### Create a `SessionLocal` class

Each instance of the `SessionLocal` class will be a database session. The class itself is not a database session yet.

But once we create an instance of the `SessionLocal` class, this instance will be the actual database session.

                """
            ),
        ] = True,
    ):
        self.model: APIKey = APIKey(
            **{"in": APIKeyIn.query},  # type: ignore[arg-type]
            name=name,
            description=description,
        )
        self.scheme_name = scheme_name or self.__class__.__name__
        self.auto_error = auto_error

    async def __call__(self, request: Request) -> Optional[str]:

class Person:
    def __init__(self, name: str):
        self.name = name


class Pet:
    def __init__(self, owner: Person, name: str):
        self.owner = owner
        self.name = name


@dataclass
class Item:
    name: str
    count: int


class DictablePerson(Person):
    def __iter__(self):
        return ((k, v) for k, v in self.__dict__.items())


class DictablePet(Pet):

# 异步 SQL 关系型数据库

**FastAPI** 使用 <a href="https://github.com/encode/databases" class="external-link" target="_blank">`encode/databases`</a> 为连接数据库提供异步支持（`async` 与 `await`）。

`databases` 兼容以下数据库：

* PostgreSQL
* MySQL
* SQLite

本章示例使用 **SQLite**，它使用的是单文件，且 Python 内置集成了 SQLite，因此，可以直接复制并运行本章示例。

生产环境下，则要使用 **PostgreSQL** 等数据库服务器。

!!! tip "提示"

class ValidationException(Exception):
    def __init__(self, errors: Sequence[Any]) -> None:
        self._errors = errors

    def errors(self) -> Sequence[Any]:
        return self._errors


class RequestValidationError(ValidationException):
    def __init__(self, errors: Sequence[Any], *, body: Any = None) -> None:
        super().__init__(errors)
        self.body = body

                """
            ),
        ] = True,
    ):
        self.model = OpenIdConnectModel(
            openIdConnectUrl=openIdConnectUrl, description=description
        )
        self.scheme_name = scheme_name or self.__class__.__name__
        self.auto_error = auto_error

    async def __call__(self, request: Request) -> Optional[str]:
        authorization = request.headers.get("Authorization")

    if the overridden default value was truthy.
    """

    def __init__(self, value: Any):
        self.value = value

    def __bool__(self) -> bool:
        return bool(self.value)

    def __eq__(self, o: object) -> bool:
        return isinstance(o, DefaultPlaceholder) and o.value == self.value


DefaultType = TypeVar("DefaultType")

        self.path_regex, self.path_format, self.param_convertors = compile_path(path)
        self.dependant = get_dependant(path=self.path_format, call=self.endpoint)
        for depends in self.dependencies[::-1]:
            self.dependant.dependencies.insert(
                0,
                get_parameterless_sub_dependant(depends=depends, path=self.path_format),
            )

        @property
        def alias(self) -> str:
            a = self.field_info.alias
            return a if a is not None else self.name

        @property
        def required(self) -> bool:
            return self.field_info.is_required()

        @property
        def default(self) -> Any:
            return self.get_default()

        @property
        def type_(self) -> Any:

You can also create an environment variable only for a specific program invocation, that is only available to that program, and only for its duration.

To do that, create it right before the program itself, on the same line:

<div class="termy">

```console
// Create an env var MY_NAME in line for this program call
$ MY_NAME="Wade Wilson" python main.py

// Now it can read the environment variable