Subject: Re: __getitem__ and lookups in templates - msg#02159

On Mon, Aug 31, 2009 at 3:48 PM, efege <fjgomez@xxxxxxxxx> wrote:

Hi,

According to the docs,

"... when the template system encounters a dot in a variable name, it
tries the following lookups, in this order:

   * Dictionary lookup. Example: foo["bar"]
   * Attribute lookup. Example: foo.bar
   * Method call. Example: foo.bar()
   * List-index lookup. Example: foo[bar]

The template system uses the first lookup type that works. It's short-
circuit logic."

I wonder what's the exact meaning of "works" in the last paragraph...

This is my problem: my template's variables are instances of a class
which has a special method __getitem__ defined. It also has a method
foo. So, when I want my template to invoke the method foo of that same
class, I wrote something like

   someobject.foo

And guess what... dictionary lookup, i.e. __getitem__('foo'), takes
precedence, and so the method foo is not invoked!

In my case, __getitem__ only accepts as valid certain specific
arguments, and 'foo' is not valid, so that __getitem__('foo') might
raise an exception, or return None. But in any case, the fact is that
Django *stops* trying the other possible lookups.

Your __getitem__ might raise an exception, or return None?  Which does it do?

Returning None will be seen by the template system as "working", as it is perfectly valid for None to be a value stored in a dictionary.  You will need to make this method raise an appropriate exception to make the template system consider the dictionary lookup to have failed.  Here's where the template code tries these different lookups in order:

http://code.djangoproject.com/browser/django/tags/releases/1.1/django/template/__init__.py#L712

Based on that code, a dictionary lookup that raises TypeError, AttributeError, or KeyError will cause the template code to continue with trying an attribute lookup, so it is one of those three specifically that you will need to raise in the case where you want the dictionary lookup to be seen as failing.

Karen

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