使用Python的元类实现一个简易orm

orm.py

import numbers


class Field:
    pass


class CharField(Field):
    def __init__(self, db_column, max_length: int):
        self.db_column = db_column
        self._value = ''

        if max_length <= 0:
            raise ValueError('max_length must be larger than 0')

        self.max_length = max_length

    def __get__(self, instance, owner):
        return self._value

    def __set__(self, instance, value):
        if value is None:
            value = ''

        if not isinstance(value, str):
            raise ValueError('')

        if len(value) > self.max_length:
            raise ValueError("value's length must be larger than max_length")

        self._value = value


class IntField(Field):
    def __init__(self, db_column, min_value: int=None, max_value: int=None):
        self.db_column = db_column
        self._value = 0

        if min_value is not None:
            if not isinstance(min_value, numbers.Integral):
                raise ValueError("min_value must be int")

        if max_value is not None:
            if not isinstance(max_value, numbers.Integral):
                raise ValueError("max_value must be int")

        if min_value is None:
            min_value = 0
        if max_value is None:
            max_value = (1 << 31) - 1

        if max_value < min_value:
            raise ValueError("max_value must be larger than min_value")

        self.min_value = min_value
        self.max_value = max_value

    def __get__(self, instance, owner):
        return self._value

    def __set__(self, instance, value:int):
        if not isinstance(value, numbers.Integral):
            raise ValueError('value must be int')

        if value < self.min_value or value > self.max_value:
            raise ValueError('')

        self._value = value


class MetaModel(type):
    def __new__(mcs, name, bases, kwargs):
        if name == 'Model':
            return super().__new__(mcs, name, bases, kwargs)

        fields = {}
        for k, v in kwargs.items():
            if isinstance(v, Field):
                fields[k] = v

        attrs_meta = kwargs.get('Meta', None)
        _meta = {}
        db_table = name.lower()
        if attrs_meta is not None:
            db_table = getattr(attrs_meta, 'db_table', db_table)

        _meta['db_table'] = db_table
        kwargs['fields'] = fields
        kwargs['_meta'] = _meta
        del kwargs['Meta']

        return super().__new__(mcs, name, bases, kwargs)


class Model(metaclass=MetaModel):
    def __init__(self, *args, **kwargs):
        for k, v in kwargs.items():
            setattr(self, k, v)

        super().__init__(*args, **kwargs)

    def save(self):
        pass


class User(Model):
    name = CharField(db_column="name", max_length=20)
    age = IntField(db_column="age", min_value=1, max_value=200)

    class Meta:
        db_table = "user"


if __name__ == '__main__':
    user = User()
    user.name = "fpgayu"
    user.age = 26
    # user.save()
    print(user.fields)