Django template tag setting variable to context with an if/else condition

if_else_as.py

from django.template.defaulttags import TemplateIfParser
from django import template

register = template.Library()

class IfElseAs(template.Node):
    def __init__(self, condition, if_var, else_var, var_name):
        self.condition = condition
        self.if_var = template.Variable(if_var)
        self.else_var = template.Variable(else_var)
        self.var_name = var_name

    def render(self, context):
        context[self.var_name] = (self.if_var if self.condition.eval(context) else self.else_var).resolve(context)
        return ""

@register.tag
def if_else_as(parser, token):
    """ Usage: 
    {% if_else_as condition then if_var else else_var as var_name %}
    where
    condition: a multi part condition separated by spaces; this uses the same evaluation as Django's own `{% if` tag
    if_var: the value or variable name used when condition is true
    else_var: the value or variable name used when condition is false
    var_name: the name of the variable added to/set in the template context
    
    Other parts of the template tags (then/else/as) are only there for readability
    
    e.g.
    {% if_else_as author == "Douglas Adams" then 42 else a_context_var as answer_to_universe %}
    {{answer_to_universe}} # outputs 42 assuming the context variable `author` is "Douglas Adams"
    """
    
    parts = token.split_contents()
    _tag_name, *expressions, _then, if_var, _else, else_var, _as, var_name = parts
    condition = TemplateIfParser(parser, expressions).parse()
    return IfElseAs(condition, if_var, else_var, var_name)