1   Flask — Templates¶

1.1   Jinja Setup¶

Unless customized, Jinja2 is configured by Flask as follows:

• autoescaping is enabled for all templates ending in .html, .htm, .xml as well as .xhtml when using render_template().
• autoescaping is enabled for all strings when using render_template_string().
• a template has the ability to opt in/out autoescaping with the {% autoescape %} tag.
• Flask inserts a couple of global functions and helpers into the Jinja2 context, additionally to the values that are present by default.

1.2   Standard Context¶

The following global variables are available within Jinja2 templates by default:

config

The current configuration object (flask.config)

New in version 0.6.

Changed in version 0.10: This is now always available, even in imported templates.

request

The current request object (flask.request). This variable is unavailable if the template was rendered without an active request context.

session

The current session object (flask.session). This variable is unavailable if the template was rendered without an active request context.

g

The request-bound object for global variables (flask.g). This variable is unavailable if the template was rendered without an active request context.

url_for()

The flask.url_for() function.

get_flashed_messages()

The flask.get_flashed_messages() function.

{% from '_helpers.html' import my_macro with context %}

1.3   Standard Filters¶

These filters are available in Jinja2 additionally to the filters provided by Jinja2 itself:

tojson()

This function converts the given object into JSON representation. This is for example very helpful if you try to generate JavaScript on the fly.

Note that inside script tags no escaping must take place, so make sure to disable escaping with |safe before Flask 0.10 if you intend to use it inside script tags:

<script type=text/javascript>
    doSomethingWith({{ user.username|tojson|safe }});
</script>

1.4   Controlling Autoescaping¶

Autoescaping is the concept of automatically escaping special characters for you. Special characters in the sense of HTML (or XML, and thus XHTML) are &, >, <, " as well as '. Because these characters carry specific meanings in documents on their own you have to replace them by so called “entities” if you want to use them for text. Not doing so would not only cause user frustration by the inability to use these characters in text, but can also lead to security problems. (see Cross-Site Scripting (XSS))

Sometimes however you will need to disable autoescaping in templates. This can be the case if you want to explicitly inject HTML into pages, for example if they come from a system that generates secure HTML like a markdown to HTML converter.

There are three ways to accomplish that:

• In the Python code, wrap the HTML string in a Markup object before passing it to the template. This is in general the recommended way.
• Inside the template, use the |safe filter to explicitly mark a string as safe HTML ({{ myvariable|safe }})
• Temporarily disable the autoescape system altogether.

To disable the autoescape system in templates, you can use the {% autoescape %} block:

{% autoescape false %}
    <p>autoescaping is disabled here
    <p>{{ will_not_be_escaped }}
{% endautoescape %}

Whenever you do this, please be very cautious about the variables you are using in this block.

1.5   Registering Filters¶

If you want to register your own filters in Jinja2 you have two ways to do that. You can either put them by hand into the jinja_env of the application or use the template_filter() decorator.

The two following examples work the same and both reverse an object:

@app.template_filter('reverse')
def reverse_filter(s):
    return s[::-1]

def reverse_filter(s):
    return s[::-1]
app.jinja_env.filters['reverse'] = reverse_filter

In case of the decorator the argument is optional if you want to use the function name as name of the filter. Once registered, you can use the filter in your templates in the same way as Jinja2’s builtin filters, for example if you have a Python list in context called mylist:

{% for x in mylist | reverse %}
{% endfor %}

1.6   Context Processors¶

To inject new variables automatically into the context of a template, context processors exist in Flask. Context processors run before the template is rendered and have the ability to inject new values into the template context. A context processor is a function that returns a dictionary. The keys and values of this dictionary are then merged with the template context, for all templates in the app:

@app.context_processor
def inject_user():
    return dict(user=g.user)

The context processor above makes a variable called user available in the template with the value of g.user. This example is not very interesting because g is available in templates anyways, but it gives an idea how this works.

Variables are not limited to values; a context processor can also make functions available to templates (since Python allows passing around functions):

@app.context_processor
def utility_processor():
    def format_price(amount, currency=u'€'):
        return u'{0:.2f}{1}'.format(amount, currency)
    return dict(format_price=format_price)

The context processor above makes the format_price function available to all templates:

{{ format_price(0.33) }}

You could also build format_price as a template filter (see 1.5   Registering Filters), but this demonstrates how to pass functions in a context processor.