How to Create Jinja2 Filters in Flask.

Flask uses Jinja2 as a template engine. Although you are free to use any other template engine, Jinja2 is a de-facto industry standard. In this post, I'll talk about filters. Jinja2 has a list of built-in filters, and Flask leverages them. For example, when you want to capitalize a string:

<p>{{ name|capitalize }}</p>

This way capitalize function receives the name value as parameter and returns a formatted string. You can also invoke it as a function to pass extra parameters, like with default:

<p>{{ name|default("Robot") }}</p>

How to Register Custom Jinja2 Filter?

If you want to register your filter there are two ways to do this in Flask. You can either use template_filter decorator, or you can put them to jinja_env. Decorators can be defined app-wise or at the blueprint level. I prefer to define filters globally via jinja_env.

Extending jinja_env

def locale_code_to_name_filter(locale_code: str) -> str:
    if locale_code == "en_us":
        return "English (US)"

    return locale_code


app.jinja_env.filters['lc_name'] = locale_code_to_name_filter

Using template_filter decorator

@app.template_filter("lc_name")
def locale_code_to_name_filter(locale_code: str) -> str:
    if locale_code == "en_us":
        return "English (US)"

    return locale_code

Then in your templates:

<p>{{ locale|lc_name }}</p>

• Builtin Filters