python - 为什么 TypedDict 与匹配的 Mapping 不兼容？

Question

考虑以下代码:

from typing import Any, Mapping, TypedDict


class MyDict(TypedDict):
    foo: bool


def my_func_any(a: Mapping[str, Any]) -> None:
    print(a)


def my_func_bool(a: Mapping[str, bool]) -> None:
    print(a)


d: MyDict = {
    'foo': True
}

my_func_any(d)
my_func_bool(d)  # line 21

使用 mypy==0.761 检查时会出现以下错误:

test.py:21: error: Argument 1 to "my_func_bool" has incompatible type "MyDict"; expected "Mapping[str, bool]"

我希望两者都是 my_func_any(d)和 my_func_bool(d)没关系，但后者是一个错误。这是一个错误还是我错过了什么？

Answer 1

引用 PEP 589 :

Type Consistency

First, any TypedDict type is consistent with Mapping[str, object].

[...]

• A TypedDict with all int values is not consistent with Mapping[str, int], since there may be additional non-int values not visible through the type, due to structural subtyping. These can be accessed using the values() and items() methods in Mapping, for example. Example:

  class A(TypedDict):
      x: int
  
  class B(TypedDict):
      x: int
      y: str
  
  def sum_values(m: Mapping[str, int]) -> int:
      n = 0
      for v in m.values():
          n += v  # Runtime error
      return n
  
  def f(a: A) -> None:
      sum_values(a)  # Error: 'A' incompatible with Mapping[str, int]
  
  b: B = {'x': 0, 'y': 'foo'}
  f(b)