如何在python中定义通过ForwardRef延迟计算的类型别名（TypeAlias）？

问：

我的目标是防止在使用类型别名时在运行时导入昂贵的模块。

如果没有别名，可以将昂贵的模块隐藏在后面，并使导入成为相应函数的本地：typing.TYPE_CHECKING

# utils.py -- module that must be very lightweight to import
from typing import TYPE_CHECKING
if TYPE_CHECKING:
    import tensorflow as tf  # lot's of unwanted side-effects

def rarely_used_function(t: "tf.Tensor"):  # <-- note the lazily evaluated type hint
   import tensorflow as tf
   ...

在定义类型别名时，如何实现相同的效果？

# types.py  -- a collection of our own type aliases
from typing import TYPE_CHECKING, TypeAlias

if TYPE_CHECKING:
    import tensorflow as tf

MyTensorAlias = "tf.Tensor"  # <-- now this is just a string assignment and will not evaluate to a `ForwardRef`


# Notes of things that do not work:
# 1) annotating with TypeAlias
MyOtherTensorAlias: TypeAlias = "tf.Tensor"  # <-- still just a str and will cause runtime errors

# 2) Aliases behind `TYPE_CHECKING`
if TYPE_CHECKING:
    MyLazyTensorAlias = tf.Tensor  # <-- this is fine for static type checkers, but raises at runtime when `types has no member MyLazyTensorAlias` anymore.

可以使用 实现延迟评估，但是子类型不是我想要的（因为这样我需要强制转换所有用户，例如 .typing.NewType()Subtype(tf.Tensor(...))

编辑：

它在运行时失败的方式特定于新运算符。 考虑：|

Python 3.10.8 (main, Nov 24 2022, 08:09:04) [Clang 14.0.6 ]
Type 'copyright', 'credits' or 'license' for more information
IPython 8.7.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: from typing import TypeAlias
   ...: Alias: TypeAlias = "Original"
   ...: def func(a: Alias | None):
   ...:     pass
   ...:
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[1], line 3
      1 from typing import TypeAlias
      2 Alias: TypeAlias = "Original"
----> 3 def func(a: Alias | None):
      4     pass

TypeError: unsupported operand type(s) for |: 'str' and 'NoneType'

In [2]: from typing import TypeAlias,Optional
   ...:
   ...: Alias: TypeAlias = "Original"
   ...: def func(a: Optional[Alias]):
   ...:     pass
   ...:

In [3]: class Original:
   ...:     pass
   ...:

In [4]: def func(a: Original | None):
   ...:     pass
   ...: