To decorate an imported function in Python you would do something like this

# in ./lib.py
def function_name():
    # function body

And then in your program you could decorate it like this

from lib import function_name
function_name = decorator_expression(function_name)

It's a little different in Flask because we have an app context.

Normally, you would create a custom filter like this

app = Flask(__name__)
@app.template_filter('function_name')
def function_name():
    # do something

To accomplish the same end result with custom filters that are imported, we will register our custom filter in jinja_env.filters

from flask import Flask
from lib import function_name

app = Flask(__name__)
app.jinja_env.filters['function_name'] = function_name

This will let us use our function_name filter in our templates

If have a lot of filters you could loop over an array like so

for f in [function_name, other_function_name]:
    app.jinja_env.filters[f.__name__] = f

Using the __name__ method on the function gives us the method name as a string that will be available as our template filter.

We could use it in templates the same way

{{'hello world' | function_name}}