Frequently Asked Questions¶
Why is it called Jinja?¶
How fast is Jinja?¶
Jinja is relatively fast among template engines because it compiles and caches template code to Python code, so that the template does not need to be parsed and interpreted each time. Rendering a template becomes as close to executing a Python function as possible.
Jinja also makes extensive use of caching. Templates are cached by name after loading, so future uses of the template avoid loading. The template loading itself uses a bytecode cache to avoid repeated compiling. The caches can be external to persist across restarts. Templates can also be precompiled and loaded as fast Python imports.
We dislike benchmarks because they don’t reflect real use. Performance depends on many factors. Different engines have different default configurations and tradeoffs that make it unclear how to set up a useful comparison. Often, database access, API calls, and data processing have a much larger effect on performance than the template engine.
Isn’t it a bad idea to put logic in templates?¶
Without a doubt you should try to remove as much logic from templates as
possible. With less logic, the template is easier to understand, has
fewer potential side effects, and is faster to compile and render. But a
template without any logic means processing must be done in code before
rendering. A template engine that does that is shipped with Python,
string.Template, and while it’s definitely fast it’s not
Jinja’s features such as blocks, statements, filters, and function calls make it much easier to write expressive templates, with very few restrictions. Jinja doesn’t allow arbitrary Python code in templates, or every feature available in the Python language. This keeps the engine easier to maintain, and keeps templates more readable.
Some amount of logic is required in templates to keep everyone happy. Too much logic in the template can make it complex to reason about and maintain. It’s up to you to decide how your application will work and balance how much logic you want to put in the template.
Why is HTML escaping not the default?¶
Jinja provides a feature that can be enabled to escape HTML syntax in rendered templates. However, it is disabled by default.
While automatic escaping means that you are less likely have an XSS problem, it also requires significant extra processing during compiling and rendering, which can reduce performance. Jinja uses MarkupSafe for escaping, which provides optimized C code for speed, but it still introduces overhead to track escaping across methods and formatting.