Template language review

First, let's quickly review a number of terms introduced in Chapter 3, Templates:

• A template is a text document, or a normal Python string, that is marked up using the Django template language. A template can contain template tags and variables.

• A template tag is a symbol within a template that does something. This definition is deliberately vague. For example, a template tag can produce content, serve as a control structure (an if statement or for loop), grab content from a database, or enable access to other template tags.

Template tags are surrounded by {% and %}:

        {% if is_logged_in %} 
            Thanks for logging in! 
        {% else %} 
            Please log in. 
        {% endif %} 

• A variable is a symbol within a template that outputs a value.

• Variable tags are surrounded by {{ and }}:

• A context is a name->value mapping (similar to a Python dictionary) that is passed to a template.

• A template renders a context by replacing...