【7.6】通过元类实现简单的orm

#!/user/bin/env python
# -*- coding:utf-8 -*-
# 需求
import numbers


class Field:
    pass


class IntField(Field):
    def __init__(self, db_column, min_value=None, max_value=None):
        self._value = None
        self.db_column = db_column
        self.min_value = min_value
        self.max_value = max_value
        if min_value is not None:
            if not isinstance(min_value, numbers.Integral):
                raise ValueError('min_value must be int')
            elif min_value < 0:
                raise ValueError('min_value must be positive int')
        if max_value is not None:
            if not isinstance(max_value, numbers.Integral):
                raise ValueError('max_value must be int')
            elif min_value < 0:
                raise ValueError('max_value must be positive int')
        if min_value is not None and max_value is not None:
            if min_value > max_value:
                raise ValueError('min_value must be smaller than max_value')

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

    def __set__(self, instance, value):
        if not isinstance(value, numbers.Integral):
            raise ValueError('int value need')
        if value < self.min_value or value > self.max_value:
            raise ValueError('value must between min_value and max_value')
        self._value = value


class CharField(Field):
    def __init__(self, db_column, max_length=None):
        self._value = None
        self.db_column = db_column
        if max_length is None:
            raise ValueError('you must spcify max_length for CharField')
        self.max_length = max_length

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

    def __set__(self, instance, value):
        if not isinstance(value, str):
            raise ValueError('string value need')

        if len(value) > self.max_length:
            raise ValueError('value len excess len of max_length')
        self._value = value


class ModelMetaClass(type):
    def __new__(cls, name, bases, attrs, **kwargs):
        if name == 'BaseModel':
            return super().__new__(cls, name, bases, attrs, **kwargs)
        fields = {}
        for key, value in attrs.items():
            if isinstance(value, Field):
                fields[key] = value
        attrs_meta = attrs.get('Meta', None)
        _meta = {}
        db_table = name.lower()
        if attrs_meta is not None:
            table = getattr(attrs_meta, 'db_table', None)
            if table is None:
                db_table = table
        _meta['db_table'] = db_table
        attrs['_meta'] = _meta
        attrs['fields'] = fields
        del attrs['Meta']
        return super().__new__(cls, name, bases, attrs, **kwargs)


class BaseModel(metaclass=ModelMetaClass):
    def __init__(self, *args, **kwargs):
        for key, value in kwargs.items():
            setattr(self, key, value)
        return super().__init__()

    def save(self):
        fields = []
        values = []
        for key, value in self.fields.items():
            db_column = value.db_column
            if db_column is None:
                db_column = key.lower()
            fields.append(db_column)
            value = getattr(self, key)
            values.append(str(value))

        sql = 'insert {db_table}({fields}) value({values})'.format(db_table=self._meta['db_table'], fields=','.join(fields), values=','.join(values))
        print(sql)


class User(BaseModel):
    name = CharField(db_column='name', max_length=10)
    age = IntField(db_column='age', min_value=0, max_value=100)

    class Meta:
        db_table = 'user'


if __name__ == '__main__':
    user = User()
    user.name = 'zy'
    user.age = 21
    user.save()

insert user(name,age) value(zy,21)