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Django - URL Dispatcher

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• Django - URL Dispatcher
  • URLconf
  • How Django processes a request
  • What the URLconf searches against
  • Sample URLconf
  • urls.py
  • URL namespaces
    • Example: Namespace
    • Example: Admin page

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URLconf

• URLconf / URL configuration:
  • a Python module to design URLs for an app
  • a mapping between URL path expressions to Python functions(views).

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How Django processes a request

• When a user requests a page from a Django-powered site, this is the algorithm the system follows to determine which Python code to execute:
  • 1. Django determines the root URLconf module to use.
       • Ordinarily, this is the value of the ROOT_URLCONF setting, but if the incoming HttpRequest object has a urlconf attribute (set by middleware), its value will be used in place of the ROOT_URLCONF setting.
  • 1. Django loads that Python module and looks for the variable urlpatterns.
       • This should be a sequence of django.urls.path() and/or django.urls.re_path() instances.
  • 1. Django runs through each URL pattern, in order, and stops at the first one that matches the requested URL, matching against path_info.
  • 1. Once one of the URL patterns matches, Django imports and calls the given view, which is a Python function (or a class-based view).
       • The view gets passed the following arguments:
         • An instance of HttpRequest.
         • If the matched URL pattern contained no named groups, then the matches from the regular expression are provided as positional arguments.
         • The keyword arguments are made up of any named parts matched by the path expression that are provided, overridden by any arguments specified in the optional kwargs argument to django.urls.path() or django.urls.re_path().
  • 1. If no URL pattern matches, or if an exception is raised during any point in this process, Django invokes an appropriate error-handling view.

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What the URLconf searches against

• The URLconf searches against the requested URL, as a normal Python string.

• This does not include GET or POST parameters, or the domain name.

  • For example, in a request to https://www.example.com/myapp/, the URLconf will look for myapp/.

  • In a request to https://www.example.com/myapp/?page=3, the URLconf will look for myapp/.

• The URLconf doesn’t look at the request method.

  • In other words, all request methods – POST, GET, HEAD, etc. – will be routed to the same function for the same URL.

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Sample URLconf

from django.urls import path

from . import views

urlpatterns = [
    path("articles/2003/", views.special_case_2003),
    path("articles/<int:year>/", views.year_archive),
    path("articles/<int:year>/<int:month>/", views.month_archive),
    path("articles/<int:year>/<int:month>/<slug:slug>/", views.article_detail),
]

• Notes:

  • To capture a value from the URL, use angle brackets. eg: <int:year>
  • Captured values can optionally include a converter type.
    • For example, use <int:name> to capture an integer parameter.
    • If a converter isn’t included, any string, excluding a / character, is matched.
    • There’s no need to add a leading slash, because every URL has that.
      • For example, it’s articles, not /articles.

• Example requests:

  • A request to /articles/2005/03/ would match the third entry in the list. Django would call the function views.month_archive(request, year=2005, month=3).
  • /articles/2003/ would match the first pattern in the list, not the second one, because the patterns are tested in order, and the first one is the first test to pass. Here, Django would call the function views.special_case_2003(request).
  • /articles/2003/03/building-a-django-site/ would match the final pattern. Django would call the function views.article_detail(request, year=2003, month=3, slug="building-a-django-site").
  • /articles/2003 would not match any of these patterns, because each pattern requires that the URL end with a slash.

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urls.py

• a .py file to store URLconf variable urlpatterns.

• urlpatterns should be a sequence of path() and/or re_path() instances.

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URL namespaces

• URL namespaces
  • allow to uniquely reverse named URL patterns even if different applications use the same URL names.
  • It’s a good practice for third-party apps to always use namespaced URLs.
• A URL namespace comes in two parts:
  • application namespace
    • the name of the application that is being deployed.
    • Every instance of a single application will have the same application namespace.
  • instance namespace
    • identifies a specific instance of an application
    • should be unique across your entire project.
    • The instance namespce for the default instance of an application can be the same as the application namespace.
• Namespaced URLs:
  • specified using the : operator.
    • 'admin:index': indicates a namespace of ‘admin’, and a named URL of ‘index’.
  • Namespaces can also be nested.
    • 'sports:polls:index': look for a pattern named ‘index’ in the namespace ‘polls’ that is itself defined within the top-level namespace ‘sports’.
• 常见错误:
  • 引号内多余的空格: ' app_name:url' , 'app_name:url ', 'app_name :url', or 'app_name: url'都不能匹配'app_name:url'
• Application namespaces of included URLconfs can be specified using app_name = '<app_name>'

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Example: Namespace

• urls.py at the project level

# both url refering to the same urls.py file. 
# But due to the defferent namespace, urls have different prefix.
path('employee/', include("EmpApp.urls",namespace="employee")),
path('department/', include("EmpApp.urls",namespace="department")),

• urls.py within EmpApp application

app_name = "EmpApp"

urlpatterns = [
    path("", view=views.emp_list, name="index"),
    path("emp/list/", view=views.emp_list, name="emp_list"),
    path("emp/update/<int:emp_id>", view=views.emp_update, name="emp_update"),

    path("dept/list/", view=views.dept_list, name="dept_list"),
    path("dept/update/<int:dept_id>", view=views.dept_update, name="dept_update"),
]

• Template:

<!-- 
  Using `employee:url`, to reverese the url as '/employee/'.
  Then further into '/employee/emp/list/
 -->
<a href="-/ url 'employee:emp_list' -\">List</a>

<!-- 
  Using `department:url`, to reverese the url as '/department/'.
  Then further into '/department/dept/list/
 -->
<a href="-/ url 'department:dept_list' -\">List</a>


<!-- 
  EmpApp is an application namespace which defined in the EmpApp's urls.py.
  Djnago locate this url file to find emp_detail url
 -->
<a href="-/ url 'EmpApp:emp_detail' emp.id -\">Edit</a>

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Example: Admin page

• Admin page can be call by
  • Using the row url:
    • <a href='/admin/'>Admin</a>
  • Using namespaceadmin:
    • <a href='-/ url 'admin:index' -\'>Admin</a>

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