Expr Tag

• version: 0.2
• author: limodou@gmail.com

Updates

• 2006/12/03 If you don't add "as variable", then the result will directly output to template.

Code

from django import template
from django.utils.translation import gettext_lazy as _
import re

register = template.Library()

class ExprNode(template.Node):
    def __init__(self, expr_string, var_name):
        self.expr_string = expr_string
        self.var_name = var_name
    
    def render(self, context):
        try:
            clist = list(context)
            clist.reverse()
            d = {}
            d['_'] = _
            for c in clist:
                d.update(c)
            if self.var_name:
                context[self.var_name] = eval(self.expr_string, d)
                return ''
            else:
                return str(eval(self.expr_string, d))
        except:
            raise

r_expr = re.compile(r'(.*?)\s+as\s+(\w+)', re.DOTALL)    
def do_expr(parser, token):
    try:
        tag_name, arg = token.contents.split(None, 1)
    except ValueError:
        raise template.TemplateSyntaxError, "%r tag requires arguments" % token.contents[0]
    m = r_expr.search(arg)
    if m:
        expr_string, var_name = m.groups()
    else:
        if not arg:
            raise template.TemplateSyntaxError, "%r tag at least require one argument" % tag_name
            
        expr_string, var_name = arg, None
    return ExprNode(expr_string, var_name)
do_expr = register.tag('expr', do_expr)

How to use it

{% expr "1" as var1 %}
{% expr [0, 1, 2] as var2 %}
{% expr _('Menu') as var3 %}
{% expr var1 + "abc" as var4 %}

for 0.2 version

{% expr 3 %}
{% expr "".join(["a", "b", "c"]) %}

Will directly output the result to template

Syntax

{% expr python_expression as variable_name %}

python_expression can be valid python expression, and you can even use _() to translate a string. Expr tag also can used context variables.