[issue27985] Implement PEP 526

[Ivan Levkivskyi]

Ivan Levkivskyi added the comment:

Guido,

I fixed __annotations__ in interactive REPL, will fix other minor things and submit a new patch tomorrow morning.

There are two important questions in typing.py:

> Why is the second isinstance() needed? Isn't _ClassVar a subclass of
> TypingMeta

and 

> This being a metaclass, the name should end in Meta, like most
> other subclasses of TypingMeta. (And they don't have a leading _,
> though they're still not public.)

The problem with ClassVar is that it is not actually a class, and _ClassVar is not a metaclass, it is just a class. I deviated from the common pattern in module for the following reason. ClassVar is going to be extensively used at class scope, where all annotations are evaluated. Therefore I wanted not to create new class objects by __getitem__ like it is going for other classes like Union etc (I tried to timeit this and ClassVar is almost ten times faster than Union)

What do you think?
Should I keep it like this, or rewrite in a common pattern for the module?

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