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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 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