In chapter 1 you learned about the core building blocks in Django, including what are views, models and urls. In this chapter, you'll learn more about Django urls which are the entry point into a Django application workflow. You'll learn how to create url paths using basic strings and regular expressions, how to use url values in view methods & templates, how to structure & manage urls and how to name urls.
After urls, Django views represent the next step in almost all Django workflows, where views are charged with inspecting requests, executing business logic, querying a database and validating data, as well as generating responses. In this chapter, you'll learn how to create Django views with optional parameters, the structure of view requests & responses, how to use middleware with views and how to create class based views.
In the Set up content, understand Django urls, templates and apps section in Chapter 1, you learned how Django projects use the
urls.py file to define url paths in two parts: a basic string that matches a url request and an action to take when said string is matched. For example, specifically in listing 1-20 you learned how an empty string
'' represents a site's root path or homepage that can be set to serve a static template, as well as how a string like
'admin/' can match all url requests prefixed with
/admin/ and be processed by yet another url file with more specific strings.
In most circumstances, Django uses the
django.urls.path method to achieve this url path matching/action mechanism, however, Django also offers the
django.urls.re_path method. So what's the difference between the
django.urls.re_path methods ? The
path method is designed to perform matches against exact strings, whereas the
re_path method is designed to perform matches against patterned strings based on regular expressions.
Exact urls with
path and patterned urls with
There's little explanation required to understand the
path method because for the most part it works with exact url paths. For example, if you need to perform an action on the
/contact/ url path you use the
path('contact/',...) syntax and if you need to perform an action on the
/contact/email/ url path you use the
path('contact/email/',...) syntax. The note below briefly describes the exceptions for the
path method working with exact paths, exceptions which are explained in detail later in this chapter.
pathworks with exact paths deserves some clarification for its edge cases. You'll often find the
pathmethod chained to other
pathmethods, so it might not be obvious you're working with exact urls because url parts are spread out across multiple url files, this is the case for the Django admin url path described earlier (e.g.
path('admin/', admin.site.urls), where
pathdeclarations), as well as other examples presented in the Url consolidation and modularization section later in this chapter.
Another variation that doesn't precisely work with exact urls are
pathstrings with url parameters. For example, the
path('stores/<int:store_id>/',...)syntax defines a url parameter named
store_idthat matches paths like
/stores/2/or any other url that begins with
/stores/and is followed by an integer. Although loosely speaking urls in the form
/stores/<any_integer>/can be considered exact, semantics aside, because
pathstrings with url parameters also require special view method handling,
pathstrings with url parameters are described in the Url parameters, extra options & query strings section later in this chapter.
The main limitation of the
path method is it requires defining exact url paths or url parameters that must be handled by view methods -- the latter described in the previous note. So what if you want to handle more complex url paths ? For example, dynamic url paths that don't require url parameters to forgo the extra handling in view methods ? Enter the
re_path method and regular expressions.
Regular expressions provide a
powerful approach in all programming languages to determine
patterns, but with power also comes complexity, to the point
there are entire books written on the topic of regular
expressions. Although most Django urls will
never require a fraction of the complexity illustrated in many
regular expression books, it's important to take a closer look at the most common regular expressions patterns used by Django urls, as well as the most important behaviors of the
re_path behaviors: Precedence rule (Granular urls first, broad urls last) and exact url patterns (Forgoing broad matching)
The order in which url paths are declared with
path statements is irrelevant because this method operates on the premise of exact matches. For example, the
path('contact/',...) statement is unequivocally different than the
path('contact/email/',...) statement, so there's no way for the former syntax to be confused with the latter syntax even if the
path statements are made in different order. This however is not the case for the
re_path method which operates with regular expressions.
By design, regular expressions use a very specific syntax to indicate whether to match specific or broad patterns, a behavior that can have unintended consequences if you use the
re_path method and don't take care of declaring more granular url regular expressions first and broader url regular expressions last. Since Django url resolution triggers the action of the first matching url statement -- whether it's a
re_path statement -- you can end up never reaching an intended action if broad url regular expressions are placed first.
You shouldn't underestimate how
easy it can be to introduce two url regular expressions that match
the same pattern, particularly if you've never worked with regular
expressions which have cryptic syntax. Listing 2-1
illustrates the right way to declare
re_path url regular expressions statements, with more
granular regular expressions toward the top and broad regular
expressions toward the bottom.
Listing 2-1. Correct precedence for re_path url regular expressions
from django.urls import re_path from django.views.generic import TemplateView urlpatterns = [ re_path(r'^about/index',TemplateView.as_view(template_name='index.html')), re_path(r'^about/',TemplateView.as_view(template_name='about.html')), ]
Tip To learn more about strings prefixed with
r''(a.k.a. raw strings) which are common in Python regular expressions, see Appendix A 'Strings, unicode and other annoying text behaviors'.
Based on listing 2-1, lets walk
through what happens if Django receives a request for the url
/about/index. Initially Django reads the first
regular expression which says match
^about/index, therefore this is an exact match for
/about/index url and therefore control is sent to the
Now let's walk through a request
/about/ url. Initially Django reads the first
regular expression and determines it's too specific for the
/about/ url and continues to the next url regular expression statement.
re_path regular expression says match
^about/, therefore this is an exact match for
/about/ request url and therefore control is sent to the
As you can see, listing 2-1
produces what can be said to be expected behavior. But now let's
invert the order of the
re_path statements -- as shown in
listing 2-2 -- and break down why declaring more granular regular
expressions toward the bottom is the wrong way to declare Django
re_path url regular expressions.
Listing 2-2. Wrong precedence for re_path url regular expressions
from django.urls import re_path from django.views.generic import TemplateView urlpatterns = [ re_path(r'^about/',TemplateView.as_view(template_name='about.html')), re_path(r'^about/index',TemplateView.as_view(template_name='index.html')), ]
The issue in listing 2-2 comes
when a request is made for the
Initially Django reads the first
re_path regular expression statement
^about which turns out to be a broad match for the requested
/about/index url and therefore control is sent right away to the
about.html template! Before it reaches the next and likely intended
^about/index regular expression. The thing is the
^about/ regular expression simply says 'match anything that begins with
about/', therefore a request made for the
/about/index url produces a match, so you must be very careful when using
re_path statements to declare more granular url regular expressions towards the top and broader url regular expressions towards the bottom.
I'll admit I intentionally used regular expressions that allowed broad url matching to prove my point. But in my experience, as Django projects grow you'll eventually face the need to use this type of broad url regular expressions -- but more on why this is so, shortly. Another possibility to avoid falling into this broad matching regular expression pitfall -- besides being careful with regular expression precedence/order -- is to use exact url regular expressions to remove any ambiguity introduced by the order of
re_path url regular expression statements.
Lets rework the url regular expressions from listing 2-2 and make them exact regular expressions so their order doesn't matter. Listing 2-3 illustrates exact regular expressions on basis of those in listing 2-2.
Listing 2-3. Exact regular expressions, where url order doesn't matter.
from django.views.generic import TemplateView from django.urls import path, re_path urlpatterns = [ re_path(r'^about/$',TemplateView.as_view(template_name='about.html')), re_path(r'^about/index$',TemplateView.as_view(template_name='index.html')), ] # Equivalent path() method statements # urlpatterns = [ # path('about/',TemplateView.as_view(template_name='about.html')), # path('about/index',TemplateView.as_view(template_name='index.html')),
Notice the regular expressions in
listing 2-3 end with the
$ character. This is the
regular expression symbol for end of line, which means the regular
expression urls only match an exact pattern.
For example, if Django receives a
request for the
/about/index url it only matches
the last regular expression in listing 2-3 which says 'match
^about/index/$. However, it doesn't match the first
^/about/$ regular expression because this regular
expression says match
about/ exactly with
nothing else after, since the
$ indicates the end of
In addition, notice toward the bottom of listing 2-3 the
urlpatterns definition with
path statements, which is equivalent to the
urlpatterns definition one before it that uses
re_path statements. It turns out that by using exact url regular expressions with
re_path statements, you obtain identical behavior to that of
path statements which also operate on the premise of exact matches. The takeaway here is the
path method offers a more simplified approach to define exact urls than the
re_path method which offers a more general purpose approach capable of defining both exact and broad matching url patterns.
django.urls.pathwhen you have the flexibility of
django.urls.pathwas created for simplicity. Prior to Django 2.0, the only way to define urls paths was through regular expressions and the
django.conf.urls.urlmethod, something that created an unnecessary hurdle for newcomers. Therefore the existence of both
django.urls.re_pathis rooted in Django's past, with the former designed to welcome newcomers and the latter designed to support more powerful url patterns & Django old-timers accustomed to defining url paths with regular expressions.
Feel free to use whatever url definition method suites your project needs or personal tastes better.
While you might be tempted to think it's always best to declare all
re_path regular expressions with the
$ character to make them exact url regular expressions and avoid any problems, it's important to take a look at some beneficial cases where broad url regular expressions are helpful. If you plan to run the same logic on different urls -- a technique that's common in Search Engine Optimization (SEO) and A/B testing -- strict regular expressions ending with
$ used by
re_path statements or
path statements that by design are strict, can lead to a lot more work.
For example, if you start to use
/about/address and they
all use the same template or view for processing, exact regular
expressions just make the amount of urls you declare larger.
Similarly, if you use A/B testing or SEO where lengthier variations
of the same url are processed in the same way (e.g.
/about/the+coffeehouse+in+san+diego/) broad url
matching is much simper than declaring exact url patterns.
In the end, whether you opt to
use exact url regular expression ending in
re_path statements or just use
path statements, I would
still recommend you maintain the practice of keeping finer grained
url patterns at the top and broader ones at the bottom, as this
avoids the unexpected behaviors described in listing 2-2.
Common url patterns
Although url regular expressions can have limitless variations -- making it next to impossible to describe each possibility -- I'll provide examples on some of the most common url patterns you're more likely to use. Table 2-1 shows individual regular expression characters for Django urls and table 2-2 shows a series of more concrete examples with url patterns.
Table 2-1. Regular expression syntax for Django urls: Symbol (Meaning)
Table 2-2. Common Django url patterns for
django.urls.re_path and their regular
expressions, with samples
|Url regular expression||Description||Sample urls|
|Empty string (Home page)||Matches:
|Any trailing characters||Matches:
|Exact, no trailing characters||Matches:
http://127.0.0.1/stores/2/ http://127.0.0.1/stores/34/Doesn't match:
|Word options, any trailing characters||Matches:
http://127.0.0.1/drinks/mocha/ http://127.0.0.1/drinks/mochaccino/ http://127.0.0.1/drinks/espresso/Doesn't match:
|Word options exact, no trailing characters||Matches:
|Word characters (Any letter lower-upper case, number or underscore)||Matches:
http://127.0.0.1/stores/sandiego/ http://127.0.0.1/stores/LA/ http://127.0.0.1/stores/1/Doesn't match:
|Word characters or dash||Matches:
|Two upper case letters||Matches:
On certain urls -- those made by HTTP GET requests, common in HTML forms or REST services -- parameters are added to urls with
/drinks/mocha/?type=cold&size=large). These set of values are known as query strings and Django ignores them for the purpose of url pattern matching.
If you need to make use of these values as url parameters -- a topic explored in the next section -- you can access these values in Django view methods through the request reference. Another alternative is to change the url structure to accommodate regular expressions with the
re_pathmethod or basic string matching with the