django-fastdev

May 15, 2025 · View on GitHub

:code:django-fastdev is an app that makes it safer, faster and more fun to develop Django apps.

Features

Error on non-existent template variables


Django templates by default hide errors, and when it does show an error it's often not very helpful. This app changes this behavior to provide more informative error messages. For example, if you use:

.. code:: html

    {{ does_not_exist }}

instead of rendering that as an empty string, this app will raise a :code:`FastDevVariableDoesNotExist` error with a detailed message:

.. code::

    does_not_exist does not exist in context. Available top level variables:

        DEFAULT_MESSAGE_LEVELS
        False
        None
        True
        bar
        csrf_token
        foo
        messages
        perms
        request
        user

There are more specialized error messages for accessing non-existent keys in a :code:`dict` or attributes of an object several levels deep, such as :code:`foo.bar.baz` (where :code:`baz` doesn't exist).

**Handling `default` and `default_if_none` Filters**

When using the :code:`default` or :code:`default_if_none` filters, :code:`django-fastdev` will not raise an exception for non-existent variables. Instead, it behaves as one might intuitively expect by populating the context variable with the result of the filter operation. For example:

.. code:: html

    {{ does_not_exist|default:"N/A" }}
    {{ does_not_exist|default_if_none:"" }}

In these cases:
  * If :code:`does_not_exist` is undefined, :code:`default:"N/A"` will render as :code:`N/A`, and :code:`default_if_none:""` will render as an empty string (:code:`""`).
  * This ensures that templates using these filters handle missing variables gracefully, aligning with Django's built-in behavior while maintaining :code:`django-fastdev`'s strict checking for other cases.

By default, :code:`django-fastdev` only checks templates that exist within your project directory. To check ALL templates, including stock Django templates and templates from third-party libraries, add :code:`FASTDEV_STRICT_TEMPLATE_CHECKING = True` to your project :code:`settings.py`.


Improved TemplateDoesNotExist errors
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Good suggestions for what you wanted to do, and a complete list of all valid values makes it very easy to fix `TemplateDoesNotExist` errors.


NoReverseMatch errors
~~~~~~~~~~~~~~~~~~~~~

Have you ever gotten this error?

.. code::

    django.urls.exceptions.NoReverseMatch: Reverse for 'view-name' with arguments '('',)' not found. 1 pattern(s) tried:


It's because you have :code:`{% url 'view-name' does_not_exist %}`. Django sees
:code:`does_not_exist` and evaluates it to the empty string because it doesn't exist.
So that's why you get an error message that makes no sense. :code:`django-fastdev` will
make your code crash on the actual error: :code:`does_not_exist` doesn't exist.


Error if you have non-space text outside a block when extending

A common mistake for beginners that can be hard to spot is when they do:

.. code-block:: html

<html>
    {% extends "something.html" %}
    stuff here
</html>

Django silently throws away :code:stuff here and :code:</html>. :code:django-fastdev makes this an error.

Error on invalid block names when using :code:{% extends "..." %}


If you have a base template:

..  code-block:: html

    <html>
        <body>
            {% block content %}{% endblock %}
        </body>
    </html>

and then write a template like this:

..  code-block:: html

    {% extends "base.html" %}

    {% block contents %}
        hello!
    {% endblock %}


Django will silently throw away `hello!` because you wrote :code:`contents` instead
of :code:`content`. :code:`django-fastdev` will turn this into an error which lists the
invalid and valid block names in alphabetical order.


Better error messages for reverse
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The standard error message for a bad :code:`reverse()/{% url %}` are rather sparse.
:code:`django-fastdev` improves them by listing valid patterns so you can easily see
the problem.


Better error messages for QuerySet.get()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The error message for :code:`QuerySet.get()` is improved to give you the query
parameters that resulted in the exception.


Validate clean_* methods
~~~~~~~~~~~~~~~~~~~~~~~~

A common mistake is to make a form clean method and make a spelling error. By
default Django just won't call the function. With :code:`django-fastdev` you will get
an error message telling you that your clean method doesn't match anything.

This is also very useful during refactoring. Renaming a field is a lot safer
as if you forget to rename the clean method :code:`django-fastdev` will tell you!

By default, :code:`django-fastdev` will check only forms that exist within your project,
and not third-party libraries. If you would like to enable stricter validation that will
extend to ALL forms, you can set this by configuring :code:`FASTDEV_STRICT_FORM_CHECKING`
to :code:`True` in your Django settings.


Faster startup
~~~~~~~~~~~~~~

The initial model checks can be quite slow on big projects. :code:`django-fastdev`
will move these checks to a separate thread, so the runserver startup time is
lowered, so you don't have to wait for the runserver restart as long.


Usage
------

First install: :code:`pip install django-fastdev`

In :code:`settings.py` add :code:`django_fastdev` to INSTALLED_APPS:

.. code:: python

    INSTALLED_APPS = [
        # ...
        'django_fastdev',
   ]


Enjoy a nicer Django experience!


License
-------

BSD