django - What is a NoReverseMatch error, and how do I fix it?

Question

I have some code and when it executes, it throws a NoReverseMatch, saying:

NoReverseMatch at /my_url/ Reverse for 'my_url_name' with arguments '()' and keyword arguments '{}' not found. n pattern(s) tried: []

What does this mean, and what can I do about it?

Answer 1

The NoReverseMatch error is saying that Django cannot find a matching url pattern for the url you've provided in any of your installed app's urls.

The NoReverseMatch exception is raised by django.core.urlresolvers when a matching URL in your URLconf cannot be identified based on the parameters supplied.

To start debugging it, you need to start by disecting the error message given to you.

• NoReverseMatch at /my_url/

  This is the url that is currently being rendered, it is this url that your application is currently trying to access but it contains a url that cannot be matched

• Reverse for 'my_url_name'

  This is the name of the url that it cannot find

• with arguments '()' and

  These are the non-keyword arguments its providing to the url

• keyword arguments '{}' not found.

  These are the keyword arguments its providing to the url

• n pattern(s) tried: []

  These are the patterns that it was able to find in your urls.py files that it tried to match against

Start by locating the code in your source relevant to the url that is currently being rendered - the url, the view, and any templates involved. In most cases, this will be the part of the code you're currently developing.

Once you've done this, read through the code in the order that django would be following until you reach the line of code that is trying to construct a url for your my_url_name. Again, this is probably in a place you've recently changed.

Now that you've discovered where the error is occuring, use the other parts of the error message to work out the issue.

The url name

• Are there any typos?
• Have you provided the url you're trying to access the given name?
• If you have set app_name in the app's urls.py (e.g. app_name = 'my_app') or if you included the app with a namespace (e.g. include('myapp.urls', namespace='myapp'), then you need to include the namespace when reversing, e.g. {% url 'myapp:my_url_name' %} or reverse('myapp:my_url_name').

Arguments and Keyword Arguments

The arguments and keyword arguments are used to match against any capture groups that are present within the given url which can be identified by the surrounding () brackets in the url pattern.

Assuming the url you're matching requires additional arguments, take a look in the error message and first take a look if the value for the given arguments look to be correct.

If they aren't correct:

• The value is missing or an empty string

  This generally means that the value you're passing in doesn't contain the value you expect it to be. Take a look where you assign the value for it, set breakpoints, and you'll need to figure out why this value doesn't get passed through correctly.

• The keyword argument has a typo

  Correct this either in the url pattern, or in the url you're constructing.

If they are correct:

• Debug the regex

  You can use a website such as regexr to quickly test whether your pattern matches the url you think you're creating, Copy the url pattern into the regex field at the top, and then use the text area to include any urls that you think it should match against.

  Common Mistakes:

  • Matching against the . wild card character or any other regex characters

    Remember to escape the specific characters with a prefix

  • Only matching against lower/upper case characters

    Try using either a-Z or w instead of a-z or A-Z

• Check that pattern you're matching is included within the patterns tried

  If it isn't here then its possible that you have forgotten to include your app within the INSTALLED_APPS setting (or the ordering of the apps within INSTALLED_APPS may need looking at)

Django Version

In Django 1.10, the ability to reverse a url by its python path was removed. The named path should be used instead.

---

If you're still unable to track down the problem, then feel free to ask a new question that includes what you've tried, what you've researched (You can link to this question), and then include the relevant code to the issue - the url that you're matching, any relevant url patterns, the part of the error message that shows what django tried to match, and possibly the INSTALLED_APPS setting if applicable.