On Thu, 14 Sep 2017 at 12:09 Ivan Levkivskyi <levkivskyi@gmail.com> wrote:

On 14 September 2017 at 01:13, Guido van Rossum <guido@python.org> wrote:

That last sentence is a key observation. Do we even know whether there are (non-toy) things that you can do *in principle* with __class__ assignment but which are too slow *in practice* to bother? And if yes, is __getattr__ fast enough? @property?

I myself have never implemented deprecation warnings management nor lazy loading,
so it is hard to say if __class__ assignment is fast enough.

For lazy loading it is, but that's because after the lazy load, module.__class__ is set back to its original value, so any performance penalty is paid only once upon triggering __getattribute__ on the lazy module.

IOW no one has complained and I know plenty of places using the lazy loading mechanism in importlib at scale. :)

-Brett

For me it is more combination
of three factors:

* modest performance improvement
* some people might find __getattr__ clearer than __class__ assignment
* this would be consistent with how stubs work

IMO we're still looking for applications.

How about this

def allow_forward_references(*allowed):
    caller_globals = sys._getframe().__globals__
    def typing_getattr(name):
        if name in allowed:
            return name
        raise AttributeError(...)
    caller_globals.__getattr__ = typing_getattr

from typing_extensions import allow_forward_references
allow_forward_references('Vertex', 'Edge')

T = TypeVar('T', bound=Edge)

class Vertex(List[Edge]):
def copy(self: T) -> T:
...

class Edge:
ends: Tuple[Vertex, Vertex]
    ...

Look mum, no quotes! :-)

--
Ivan

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