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

[Steven D'Aprano]

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. So given:

def maker():
    class MyClass: pass
    return MyClass

x = maker()()
y = maker()()

assert isinstance(x, type(y))
assert isinstance(y, type(x))


both assertions will fail, which may confuse some people. 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?



-- 
Steven