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Using custom Django URL path converters

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You will very likely have seen Django's standard converters when building your URL patterns.

For example:

  # urls.py

from django.urls import path

from myapp.views import post_detail

urlpatterns = [
path('posts/<int:post_id>/', post_detail, name="post_detail"),
]

The parameter post_id will be automatically converted to an int in your view args:

  def post_detail(request, post_id: int):
...

This functionality is handled by a converter, in this case the IntConverter, mapped to the namespace int:

  class IntConverter:
regex = '[0-9]+'

def to_python(self, value):
return int(value)

def to_url(self, value):
return str(value)

The converter class must provide a to_python() method that takes a string and converts it to some other value (in this case an int) and a to_url() method that takes a Python value and converts it into a URL path-friendly string. You also need a regex property that allows Django's router to identify this converter when parsing the inbound URL.

You can see other examples here.

Suppose you want to make your own custom converter? Let's say you have a use case where you want to add a date string to your path, and have Django convert that to a datetime.date instance as a view parameter. For example:

  http://example.com/posts/archive/2022-02-17/

The string 2022-02-17 is automatically converted to a datetime.date object in your view:

  def post_archive(request, archive_date: date):
...

To get this to work, we need to create a custom converter class, and then register it with Django:

  # urls.py

from django.urls import path
from datetime import date, datetime

from myapp.views import post_detail, post_archive

class DateConverter:
regex = r"\d{4}-\d{1,2}-\d{1,2}"
format = "%Y-%m-%d"

def to_python(self, value: str) -> date:
return datetime.strptime(value, self.format).date()

def to_url(self, value: date) -> str:
return value.strftime(self.format)

register_converter(DateConverter, "date")

urlpatterns = [
path('posts/<int:post_id>/', post_detail, name="post_detail"),
path('posts/archive/<date:archive_date>/', post_archive, name="post_archive"),
]

The DateConverter class is registered with the register_converter() function with the namespace "date". The regex will look for the pattern YYYY-MM-DD in the URL, and the parameter will be automatically converted into a datetime.date object.

The to_url() method on the other hand works in reverse. For example you can write:

  from datetime import date
from django.urls import reverse

reverse("post_archive", args=[date(year=2022, month=2, day=17)])

and you will get the output /posts/archive/2022-02-17/.

One further note: if the to_python() method raises a ValueError, Django will raise a 404 NOT FOUND. For example, if you try and create a datetime from an invalid date string, strptime() raises a ValueError:

  from datetime import datetime
datetime.strptime("2022-2-33", "%Y-%m-%d")

So this URL, even though it matches the DateConverter regex, will still raise a 404:

  http://example.com/posts/archive/2022-02-33/

Therefore take care that your to_python() method either implicitly or explicitly raises a ValueError if the conversion fails.