Flask uses the MVC (Model View Controller) Framework. Just to make this as obvious as possible, I like my Flask apps to have the following.

• Model -> models.py
• View -> views.py
• Controller -> __init__.py

Typically, your CSS files will be stored in the static directory, something like this.

├── main.py
├── database (directory)
│   ├── example.db
├── my-project (directory)
│   ├── __init__.py
│   ├── views.py
│   ├── models.py
│   ├── templates (directory)
│   │   ├── base.html
│   │   ├── home.html
│   │   ├── results.html
│   └── static (directory)
│       └── custom.css

In your view (views.py in this example), here is how you can create a function that can be used in your HTML files.

@views.context_processor
def utility_processor():
    def greeting():
      return 'Hello World'
    def return_long_month_name(month):
        if month == '01':
            return 'January'
        elif month == '02':
            return 'February'
        elif month == '03':
            return 'March'
        elif month == '04':
            return 'April'
        elif month == '05':
            return 'May'
        elif month == '06':
            return 'June'
        elif month == '07':
            return 'July'
        elif month == '08':
            return 'August'
        elif month == '09':
            return 'September'
        elif month == '10':
            return 'October'
        elif month == '11':
            return 'November'
        elif month == '12':
            return 'December'
        else:
            return 'Unknown'
    return dict(greeting=greeting, return_long_month_name=return_long_month_name)

And here is how you would use these functions in your HTML templates.

<p> {{ greeting() }} </p>
<p> {{ return_long_month_name('01') }} </p>