[Python-Dev] Python-Dev Digest, Vol 85, Issue 71

[John Nagle]

On 8/24/2010 12:40 AM, python-dev-request at python.org wrote:
> Message: 4 Date: Mon, 23 Aug 2010 17:21:50 -0700 From: Brett Cannon
> <brett at python.org>
> It is also non-obvious to any beginner. Are we really going to want to
> propagate the knowledge of this trick as a fundamental idiom? I would
> rather leave hasattr in that instance. But I'm +1 on only swallowing
> AttributeError.

    I'd argue that since the ability to inherit a class from "dict"
was added, dynamically adding attributes is somewhat obsolete.
An object instance is not a dictionary.  Especially since its
namespace interacts with the namespace of its class.

    I've been using Google Code Search to look at the actual use
cases for "setattr".  The main uses are:

    1.  Copying.  Object copying is done with "setattr".
        All the "setattr" objects occur during object
        construction, or shortly after.

    2.  Creating proxy objects for remote access.  This is
        much like copying,

    3.  Representing HTML objects as
        Python object.  This usually requies gyrations to
        avoid clashes with Python built-in names and
        functions; "class" is a common attribute in
        HTML, and a reserved word in Python, and some hack
        is necessary to make that work.  BeautifulSoup
        does this.

It's rare that attributes are added long after object
construction.  Perhaps a mechanism should be provided for
dynamically constructing an object.  Something like

	class foo(object) :
		pass

	attrdict = { a : 1, b : 2}
	make_object(foo, attrdict)

This covers most of the use cases for "setattr".

					John Nagle