python - 变量类型提示在函数内部未验证

Question

当您执行此代码时:

from typing import Dict

bar: Dict[int, int, int] = dict()

异常TypeError，并带有消息Too muchparametersfortyping.Dict;实际为 3，预期为 2。但是当您在函数内定义变量时:

from typing import Dict

def foo():
    bar: Dict[int, int, int] = dict()

foo()

这次没有引发异常。这是预期行为还是错误？

Answer 1

这是预期行为，定义于 PEP 526 -- Syntax for Variable Annotations #Runtime Effects of Type Annotations .

Annotating a local variable will cause the interpreter to treat it as a local, even if it was never assigned to. Annotations for local variables will not be evaluated:

def f():
    x: NonexistentName  # No error.

However, if it is at a module or class level, then the type will be evaluated:

x: NonexistentName  # Error!
class X:
    var: NonexistentName  # Error!

此外，PEP 563 -- Postponed Evaluation of Annotations定义在 python 3.7+ 中使用 from __future__ import 注解 会阻止对这些注解进行求值。

This PEP proposes changing function annotations and variable annotations so that they are no longer evaluated at function definition time. Instead, they are preserved in __annotations__ in string form.

from __future__ import annotations
from typing import Dict

bar: Dict[int, int, int] = dict()  # no errors