[Python-ideas] Classes inside functions [was Re: Add __parent__ to all classes, functions, and modules]

[Andrew Barnert]

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On Oct 6, 2014, at 10:23, Guido van Rossum <guido at python.org> wrote:

> On Mon, Oct 6, 2014 at 8:07 AM, Andrew Barnert <abarnert at yahoo.com.dmarc.invalid> wrote:
>> On Oct 5, 2014, at 20:04, Guido van Rossum <guido at python.org> wrote:
>>> On Sun, Oct 5, 2014 at 7:36 PM, Steven D'Aprano <steve at pearwood.info> wrote:
>>>> On Sun, Oct 05, 2014 at 02:43:13PM -0700, Guido van Rossum wrote:
>>>> 
>>>> > An alternative that wouldn't even require a metaclass would be to write a
>>>> > helper function that looks the class object up by name after parsing
>>>> > __qualname__. There are some limitations to this, e.g. classes defined
>>>> > inside functions -- but that's already a suspect pattern anyway.
>>>> 
>>>> I understand that "suspect pattern" doesn't mean "don't ever do this",
>>>> but I wonder what aspect of defining classes inside a function is
>>>> suspect. I think the suspect part is that each invocation of the
>>>> function creates a new class which (at least in the naive case) has the
>>>> same name, same functionality, and looks identical to the casual
>>>> glance but actually isn't.
>>>> …
>>>> There's
>>>> probably also implications for pickling as well. Are these the only
>>>> "suspect" parts of defining classes inside functions, or have I missed
>>>> something else?
>>> 
>>> Those (and their variations, e.g. using these in except clauses) are the main concerns; it also generally bothers me when something "smallish" like a function contains something "larger" like a class, especially when the class is substantial. It's another case of "flat is better than nested".
>>> 
>>> The only non-evil use of this pattern that I can recall is in unit tests
>> 
>> I agree that returning a class from a function is an uncommon special case, and any code that does so ought to have a good and obvious reason. But to say that such functions are evil doesn't seem right.
> 
> I didn't say *all* uses (or all uses outside unit tests) are evil. I still maintain that most non-test code doing this has a bad smell and is difficult to maintain. 

OK, I think I just misinterpreted you here. If you're just saying that this is enough of a red flag that you need to pause and prove to yourself (and ideally your code's readers) that it's a "necessary evil" whenever you find yourself writing a function that returns a class, then that makes total sense. Apologies.

>> What about the functional constructors for namedtuple, or the two kinds of enums? Or a custom kind of enum (strings, bitsets, ...), which the PEP specifically suggests building on top of the stdlib support? Or something more inherently dynamic like a bridge library? Even in cases like generating an ORM from a SQL dump or a client from an RPC discovery call, which could be moved to a separate "pre-compilation" phase of building your application, is a function that generates source code for a class less suspect than one that just generates a class?
> 
> How namedtuple is implemented should be nobody's business, except Raymond's, and it certainly isn't a pattern to be recommended. That's why it's in the stdlib -- so you don't have to write such code yourself.

Sure, but I don't think you want every bridge library, dynamic RMDB/GUI/RPC/etc. binding library, etc. to be in the stdlib. _Someone_ has to write them, and the fact that Python makes such code writable (if not easy) is a strength of the language.

Still, I take your point; I wouldn't trust a dynamic binding library that isn't being used by multiple independent projects, because it's so hard to verify by reading that it's gotten all the details right. And a proposal that makes such things slightly easier to write (while still leaving them very hard to get right) probably doesn't add much to the language.
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