Python：类属性定义上的代码重复

Question

我正在尝试用 python 实现一个简单的 ORM。我面临代码重复问题，但我不知道如何解决。 这是我的项目中的类的简化示例：

class Person:

    TABLE_NAME = 'person'

    FIELDS = [
        ('name', 'VARCHAR(50)'),
        ('age', 'INTEGER')
    ]

    # CODE DUPLICATION: the two next lines shoudl be genereated with FIELDS not hard coded...
    name: str
    age: int

    def __init__(self, **kwargs):
        self.__dict__ = kwargs

    @classmethod
    def create_sql_table(cls):
        # use TABLE_NAME and FIELDS to create sql table
        pass


alice = Person(name='Alice', age=25)

print(alice.name)

如果我删除两行 name: str 和 age: int 我会丢失自动完成功能并收到 mypy 错误在打印行上（错误：人没有属性名称）

但是如果我保留它，则会出现代码重复（每个字段名称我写两次）。

有没有办法避免代码重复（例如通过使用 FIELDS 变量生成这两行）？

或者实现此类避免代码重复的另一种方法（没有 mypy 错误和自动完成丢失）？

Answer 1

您可以使用描述符：

from typing import Generic, TypeVar, Any, overload, Union

T = TypeVar('T')

class Column(Generic[T]):
    sql_type: str  # the field type used for this column

    def __init__(self) -> None:
        self.name = ''  # the name of the column

    # this is called when the Person class (not the instance) is created
    def __set_name__(self, owner: Any, name: str) -> None:
        self.name = name  # now contains the name of the attribute in the class

    # the overload for the case: Person.name -> Column[str]
    @overload
    def __get__(self, instance: None, owner: Any) -> 'Column[T]': ...

    # the overload for the case: Person().name -> str
    @overload
    def __get__(self, instance: Any, owner: Any) -> T: ...

    # the implementation of attribute access
    def __get__(self, instance: Any, owner: Any) -> Union[T, 'Column[T]']:
        if instance is None:
            return self
        # implement your attribute access here
        return getattr(instance, f'_{self.name}')  # type: ignore

    # the implementation for setting attributes
    def __set__(self, instance: Any, value: T) -> None:
        # maybe check here that the type matches
        setattr(instance, f'_{self.name}', value)

现在我们可以为每个列类型创建专门化：

class Integer(Column[int]):
    sql_type = 'INTEGER'

class VarChar(Column[str]):
    def __init__(self, size: int) -> None:
        self.sql_type = f'VARCHAR({size})'
        super().__init__()

当您定义 Person 类时，我们可以使用列类型

class Person:
    TABLE_NAME = 'person'

    name = VarChar(50)
    age = Integer()

    def __init__(self, **kwargs: Any) -> None:
        for key, value in kwargs.items():
            setattr(self, key, value)


    @classmethod
    def create_sql_table(cls) -> None:
        print("CREATE TABLE", cls.TABLE_NAME)
        for key, value in vars(cls).items():
            if isinstance(value, Column):
                print(key, value.sql_type)


Person.create_sql_table()

p = Person(age=10)
print(p.age)
p.age = 20
print(p.age)

这将打印：

创建表人员

名称 VARCHAR(50)

年龄整数

10

20

您可能还应该创建一个基 Model 类，其中包含 __init__ 和 Person 的类方法code>

您还可以扩展Column 类以允许可为空的列并添加默认值。

Mypy 不会抱怨，并且可以正确地将 Person.name 的类型推断为 str，将 Person.age 推断为 int。

Answer 2

好吧，我最终得到了这个

class Person:
    # this is not full, you need to fill other types you use it with the correct relationship
    types = {
        str: 'VARCHAR(50)',
        int: 'INTEGER',
    }  # you should extract that out if you use it elsewhere


    TABLE_NAME = 'person'
    

    # NOTE: the only annotated fields should be these. if you annotate anything else, It will break
    name: str
    age: int

    def __init__(self, **kwargs):
        self.__dict__ = kwargs

    @property
    def FIELDS(cls):
        return [(key, cls.types[value]) for key, value in cls.__annotations__.items()]


alice = Person(name='Alice', age=25)

print(alice.FIELDS)  # [('name', 'VARCHAR(50)'), ('age', 'INTEGER')]

并且

>>> mypy <module>
>>> Success: no issues found in 1 source file

Answer 3

在 Person 类中尝试在构造函数中添加数据类型