On 15 Sep 2014 05:09, "Chris Angelico" wrote:

By the way:

On Mon, Sep 15, 2014 at 4:48 AM, John Wong wrote:

...

The main challenge:

Where and how do we actually look for what attributes are relevant? namedtuple can do it because it has __slot__ and we know in advance how

many

...

attributes are set. In regular class, we deal with dynamic attribute setting, single and inheritances. I don't have an answer for this simply because I lack of experience. We can certainly start with the attributes set in the main instance and one level up in the inheritance chain.

This is a fundamentally hard problem. Obviously it's easy to see what attributes are set, but figuring out which are relevant is usually a job for the class itself. So what you might want to do is have a class attribute, and then have your custom __repr__ scan through all of __bases__, collecting up these "important attributes".

More generally, this is the kind of situation where we're more likely to provide better tools for *writing* these kinds of representations, rather than providing them by default (the latter approach is too likely to go wrong). There are currently two such key plumbing modules: pprint: https://docs.python.org/3/library/pprint.html reprlib: https://docs.python.org/3/library/reprlib.html Redesigning the way pprint works to make it easier to customise is what Łukasz had in mind when writing PEP 443 to add functools.singledispatch to Python 3.4. It also makes it easier to write your own pretty printers and custom repr functions that fall back to object introspection to provide more details. reprlib is currently focused on providing object representations that aren't overwhelmingly long, even for large (or recursively nested) containers. However, I seem to recall also seeing proposals on the tracker to adding functions there that make it easier to emit named tuple style representations for objects. More generally, the idea of a "functionally equivalent named tuple" is also relevant to implementing hashing, equality comparisons and ordering operations in a sensible way, so there's actually potential here for a third party module dedicated specifically to making it easier to write classes that behave that way. Regards, Nick.