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This chapter is from the book

This chapter is from the book

5.6 Shortening the Development Process with Django View Shortcuts

We now have a two-function view in /organizer/views.py, which currently reads as shown in Example 5.16.

Example 5.16: Project Code

organizer/views.py in 294dabd8cc

 1  from django.http.response import (
 2      Http404, HttpResponse)
 3  from django.template import Context, loader
 4
 5  from .models import Tag
 6
 7
 8  def homepage(request):
 9      tag_list = Tag.objects.all()
10      template = loader.get_template(
11          'organizer/tag_list.html')
12      context = Context({'tag_list': tag_list})
13      output = template.render(context)
14      return HttpResponse(output)
15
16
17  def tag_detail(request, slug):
18      try:
19          tag = Tag.objects.get(slug--iexact=slug)
20      except Tag.DoesNotExist:
21          raise Http404
22      template = loader.get_template(
23          'organizer/tag_detail.html')
24      context = Context({'tag': tag})
25      return HttpResponse(template.render(context))

That is a lot of code for two simple webpages. We also have a lot of duplicate code in each function, which is not in keeping with the DRY philosophy. Luckily for developers, Django provides shortcut functions to ease the development process and to significantly shorten code such as the preceding.

5.6.1 Shortening Code with get_object_or_404()

Our first shortcut, get_object_or_404(), is a complete replacement for the try...except block that currently exists in our tag_detail() function.

Let’s start by importing it into our /organizer/views.py file, as in Example 5.17.

Example 5.17: Project Code

organizer/views.py in 5705e49877

2   from django.shortcuts import get_object_or_404

We can then delete the following lines, as in Example 5.18.

Example 5.18: Project Code

organizer/views.py in 294dabd8cc

18      try:
19          tag = Tag.objects.get(slug--iexact=slug)
20      except Tag.DoesNotExist:
21          raise Http404

We replace the content in Example 5.18 with the code in Example 5.19.

Example 5.19: Project Code

organizer/views.py in 5705e49877

18      tag = get_object_or_404(
19          Tag, slug--iexact=slug)

The get_object_or_404() shortcut expects to have the model class and the desired query passed as arguments and will return the object if it finds one. If not, it raises Http404, just as we had programmed before. Because we are passing in the Tag object and using exactly the same query, the behavior of our shortened code is exactly the same as that of our original code.

Our tag_detail() thus reads as in Example 5.20.

Example 5.20: Project Code

organizer/views.py in 5705e49877

17   def tag_detail(request, slug):
18       tag = get_object_or_404(
19           Tag, slug--iexact=slug)
20       template = loader.get_template(
21           'organizer/tag_detail.html')
22       context = Context({'tag': tag})
23       return HttpResponse(template.render(context))

5.6.2 Shortening Code with render_to_response()

Most views must do the following:

  1. Load a template file as a Template object.
  2. Create a Context from a dictionary.
  3. Render the Template with the Context.
  4. Instantiate an HttpResponse object with the rendered result.

Django supplies not one but two shortcuts to perform this process for us. The first is the render_to_response() shortcut. The shortcut replaces the behavior that we currently have in our views, performing all four tasks listed above. Let’s start by importing it, adding it to the end of our pre-existing shortcut import, as shown in Example 5.21.

Example 5.21: Project Code

organizer/views.py in 5ff3dee4fa

1   from django.shortcuts import (
2       get_object_or_404, render_to_response)

We can now use render_to_response() to shorten our code. In our homepage() view, for instance, we can remove the code shown in Example 5.22.

Example 5.22: Project Code

organizer/views.py in 5705e49877

 7  def homepage(request):
 8      tag_list = Tag.objects.all()
 9      template = loader.get_template(
10          'organizer/tag_list.html')
11      context = Context({'tag_list': tag_list})
12      output = template.render(context)

The code in Example 5.22 is easily replaced with the code in Example 5.23.

Example 5.23: Project Code

organizer/views.py in 5ff3dee4fa

 7   def homepage(request):
 8       return render_to_response(
 9           'organizer/tag_list.html',
10           {'tag_list': Tag.objects.all()})

Observe how we pass in the same path to the template and a simple dictionary with the (identical) values to populate the template. The shortcut does the rest for us: the behaviors in the preceding two code examples are exactly the same.

The process to shorten tag_detail() is exactly the same. We start by removing the code in Example 5.24.

Example 5.24: Project Code

organizer/views.py in 5705e49877

20      template = loader.get_template(
21          'organizer/tag_detail.html')
22      context = Context({'tag': tag})
23      return HttpResponse(template.render(context))

Then, in Example 5.25, we write a call to render_to_response(), passing in the same values seen in the previous code: the same template path and the same dictionary passed to our Context instantiation.

Example 5.25: Project Code

organizer/views.py in 5ff3dee4fa

16     return render_to_response(
17         'organizer/tag_detail.html',
18         {'tag': tag})

Our entire file has been reduced to the code shown in Example 5.26.

Example 5.26: Project Code

organizer/views.py in 5ff3dee4fa

 1   from django.shortcuts import (
 2       get_object_or_404, render_to_response)
 3
 4   from .models import Tag
 5
 6
 7   def homepage(request):
 8       return render_to_response(
 9           'organizer/tag_list.html',
10           {'tag_list': Tag.objects.all()})
11
12
13   def tag_detail(request, slug):
14       tag = get_object_or_404(
15           Tag, slug--iexact=slug)
16       return render_to_response(
17           'organizer/tag_detail.html',
18           {'tag': tag})

The code in Example 5.26 was the original way to shorten code and, while still frequently seen on the Internet and in older projects, is no longer the best way to shorten a simple view. Instead, you’ll want to use render().

5.6.3 Shortening Code with render()

Before introducing the render_to_response() shortcut, our /organizer/views.py read as shown in Example 5.27.

Example 5.27: Project Code

organizer/views.py in 4d36d603db

 1  from django.http.response import HttpResponse
 2  from django.shortcuts import get_object_or_404
 3  from django.template import Context, loader
 4
 5  from .models import Tag
 6
 7
 8  def homepage(request):
 9      tag_list = Tag.objects.all()
10      template = loader.get_template(
11          'organizer/tag_list.html')
12      context = Context({'tag_list': tag_list})
13      output = template.render(context)
14      return HttpResponse(output)
15
16
17  def tag_detail(request, slug):
18      tag = get_object_or_404(
19          Tag, slug--iexact=slug)
20      template = loader.get_template(
21          'organizer/tag_detail.html')
22      context = Context({'tag': tag})
23      return HttpResponse(template.render(context))

Example 5.27 is sufficient for the simple views we are currently building but will prove to be inadequate in the long run. Specifically, we are not using Django context processors.

At the moment, our views are rendering Template instances with Context instances and passing the result to an HttpResponse object. The problem with this approach is that sometimes Django needs to make changes to the values within the Context objects. To enable Django to make changes to data that render a Template, we must use a RequestContext instead of a Context object. When a Template renders with a RequestContext, Django uses the HttpRequest object to add data to the RequestContext, providing information not available to Context. To do so, Django calls the context processors, which are simply functions that are listed in the TEMPLATES options of /suorganizer/settings.py (Example 5.28).

Example 5.28: Project Code

suorganizer/settings.py in 4d36d603db

58  TEMPLATES = [{
 .      ...
64      'OPTIONS': {
65          'context_processors': [
66              'django.template.context_processors.debug',
67              'django.template.context_processors.request',
68              'django.contrib.auth.context_processors.auth',
69              'django.contrib.messages.context_processors.messages',
70          ],
71      },
72  }]

At the moment, enabling context processors is of no use to us, but in Chapter 9, we build views and templates that rely on Django context processors. However, it behooves us to examine them now, as they provide insight into our new shortcut.

To make the change to using context processors, we need only change each use of Context to RequestContext. The only difference is that RequestContext needs the HttpRequest object, as it intends to pass it to all the context processors. We therefore pass request to RequestContext before the dictionary of values. Our code now reads as shown in Example 5.29.

Example 5.29: Project Code

organizer/views.py in c392ab707a

 1  from django.http.response import HttpResponse
 2  from django.shortcuts import get_object_or_404
 3  from django.template import RequestContext, loader
 4
 5  from .models import Tag
 6
 7
 8  def homepage(request):
 9      tag_list = Tag.objects.all()
10      template = loader.get_template(
11          'organizer/tag_list.html')
12      context = RequestContext(
13          request,
14          {'tag_list': tag_list})
15      output = template.render(context)
16      return HttpResponse(output)
17
18
19  def tag_detail(request, slug):
20      tag = get_object_or_404(
21          Tag, slug--iexact=slug)
22      template = loader.get_template(
23          'organizer/tag_detail.html')
24      context = RequestContext(
25          request,
26          {'tag': tag})
27      return HttpResponse(template.render(context))

Understanding and using RequestContext or Context has a direct effect on our choice of shortcuts. Prior to Django 1.3, developers would force the render_to_response() shortcut to use the RequestContext object by coding as shown in Example 5.30.

Example 5.30: Python Code

return render_to_response(
    'path/to/template.html',
    data_dictionary,
    context_instance=RequestContext(request))

Many examples online and older projects continue to use this method. However, starting in Django 1.3 (released March 2011), developers should instead use the render() shortcut, which is identical to render_to_response() except that it uses a RequestContext object instead of a Context object and therefore takes the HttpRequest object as a third argument. Specifically, render() does the following:

  1. Loads a template file as a Template object
  2. Creates a RequestContext from a dictionary (with HttpRequest)
  3. Calls all the context processors in the project, adding or modifying data to the RequestContext
  4. Renders the Template with the RequestContext
  5. Instantiates an HttpResponse object with the rendered result

The render() shortcut thus replaces the project code from Example 5.30, taking three arguments: request, the path to the template file, and the dictionary used to build the RequestContext object. We can follow the same replacement steps used for render_to_response() in the case of render(). Example 5.31 shows the resulting /organizer/views.py.

Example 5.31: Project Code

organizer/views.py in d2ecb7f70d

 1  from django.shortcuts import (
 2      get_object_or_404, render)
 3
 4  from .models import Tag
 5
 6
 7  def homepage(request):
 8      return render(
 9          request,
10          'organizer/tag_list.html',
11          {'tag_list': Tag.objects.all()})
12
13
14  def tag_detail(request, slug):
15      tag = get_object_or_404(
16          Tag, slug--iexact=slug)
17      return render(
18          request,
19          'organizer/tag_detail.html',
20          {'tag': tag})

Using RequestContext is slower than using Context, and therefore render() is slower than render_to_response() (when without the context argument). Nonetheless, most developers now use render() out of the box, choosing to prioritize ease of programming over performance. Using Context or render_to_response(), particularly in young projects with few users, could be considered a pre-optimization, limiting functionality in favor of performance. In addition, context processors are not typically the bottleneck on a website. By the same token, if a context processor is ever needed on a view using Context or render_to_response(), more work will be required to get the context processor working, particularly if the developer is unclear as to where the problem lies. It is therefore not a bad idea to start with RequestContext and render() and replace them if necessary (and if possible!). We reinforce this notion in Chapter 19 when we opt to use variables created by context processors on every webpage.

In keeping with this logic and with current trends, the rest of the book relies on render() as the de facto view shortcut.

As we move forward, please keep in mind that while similar, render_to_response() and render() have very different uses, and many of the examples online should be using render() instead of render_to_response(), making this latter shortcut a common pitfall for beginners when building forms (Chapter 9) or when using the contributed library.

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