ifinlist template tag

from django.template import Library, Node, NodeList, TemplateSyntaxError, Variable
from django.utils.translation import ugettext as _

register = Library()

class IfInListNode( Node ):
    """
    Implements the actual logic of the ifinlist tag.
    """
    def __init__( self, value_var, list_var, nodelist_true, nodelist_false ):
        self.value_var, self.list_var = Variable( value_var ), Variable( list_var )
        self.nodelist_true, self.nodelist_false = nodelist_true, nodelist_false

    def __repr__( self ):
        return "<IfInListNode>"

    def render( self, context ):
        try:
            value_var = self.value_var.resolve( context )
        except VariableDoesNotExist:
            list_var = None
        
        try:
            list_var = self.list_var.resolve( context )
        except VariableDoesNotExist:
            list_var = None
        
        if not isinstance( list_var, list ):
            raise
            
        if value_var in list_var:
            return self.nodelist_true.render(context)
        
        return self.nodelist_false.render(context)

def do_ifinlist( parser, token ):
    """
    Checks if a value is in a list.
    
    Sample usage:
    
    Suppose that we have the following value and list variables passed to the template:

        letter = 'C'
        letters = [ 'A', 'B', 'C', 'D' ]
    
        {% ifinlist letter letters %}
            Yes
        {% else %}
            No
        {% endifinlist %}
    """
    bits = token.contents.split()
    len_bits = len( bits )
    
    if len_bits != 3:
        raise TemplateSyntaxError( _( '%s tag requires three arguments' ) % bits[ 0 ] )
    
    nodelist_true = parser.parse( ( 'else', 'endifinlist' ) )
    token = parser.next_token()
    
    if token.contents == 'else':
        nodelist_false = parser.parse( ( 'endifinlist', ) )
        parser.delete_first_token()
    else:
        nodelist_false = NodeList()

    return IfInListNode( bits[ 1 ], bits[ 2 ], nodelist_true, nodelist_false )

do_ifinlist = register.tag( 'ifinlist', do_ifinlist )