mypy: "Optional[Mapping[Any, Any]]" 类型的值不可索引

Question

为什么mypy说databases库的fetch_one方法返回的值不可索引？在这种情况下我应该怎么做才能解决这个错误？

async def get_reaction_by_name(name: str, user_id: str) -> Optional[Mapping[Any, Any]]:
    query = reactions.select().where(
        (reactions.c.name == name) & (reactions.c.user_id == user_id)
    )
    return await database.fetch_one(query)

...

reaction = await get_reaction_by_name(name=name, user_id=user_id)
reaction["name"]

# mypy: Value of type "Optional[Mapping[Any, Any]]" is not indexable

Answer 1

mypy 告诉我们的是，由于函数 returns Optional[...] 在将其结果视为 ...（给定代码段中的 Mapping[Any, Any]）之前，我们应该检查它是否不是None 个赞

reaction = await get_reaction_by_name(name=name, user_id=user_id)
if reaction is not None:
    reaction["name"]

mypy: "Optional[Mapping[Any, Any]]" 类型的值不可索引

mypy: Value of type "Optional[Mapping[Any, Any]]" is not indexable

python

type-hinting

python-3.x

mypy