Performing a getattr() style lookup in a django template

I also had to write this code as a custom template tag recently. To handle all look-up scenarios, it first does a standard attribute look-up, then tries to do a dictionary look-up, then tries a getitem lookup (for lists to work), then follows standard Django template behavior when an object is not found.

(updated 2009-08-26 to now handle list index lookups as well)

# app/templatetags/getattribute.py

import re
from django import template
from django.conf import settings

numeric_test = re.compile("^\d+$")
register = template.Library()

def getattribute(value, arg):
    """Gets an attribute of an object dynamically from a string name"""

    if hasattr(value, str(arg)):
        return getattr(value, arg)
    elif hasattr(value, 'has_key') and value.has_key(arg):
        return value[arg]
    elif numeric_test.match(str(arg)) and len(value) > int(arg):
        return value[int(arg)]
    else:
        return settings.TEMPLATE_STRING_IF_INVALID

register.filter('getattribute', getattribute)

Template usage:

{% load getattribute %}
{{ object|getattribute:dynamic_string_var }}

I don't think so. But it wouldn't be too hard to write a custom template tag to return an attribute in the context dict. If you're simply trying to return a string, try something like this:

class GetAttrNode(template.Node):
    def __init__(self, attr_name):
        self.attr_name = attr_name

    def render(self, context):
        try:
            return context[self.attr_name]
        except:
            # (better yet, return an exception here)
            return ''

@register.tag
def get_attr(parser, token):
    return GetAttrNode(token)

Note that it's probably just as easy to do this in your view instead of in the template, unless this is a condition that is repeated often in your data.

I ended up adding a method to the model in question, and that method can be accessed like an attribute in the template.

Still, i think it would be great if a built in tag allowed you to dynamically lookup an attribute, since this is a problem a lot of us constantly have in our templates.

Keeping the distinction between get and getattr,

@register.filter(name='get')
def get(o, index):
    try:
        return o[index]
    except:
        return settings.TEMPLATE_STRING_IF_INVALID


@register.filter(name='getattr')
def getattrfilter(o, attr):
    try:
        return getattr(o, attr)
    except:
        return settings.TEMPLATE_STRING_IF_INVALID

There isn't a built-in tag, but it shouldn't be too difficult to write your own.

I have developed a more generic solution based on @fotinakis solution, it serves to find the value of a string expression whether it is an attribute or a function, and also supports objects chaining.

import re
import types

numeric_test = re.compile("^\d+$")
register = template.Library()

def get_attr(object, arg):
    if hasattr(object, str(arg)):
        attr = getattr(object, arg)
        if type(getattr(object, str(arg))) == types.MethodType:
            return attr()
        return attr
    elif hasattr(object, 'has_key') and object.has_key(arg):
        return object[arg]
    elif numeric_test.match(str(arg)) and len(object) > int(arg):
        return object[int(arg)]
    else:
        return object


@register.simple_tag(takes_context=True)
def get_by_name(context, name):
    """"Get variable by string name {% get_by_name data_name.data_func... %}"""
    print(context['instance'].get_edit_url())
    arr = name.split('.')
    obj = arr[0]
    object = context[obj]
    if len(arr) > 1:
        for ar in arr:
            object = get_attr(object, ar)
    return object