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

[Andrew Barnert]

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.

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?
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