ClassVar should not be used outside of a variable declaration within a class. ClassVar is not an annotation that describes the type of a variable, but it describes a (non-type) attribute of a variable. Most other languages offer keywords for such attributes, but Python overloads the type annotation mechanism to handle these cases. Other examples include Final, InitVar, Required, and NotRequired. These special forms should not be used within type aliases because they are not type annotations.

The use of ClassVar should be flagged as an error when used in a type alias like this. When I implemented support for ClassVar in pyright, it never occurred to me that someone would attempt to use it in a type alias, so there is no check for this error condition. But you should assume that it will not work when used in this manner.

If you need to use isinstance with the new syntax, you need to access the type alias's value (and force its evaluation) explicitly, through the __value__ attribute:

isinstance(n, Number.__value__)

Of course, this will only work if the value of the alias is compatible with isinstance, which rules out stuff like NewType or generics, or... most of the things you'd want to create a type alias for, but int | float should be fine. The old syntax had these limits too.

Python 3.5+

Since Python 3.5 you may use typing module.

Quoting docs, A type alias is defined by assigning the type to the alias:

Copy# Python 3.5-3.8
from typing import List
Vector = List[float]

# Python 3.9+
Vector = list[float] # No import needed, lower case l

To learn more about enforcing types in Python you may want to get familiar with PEPs: PEP483 and PEP484.

Python historically was using duck-typing instead of strong typing and hadn't built-in way of declaring types before 3.5 release.