What is Jinja2 and how is it coupled with Flask?

Jinja2 is a library found at http://jinja.pocoo.org/; you can use it to produce formatted text with bundled logic. Unlike the Python format function, which only allows you to replace markup with variable content, you can have a control structure, such as a for loop, inside a template string and use Jinja2 to parse it. Let's consider this example:

from jinja2 import Template
x = """
<p>Uncle Scrooge nephews</p>
<ul>
{% for i in my_list %}
<li>{{ i }}</li>
{% endfor %}
</ul>
"""
template = Template(x)
# output is an unicode string
print template.render(my_list=['Huey', 'Dewey', 'Louie'])

In the preceding code, we have a very simple example where we create a template string with a for loop control structure ("for tag", for short) that iterates over a list variable called my_list and prints the element inside a "li HTML tag" using curly braces {{ }} notation.

Notice that you could call render in the template instance...