Jinja filters are functions that can be applied to the result of an
expression in the {{ ... }} delimeters. It is applied before that
result is printed to the template.
<h2>{{ article.title|title }}</h2>In this code, the title filter will take article.title and return a
title-cased version, which will then be printed to the template. This
looks and works a lot like the UNIX practice of "piping" the output of
one program to another.
Note There are loads of built-in filters like title. See the full list in the Jinja docs.
We can define our own filters for use in our Jinja templates. As an
example, we'll implement a simple caps filter to capitalize all of the
letters in a string.
Note Jinja already has an upper filter that does this, and a capitalize
filter that capitalizes the first character and lowercases the rest.
These also handle unicode conversion, but we'll keep our example
simple to focus on the concept at hand.
We're going to define our filter in a module located at
myapp/util/filters.py. This gives us a util package in which to put
other miscellaneous modules.
# myapp/util/filters.py
from .. import app
@app.template_filter()
def caps(text):
"""Convert a string to all caps."""
return text.uppercase()In this code we are registering our function as a Jinja filter by using
the @app.template_filter() decorator. The default filter name is just
the name of the function, but you can pass an argument to the decorator
to change that.
@app.template_filter('make_caps')
def caps(text):
"""Convert a string to all caps."""
return text.uppercase()Now we can call make_caps in the template rather than caps:
{{ "hello world!"|make_caps }}.
To make our filter available in the templates, we just need to import it in our top-level init.py.
# myapp/__init__.py
# Make sure app has been initialized first to prevent circular imports.
from .util import filters