Jinja2 Documentation

# Jinja

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# Frequently Asked Questions

This page answers some of the often asked questions about Jinja.

## Why is it called Jinja?

The name Jinja was chosen because it’s the name of a Japanese temple and temple and template share a similar pronunciation. It is not named after the capital city of Uganda.

## How fast is it?

We really hate benchmarks especially since they don’t reflect much. The performance of a template depends on many factors and you would have to benchmark different engines in different situations. The benchmarks from the testsuite show that Jinja2 has a similar performance to Mako and is between 10 and 20 times faster than Django’s template engine or Genshi. These numbers should be taken with tons of salt as the benchmarks that took these numbers only test a few performance related situations such as looping. Generally speaking the performance of a template engine doesn’t matter much as the usual bottleneck in a web application is either the database or the application code.

## How Compatible is Jinja2 with Django?

The default syntax of Jinja2 matches Django syntax in many ways. However this similarity doesn’t mean that you can use a Django template unmodified in Jinja2. For example filter arguments use a function call syntax rather than a colon to separate filter name and arguments. Additionally the extension interface in Jinja is fundamentally different from the Django one which means that your custom tags won’t work any longer.

Generally speaking you will use much less custom extensions as the Jinja template system allows you to use a certain subset of Python expressions which can replace most Django extensions. For example instead of using something like this:

    {% load comments %}
    {% get_latest_comments 10 as latest_comments %}
    {% for comment in latest_comments %}
        ...
    {% endfor %}

You will most likely provide an object with attributes to retrieve comments from the database:

    {% for comment in models.comments.latest(10) %}
        ...
    {% endfor %}

Or directly provide the model for quick testing:

    {% for comment in Comment.objects.order_by('-pub_date')[:10] %}
        ...
    {% endfor %}

Please keep in mind that even though you may put such things into templates it still isn’t a good idea. Queries should go into the view code and not the template!

## Isn’t it a terrible idea to put Logic into Templates?

Without a doubt you should try to remove as much logic from templates as possible. But templates without any logic mean that you have to do all the processing in the code which is boring and stupid. A template engine that does that is shipped with Python and called string.Template. Comes without loops and if conditions and is by far the fastest template engine you can get for Python.

So some amount of logic is required in templates to keep everyone happy. And Jinja leaves it pretty much to you how much logic you want to put into templates. There are some restrictions in what you can do and what not.

Jinja2 neither allows you to put arbitrary Python code into templates nor does it allow all Python expressions. The operators are limited to the most common ones and more advanced expressions such as list comprehensions and generator expressions are not supported. This keeps the template engine easier to maintain and templates more readable.

## Why is Autoescaping not the Default?

There are multiple reasons why automatic escaping is not the default mode and also not the recommended one. While automatic escaping of variables means that you will less likely have an XSS problem it also causes a huge amount of extra processing in the template engine which can cause serious performance problems. As Python doesn’t provide a way to mark strings as unsafe Jinja has to hack around that limitation by providing a custom string class (the `Markup` string) that safely interacts with safe and unsafe strings.

With explicit escaping however the template engine doesn’t have to perform any safety checks on variables. Also a human knows not to escape integers or strings that may never contain characters one has to escape or already HTML markup. For example when iterating over a list over a table of integers and floats for a table of statistics the template designer can omit the escaping because he knows that integers or floats don’t contain any unsafe parameters.

Additionally Jinja2 is a general purpose template engine and not only used for HTML/XML generation. For example you may generate LaTeX, emails, CSS, JavaScript, or configuration files.

## Why is the Context immutable?

When writing a `contextfunction()` or something similar you may have noticed that the context tries to stop you from modifying it. If you have managed to modify the context by using an internal context API you may have noticed that changes in the context don’t seem to be visible in the template. The reason for this is that Jinja uses the context only as primary data source for template variables for performance reasons.

If you want to modify the context write a function that returns a variable instead that one can assign to a variable by using set:

    {% set comments = get_latest_comments() %}

## I don’t have the _speedups Module. Is Jinja slower now?

To achieve a good performance with automatic escaping enabled, the escaping function is also implemented in pure C and used if Jinja2 was installed with the speedups module. This happens automatically if a C compiler is available on the system during installation.

## My tracebacks look weird. What’s happening?

If the speedups module is not compiled and you are using a Python installation without ctypes (Python 2.4 without ctypes, Jython or Google’s AppEngine) Jinja2 is unable to provide correct debugging information and the traceback may be incomplete. There is currently no good workaround for Jython or the AppEngine as ctypes is unavailable there and it’s not possible to use the speedups extension.

## Why is there no Python 2.3 support?

Python 2.3 is missing a lot of features that are used heavily in Jinja2. This decision was made as with the upcoming Python 2.6 and 3.0 versions it becomes harder to maintain the code for older Python versions. If you really need Python 2.3 support you either have to use Jinja 1 or other templating engines that still support 2.3.

## My Macros are overriden by something

In some situations the Jinja scoping appears arbitrary:

layout.tmpl:

    {% macro foo() %}LAYOUT{% endmacro %}
    {% block body %}{% endblock %}

child.tmpl:

    {% extends 'layout.tmpl' %}
    {% macro foo() %}CHILD{% endmacro %}
    {% block body %}{{ foo() }}{% endblock %}

This will print `LAYOUT` in Jinja2. This is a side effect of having the parent template evaluated after the child one. This allows child templates passing information to the parent template. To avoid this issue rename the macro or variable in the parent template to have an uncommon prefix.