Home > Articles

This chapter is from the book

This chapter is from the book

5.2 The Purpose of Views and URL Configurations

A webpage consists of (1) the data contained in the webpage and (2) the URL (location) of the webpage. Django follows this abstraction by splitting the Controller into two parts. Django views give Django the data of the webpage. The URL associated with each view is listed in the URL configuration.

In the rest of this section, we expand on the nature and purpose of the URL configuration and views. To make the material more tangible, we then step through what happens when Django receives a request, detailing the actions the Controller takes.

5.2.1 Django URL Configurations

As discussed in Chapter 1, Section 1.2, webpages were originally quite basic. The webpage’s data were contained in a flat file (a text file, an HTML file, or a PDF file, for instance). The URL was literally the location of the file on the server. If a user directed his or her browser to http://awebsite.com/project1/important.pdf, the awebsite.com server would go to the project1 directory and fetch the important.pdf file to give to the user’s browser.

Because modern web frameworks generate webpages dynamically, URLs have ceased to be the actual path to the data. A URL is now an abstraction, and it represents the logical path to data. For instance, the path /startup/jambon-software obviously requests information about the JamBon Software startup, whereas the path /blog/2013/1/django-training/ is clearly a request for a blog post about Django classes published in January 2013.

The name Uniform Resource Locator is thus not quite right anymore, as we are not actually requesting the location of the data. Instead, we are simply identifying it. Appropriately, URLs are a direct subset of Uniform Resource Identifiers (URIs), as illustrated in Figure 5.2.

Figure 5.2

Figure 5.2: URLs are a subset of URIs

While there is some confusion surrounding the difference between URLs and URIs, RFC 39861 is quite clear on the topic (effectively superseding RFC 3305)2:

  • A URI can be further classified as a locator, a name, or both. The term “Uniform Resource Locator” (URL) refers to the subset of URIs that, in addition to identifying a resource, provides a means of locating the resource by describing its primary access mechanism (e.g., its network “location”).

Every URL is thus a URI. However, a URL must specify a scheme to access the data, such as http or https, while a URI does not have to. According to this definition, the string /blog/2013/1/django-training/ is a URI, but the string http://site.django-unleashed.com/blog/2013/1/django-training/ is a URL despite the fact that the URL path is not an actual location. For this reason, Django continues to refer to URLs instead of URIs.

Because of the Hollywood principle (inversion of control), the URL configuration acts as a way to direct both users and Django to data. The URL configuration connects URLs to views: Django uses the URL configuration to find views. Django does not know the existence of any view without the URL configuration.

The URL configuration is a list of URL pattens. The URL pattern represents the two parts of a webpage: it maps a URI (the route/location/identifier) to a view (the data). Formally, the URI is a regular expression pattern, whereas the view is a Python callable. A URL configuration can also point to another URL configuration instead of a view, as we discuss in more depth in Section 5.7.1.

In Figure 5.3, each arrow is a URL pattern. Multiple URIs may point to a single view, but a single URI may not be defined more than once. The regular expression pattern in each URL pattern is how Django performs its matching. When Django receives an HTTP request, it tries to match the URL of the request to each and every regular expression pattern in each and every URL pattern. Upon finding a match, Django calls the view that the regular expression pattern maps to. Django uses the first match, meaning that the order of the list of URL patterns matters if there are several potential matches. If Django does not find a match, it returns an HTTP 404 error.

Figure 5.3

Figure 5.3: URL Configuration

In the example provided by Figure 5.3, if a user requested the URI /startup/, perhaps in a URL such as http://site.django-unleashed.com/startup/, then Django would call the startup_list() function view. Django automatically strips the root slash of the URL path (to Django, /startup/ becomes startup/).

We first coded a URL pattern in Chapter 2 and then again in Chapter 4. This last one, shown in Example 5.1, should still exist in suorganizer/urls.py.

Example 5.1: Project Code

suorganizer/urls.py in 95b20c151b

23    url(r'^$', homepage),

Requesting the root path of our website causes Django to call homepage(), coded in organizer/views.py. We walk through exactly how Django does this shortly.

5.2.2 Django Views

The view is where webpage data is generated. The developer uses the view to interact with the database, load and render the template, and perform any other logic necessary to displaying a webpage.

A Django view is any Python callable (function, class, or object) that meets the following two requirements:

  • Accepts an HttpRequest object as argument
  • Returns an HttpResponse object

An HttpRequest object contains all of the information about the page requested, any data the user is passing to the website, and any data the browser is sending about the user. The HttpResponse returns an HTTP code (please see Appendix A for information about HTTP codes) as well as any data the developer chooses to return to the user.

Because the nature of a view depends solely on its input and output, any Python callable can be a view. Typically, however, you will be using either functions or Django’s supplied classes to create views. For the moment, we build views using functions and wait until the end of the chapter to look at Django’s class-based views.

Developers often refer to Django views as view functions. This is rather confusing, as views are not limited to being functions (this was not the case historically, which is where the vocabulary originates). In this book, I refer to any callable that builds a webpage as a view. Any view that is built using a function is called a function view, and any view that is an object is called a class-based view (following the documentation’s nomenclature).

We currently have a function view coded in organizer/views.py, shown in Example 5.2.

Example 5.2: Project Code

organizer/views.py in f0d1985791

 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)
13       return HttpResponse(output)

We can see how the function in Example 5.2 adheres to view requirements: it accepts an HttpRequest object as the request argument and returns an HttpResponse object with the output of a rendered template. It is also clearly dynamic, generating content based on data in the database.

In this chapter, we focus on using the database to generate dynamic pages. In Chapter 9: Controlling Forms in Views, we generate dynamic pages based on not only the database but also the contents of the HttpRequest object. In Chapter 15: Creating Webpages with Django Flatpages, we also discuss the ability to make static/flat pages with views.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020