When you installed flask into your virtualenv, along with it came a Python-based templating engine called Jinja2.

In this walkthrough, you'll see some basics about how templates work, and get to know what sorts of options they provide you for creating HTML from a Python process.

"I enjoy writing HTML in Python"

-- nobody, ever

A good framework will provide some way of generating HTML with a templating system.

There are nearly as many templating systems as there are frameworks

Each has advantages and disadvantages

Flask includes the Jinja2 templating system (perhaps because it's built by the same folks)

Let's start with the absolute basics.

(flaskenv)$ python
>>> from jinja2 import Template

>>> t1 = Template("Hello {{ name }}, how are you?")

Call the render method, providing some context:

>>> t1.render(name="Freddy")
u'Hello Freddy, how are you?'
>>> t1.render({'name': "Roberto"})
u'Hello Roberto, how are you?'
>>>

Context can either be keyword arguments, or a dictionary

Dictionaries passed in as part of the context can be addressed with either subscript or dotted notation:

>>> person = {'first_name': 'Frank',
...           'last_name': 'Herbert'}
>>> t2 = Template("{{ person.last_name }}, {{ person['first_name'] }}")
>>> t2.render(person=person)
u'Herbert, Frank'

• Jinja2 will try the correct way first (attr for dotted, item for subscript).
• If nothing is found, it will try the opposite.
• If nothing is found, it will return an undefined object.

The exact same is true of objects passed in as part of context:

>>> t3 = Template("{{ obj.x }} + {{ obj['y'] }} = Fun!")
>>> class Game(object):
...   x = 'babies'
...   y = 'bubbles'
...
>>> bathtime = Game()
>>> t3.render(obj=bathtime)
u'babies + bubbles = Fun!'

This means your templates can be a bit agnostic as to the nature of the things in context

You can apply filters to the data passed in context with the pipe ('|') operator:

t4 = Template("shouted: {{ phrase|upper }}")
>>> t4.render(phrase="this is very important")
u'shouted: THIS IS VERY IMPORTANT'

t5 = Template("confusing: {{ phrase|upper|reverse }}")
>>> t5.render(phrase="howdy doody")
u'confusing: YDOOD YDWOH'

Logical control structures are also available:

tmpl = """
... {% for item in list %}{{ item }}, {% endfor %}
... """
>>> t6 = Template(tmpl)
>>> t6.render(list=[1,2,3,4,5,6])
u'\n1, 2, 3, 4, 5, 6, '

Any control structure introduced in a template must be paired with an explicit closing tag ({% for %}...{% endfor %})

There are a number of specialized tests available for use with the if...elif...else control structure:

>>> tmpl = """
... {% if phrase is upper %}
...   {{ phrase|lower }}
... {% elif phrase is lower %}
...   {{ phrase|upper }}
... {% else %}{{ phrase }}{% endif %}"""
>>> t7 = Template(tmpl)
>>> t7.render(phrase="FOO")
u'\n\n  foo\n'
>>> t7.render(phrase="bar")
u'\n\n  BAR\n'
>>> t7.render(phrase="This should print as-is")
u'\nThis should print as-is'

Basic Python expressions are also supported:

tmpl = """
... {% set sum = 0 %}
... {% for val in values %}
... {{ val }}: {{ sum + val }}
...   {% set sum = sum + val %}
... {% endfor %}
... """
>>> t8 = Template(tmpl)
>>> t8.render(values=range(1,11))
u'\n\n\n1: 1\n  \n\n2: 3\n  \n\n3: 6\n  \n\n4: 10\n
  \n\n5: 15\n  \n\n6: 21\n  \n\n7: 28\n  \n\n8: 36\n
  \n\n9: 45\n  \n\n10: 55\n  \n'

There's more that Jinja2 templates can do, and you'll see more in class when we write templates for our Flask app.

http://jinja.pocoo.org/docs/templates/