Python lambdas in Jinja2

register it as a filter:

your_jinja_env.filters['arrow_class'] = arrow_class_from_deg

and in template:

<something class="{{ angle | arrow_class }}">blah</something>

you can use decorators to manage jinja filters easily:

class Filter(object):
    def __init__(self, filter_name=None):
         self.filter_name = filter_name

    def __call__(self, function):
         my_jinja_env.filters[self.filter_name or function.__name__] = function
         return function

@Filter()
def i_love_you(name):
    ''' say I love you to the name you entered. 
    usage: {{ "John" | i_love_you }} => "I Love You, John!"'''

    return "I Love You, %s!" %name