Python Annotations：作为实例返回类型

Question

有没有办法让返回值知道它的类型应该属于某个isinstance？

例如，def myfunction(...) -> instance[MyClass]: （这是一个非正式的语法，我只是举例我想要的语法是这样的）

这意味着它只接受属于MyClass的返回类型。

---

一个例子

下面给了我一个例子，让你更清楚我用它做什么。

考虑以下几点：

from typing import List, TypeVar, Union, Type


class Table:
    NAME: str

    def show_info(self):
        print(self.NAME)


class TableA(Table):
    NAME = 'A'

    def method_belong_a_only(self):
        print('a only')


class TableB(Table):
    NAME = 'B'

    def method_belong_b_only(self):
        print('b only')


class DBBase:
    Tables: List[Table]


class MyDB(DBBase):
    Tables = [TableA, TableB]

    def get_table(self, table_name: str) -> Union[Table, None]:
        table_list = [_ for _ in self.Tables if hasattr(_, 'NAME') and _.NAME == table_name]

        if not table_list:
            return None

        if len(table_list) > 1:
            raise ValueError('Table name conflict!')
        return table_list[0]()  # create an instance

然后如果我写以下内容，

db = MyDB()
ta = db.get_table('A')
ta.show_info()  # ok
# ta.  # IDE will show method, which belongs to Table only.
ta.method_belong_a_only()  # It still can run, but IDE show Cannot find reference 'method_belong_a_only' in Table|None

tb: TableB = db.get_table('B')  # It still can run, but IDE show ``Expected type 'TableA', got 'Optional[Table]' instead``
# tb.  # Now, It can show ``method_belong_b_only``

我不希望它（PyCharm IDE）向我显示警告，我希望它在 intelligence 上运行良好。

Answer 1

你可以使用typing.TypeVarPEP484.type-aliases

from typing import List, TypeVar, Union, Type


T_Table = TypeVar('T_Table', bound=Table)


class MyDB2(DBBase):
    Tables = [TableA, TableB]

    def get_table(self, t: Type[T_Table] = Table) -> Union[T_Table, None]:
        t_set = {cur_t for cur_t in self.Tables if cur_t.__name__ == t.__name__}

        if not t_set:
            return None

        table = t_set.pop()
        return table()   # create an instance

    def get_table_2(self, table_type_name: str) -> Union[T_Table, None]:
        t_set = {cur_t for cur_t in self.Tables if cur_t.__name__ == table_type_name}

        if not t_set:
            return None

        table = t_set.pop()
        return table()   # create an instance


db = MyDB2()
a = db.get_table(TableA)
#  a = db.get_table(str)  # <-- Expected type 'Type[T_Table]', got 'Type[str]' instead
a.method_belong_a_only()  # intellisense will work perfectly.

b: TableB = db.get_table_2('TableB')  # This is another way, if IDE felt confused, you can tell him what the type it is.
# b.  # intellisense will work perfectly.