0:
yield []
elif char_count == 1:
yield [word_chars[0]]
else:
for char_index in range(len(word_chars)):
lone_char = [word_chars[char_index]]
other_chars=word_chars[:char_index]+word_chars[char_index+1:]
for others in _generate_anagrams(other_chars):
yield lone_char + others{# File: pagination_test.html #}
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title>Anagrams of {{ original }}</title>
</head>
<body>
<h1>Anagrams of {{ original }}</h1>
<ul>
{% for anagram in anagrams %}
<li>{{ anagram }}</li>
{% endfor %}
</ul>
</body>
</html>#File: urls.py
from django.conf.urls.defaults import *

urlpatterns = patterns('views',
# Other urls ...
url(r'^test/pagination/(?P<word_string>.+)/$,
'pagination_test', name='pagination_test')
)

Here we take a string as an input from the user and return all the possible unique combinations that can be achieved by rearranging its characters. For short strings this produces manageable numbers of results but even getting up to five character strings the list starts to increase in size dramatically (something like order n^2). What we can do here though is put it through Django's pagination framework like so:

#File: views.py
def pagination_test(request, word_string, page_index=1):
word_chars = list(word_string)
anagrams = []
for anagram in _generate_anagrams(word_chars):
anag = "".join(anagram)
if anag not in anagrams:
anagrams.append(anag)
paginator = Paginator(anagrams, 10, 5)
try:
page = paginator.page(page_index)
except (EmptyPage, InvalidPage), e:
page = paginator.page(paginator.num_pages)
response_dict = {'anagrams':page,
'original':word_string,
}
context_instance = RequestContext(request, response_dict)
return render_to_response('pagination_test.html',
context_instance=context_instance)

def _generate_anagrams(word_chars):
# Same function as before

In the code above, paginator = Paginator(anagrams, 10, 5) sets up a paginator on our results list. The first argument to Paginator says that we want the items in the anagrams list to be split into pages. The second argument says that each page should contain ten results, whilst the third argument says that if there would not be at least five results on the final page they should be added to the one before (meaning the last page will contain anything from five to fourteen results). The other addition is responsible for saying which page of results should be displayed, first of all trying the value passed in and if it's a non-existent page then using the last available page.

{# File: pagination_test.html #}
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title>Anagrams of {{ original }}</title>
</head>
<body>
<h1>Anagrams of {{ original }}</h1>
<div>
Results {{ anagrams.start_index }}
to {{ anagrams.end_index }}
of {{ anagrams.paginator.count }}
</div>
<ul>
{% for anagram in anagrams.object_list %}
<li>{{ anagram }}</li>
{% endfor %}
</ul>
<div>
{% if anagrams.has_previous %}
<a href="{% url paged_pagination_test original anagrams.previous_page_number %}">Previous</a>
{% endif %}
{% if anagrams.has_next %}
<a href="{% url paged_pagination_test original anagrams.next_page_number %}">Next</a>
{% endif %}
</div>
</body>
</html>

In the template the main additions are for saying which results we're viewing through the use of start_index and end_index which are functions that belong to pagination page objects representing the first and last indices of the results on display and count which is a function on the paginator itself representing the total number of results.

In the loop that iterates over the results, it needs to be told to iterate over the object_list attribute of the pagination page as this represents the actual results to be shown on this page.

Finally the section at the bottom of the page is responsible for adding Next and Prev links. has_previous and has_next are pagination page functions that do exactly what you would expect and tell you if there is a page of results available either before or after the current one. Meanwhile previous_page_number and next_page_number return the indices of the page before and page after.

#File: urls.py
from django.conf.urls.defaults import *

urlpatterns = patterns('views',
# Other urls ...
url(r'^test/pagination/(?P<word_string>[^/]+)/$',
'pagination_test', name='pagination_test'),
url(r'^test/pagination/(?P<word_string>.+)/(?P<page_index>\d+)/$',
'pagination_test', name='paged_pagination_test'),
)

An extra line in the urls file picks up any visits to our test url that has a page index included on the end.

If you now try visiting this with a shortish word like "hop" you should be presented with a single page containing all six possible anagrams. Try again with "shop" and you should get two pages worth of results with ten on the first page and fourteen on the second. Finally try with "shops" and you should get six pages of ten results.

That's it for this weeks tutorial, tune in again next week where I will be starting to look at Django's middleware system.

Signals: Letting People Know When You've Got Something To Say

This week marks a break away from the topic of templates and instead will be focussing on signals.

Signals are Django's way of allowing you to listen out for certain events that either occur by default in the Django code or that have been specified by an application developer. The signals provided by default are explained in a fair amount of detail in the Django Built-in Signal Reference (djangoproject.com) so I won't go into too much depth on what they do, but they are as follows (skip to the first example):

pre_init

Occurs at the start of the process of initialising a model;

post_init

Occurs at the end of the process of initialising a model;

pre_save

Occurs at the start of the process of saving a model;

post_save

Occurs at the end of the process of saving a model;

pre_delete

Occurs at the start of the process of deleting a model;

post_delete

Occurs at the end of the process of deleting a model;

class_prepared

Internally used, occurring when a model class has been registered with Django;

post_syncdb

Occurs after syncdb installs an application;

request_started

Occurs just before Django starts to process a request;

request_finished

Occurs just after Django processes a request;

got_request_exception

Occurs when Django encounters an exception whilst processing a request;

template_rendered

Occurs only whilst running tests when Django renders a template;

comment_will_be_posted

Occurs if you have Django's comment app installed and a comment is just about to be saved;

comment_was_posted

Occurs if you have Django's comment app installed and a comment has just been saved;

comment_was_flagged

Occurs if you have Django's comment app installed and a comment has had a flag set (i.e. a moderator just approved it).

For my first example this week I'm going to be looking at the post_save signal and how it can be used to ensure that a User object always has a profile instance associated with it.

# File: test_signal/models.py
from django.db import models
from django.contrib.auth.models import User
from django.db.models.signals import post_save

class UserProfile(models.Model):
user = models.ForeignKey(User, unique=True)
allow_our_emails = models.BooleanField(default=False)
allow_other_emails = models.BooleanField(default=False)

def ensure_profile_exists(sender, **kwargs):
if kwargs.get('created', False):
UserProfile.objects.create(user=kwargs.get('instance'))

post_save.connect(ensure_profile_exists, sender=User)
# File: (somewhere in) settings.py
AUTH_PROFILE_MODULE = "test_signal"

As you can see this defines a fairly simple profile model (supposedly for storing a user's e-mail preferences), the key part of this comes from the function ensure_profile_exists. With all signals it can only be guaranteed that the sender keyword argument will be present therefore we also need to catch everything else using **kwargs. Inside the function we use the fact that post_save is supposed to provide the arguments created and instance to check if the save was for a new object and if so to assign the created instance to the user property of a freshly created UserProfile object.

Finally we connect our function to the post_save signal with instructions to only receive signals when the sending class is User

If the test_signal app is now added to the list of installed apps in settings.py then all new users created will now have an associated profile created also.

The second example for this week demonstrates how you can create your very own signals for code to listen to.

# File: test_signal/signals.py
from django.dispatch import Signal

form_post_received = Signal(providing_args=["path","arguments",]

Here we tell Django that we want form_post_received to be a signal and that it should provide the arguments path and arguments. Now if we add the following:

# File: test_signal/views.py
from signals import form_post_received
from django.conf import settings

def view1(request):
if request.method == "POST":
form_post_received.send(sender=request, path=request.path, arguments=request.POST)
# Continue to do stuff with the form.

def view2(request):
if request.method == "POST":
form_post_received.send(sender=request, path=request.path, arguments=request.POST)
# Continue to do stuff with the form.

def log_form_received(sender, **kwargs):
print "Form Received at %(path)s with arguments %(arguments)s" % kwargs

if settings.DEBUG:
form_post_received.connect(log_form_received)

When pointing views at view1 and view2 whenever the server is in debug mode extra information should be output to the terminal informing us of whenever a form has been posted to either view.

This is done by calling send on our signal along with an argument for sender, in this case we use the request that triggered the view, along with values for each of the other arguments we specified. Django then passes them on to each function that has connected to our signal, which in this case is our logging function (but only when the server is in debug mode, otherwise connect will never be called).

That concludes this weeks tutorial on signals. Next week we shall be looking at the useful functions Django has built in for performing pagination.

UPDATE:There is now a second part to this tutorial talking about more advanced uses of signals. Check it out at Signals - Part 2: Return to sender

Custom Template Filters: Manipulating The Results

After last weeks brief distraction on the topic of inclusion tags, the time has finally come for the tutorial on template filters.

You've probably come across several of the standard filters when writing django templates, they're the things that are appended to variables though the use of the | character. Some common examples include |length which does as it says and returns the length of the variable (I.e. Number of characters if it's a string or number of elements if it's a list or dictionary), |safe which prevents any HTML within the variable being escaped and |date:"D dS M Y" which formats a date type variable using the format string provided.

The first example we're going to be looking at today will be a filter which can be used for sorting a list (by its values) or a dictionary (by its keys). The code is as follows (this should sit in the templatetags directory along with the files for custom template tags):

#File: custom_filters.py
from django import template
from django.utils.datastructures import SortedDict

register = template.Library()

@register.filter(name='sort')
def listsort(value):
if isinstance(value,dict):
new_dict = SortedDict()
key_list = value.keys()
key_list.sort()
for key in key_list:
new_dict[key] = value[key]
return new_dict
elif isinstance(value, list):
new_list = list(value)
new_list.sort()
return new_list
else:
return value
listsort.is_safe = True

If you've been following the tutorials on custom template tags over the last few weeks then most of this should look familiar. The key new points to notice are the decorator @register.filter(name='sort') which tells django that the upcoming function is a filter and that it should go by the name of sort when it is used within a template; SortedDict which is a useful data structure that is provided by django which remembers the order that you added items to it and the line listsort.is_safe = True which gives a hint to django that we've not done anything to the variable to cause it to need escaping (if the variable needed escaping beforehand however then it will still need escaping afterwards).

Pointing a url at the following view should allow you to see the filter in action. Two numbered lists should be displayed, the first being a sorted version of test_list and the second a (key) sorted version of test_dict.

#File: views.py
from django.template import Template,Context
from django.http import HttpResponse
def filter_test(request):
test_list=['Item 1','Item 2','Third', 'Initial element', 'Item 1.5']
test_dict={'a':'First', 't':'Second', 'b':'Third', 'alpha':'Fourth'}
t = Template("""{% load custom_filters %}
<html>
<head><title>Filter Test</title></head>
<body>
<ol>
{% for item in test_list %}
<li>{{ item }}</li>
{% endfor %}
</ol>
<ol>
{% with test_dict|sort as sorted_dict %}
{% for key, value in sorted_dict.items %}
<li>{{ key }} - {{ value }}</li>
{% endfor %}
{% endwith %}
</ol>
</body>
</html>""")
return HttpResponse(t.render(Context({'test_list':test_list,
'test_dict':test_dict,})))

That now shows the basics of creating custom filters, however it's also possible to create them to take extra arguments. To demonstrate this we'll look at a filter which allows you to prefix a string to the front of other string type variables. The code is as follows (append it to the existing file):

#File: custom_filters.py
from django import template
from django.template.defaultfilters import stringfilter

register = template.Library()

@register.filter
@stringfilter
def prefix(value, prefix):
return "%s%s" % (prefix, value)

In this example there's not been any arguments passed in to @register.filter, in this case django will take the name of the function being decorated to be the name of the filter (i.e. prefix). The @stringfilter decorator has been used on the function, which informs django that we only wish to operate on strings and so django will ensure that any variable passed in to the filter has been converted into a string first. Finally we do not specify that the output is safe, as there's no guarantee that the item being prefixed wont require escaping later on and so by not declaring it as safe it will force django to check that the string is escaped later on.

Now alter the view from before as follows (changes highlighted like this) and you should see the same result as before except the elements of test_list will have "Sorted! " prefixed to it.

#File: views.py
from django.template import Template,Context
from django.http import HttpResponse
def filter_test(request):
test_list=['Item 1','Item 2','Third', 'Initial element', 'Item 1.5']
test_dict={'a':'First', 't':'Second', 'b':'Third', 'alpha':'Fourth'}
t = Template("""{% load custom_filters %}
<html>
<head><title>Filter Test</title></head>
<body>
<ol>
{% for item in test_list %}
<li>{{ item|prefix:"Sorted! " }}</li>
{% endfor %}
</ol>
<ol>
{% with test_dict|sort as sorted_dict %}
{% for key, value in sorted_dict.items %}
<li>{{ key }} - {{ value }}</li>
{% endfor %}
{% endwith %}
</ol>
</body>
</html>""")
return HttpResponse(t.render(Context({'test_list':test_list,
'test_dict':test_dict,})))

That's all for the subject of custom filters. Next time we'll take a break from template related tutorials and have a look at Signals in django.

Custom Template Tags - Part 3: A Last Minute Inclusion

After finishing last week's tutorial on block style template tags I realised that there was one template tag creation tip that I had yet to mention and that's the inclusion tag. This is quite closely related to the simple template tags described previously in that it only requires you to write one function. This time however you must provide also provide a template name which will be rendered at the end of it all. Take a look at the following example (a reworking of the simple tag example)

#File: list_formatters.py
from django import template

register = template.Library()

@register.inclusion_tag('format_list_tag.html')
def format_list(input_list):
return { 'unformatted_list':input_list, }

This file is fairly straightforward, we decorate the format_list function to say that it's an inclusion style tag and that it should use the template format_list_tag.html we then return a dictionary which will be used as the context for the template when it is processed. All that's left to include is the following template file:

{# File: format_list_tag.html #}
<ul>
{% for item in unformatted_list %}
<li>{{ item }}</li>
{% endfor %}
<ul>

This file simply provides a snippet of html describing how to display the list. Pop list_formatters.py into your templatetags directory and format_list_tag.html into your templates directory and pointing a url towards the following example (repeated from the previous tutorial) you can see the tag in action.

from django.template import Template,Context
from django.http import HttpResponse
def tag_test(request):
test_list=['Item 1','Item 2','Third time lucky']
t = Template("""{% load list_formatters %}
<html>
<head><title>Tag Test</title></head>
<body>
{% format_list test_list %}
</body>
</html>""")
return HttpResponse(t.render(Context({'test_list':test_list})))

There is one other useful feature of the inclusion tag and that is the ability to access the context. This can be done as follows (changes from the original file have been highlighted):

#File: list_formatters.py
from django import template

register = template.Library()

@register.inclusion_tag('format_list_tag.html', takes_context=True)
def format_list(context):
return { 'unformatted_list':context.get(test_list,[]), }

Whilst this example isn't really the best way to do things it should illustrate the power of what can be done by the inclusion tag and should enable you to use it in your own django sites.

Next time things will be back on track with the promised tutorial on custom filters