函数内部未验证变量类型提示

Question

当你执行这段代码时：

from typing import Dict

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

引发异常 TypeError 并显示一条消息 Too many parameters for typing.Dict; actual 3, expected 2。但是当你在函数中定义变量时：

from typing import Dict

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

foo()

这次没有异常。这是预期行为还是错误？

Answer 1

这是预期的行为，定义在 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 定义将 from __future__ import annotations 与 python 3.7+ 一起使用会阻止评估这些注释。

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

函数内部未验证变量类型提示

Variable type hints are not validated inside function

python

type-hinting