使用元類編寫一個簡單的ORM框架

使用元類編寫一個簡單的ORM框架

# -*- coding : utf-8 -*-
class Field(object):
    '''filed'''

    def __init__(self, name, column_type):
        self.name = name
        self.column_type = column_type

    def __str__(self):
        return '<%s:%s>' % (self.__class__.__name__, self.name)

# ------------------


class StringField(Field):
    '''String Field'''

    def __init__(self, name):
        super().__init__(name, 'varchar(100)')

# ------------------


class IntegerField(Field):
    '''Integer Field'''

    def __init__(self, name):
        super().__init__(name, 'bigint')

# ------------------


class ModelMetaclass(type):
    '''meta class'''
    def __new__(cls, name, bases, attrs):
        if name == 'Model':
            return type.__new__(cls, name, bases, attrs)
        print('Found Model: %s' % name)
        mappings = dict()
        for k, v in attrs.items():
            if isinstance(v, Field):
                print('Found mapping: %s ==> %s' % (k, v))
                mappings[k] = v
        for k in mappings.keys():
            attrs.pop(k)
        attrs['__mappings__'] = mappings
        attrs['__table__'] = name
        return type.__new__(cls, name, bases, attrs)
# ------------------


class Model(dict, metaclass=ModelMetaclass):
    '''model'''

    def __init__(self, **kw):
        super().__init__(**kw)

    def __getattr__(self, key):
        try:
            return self[key]
        except KeyError:
            raise AttributeError(r"'Model' object has no attribute '%s'" % key)

    def __setattr__(self, key, value):
        self[key] = value

    def save(self):
        fields = []
        params = []
        args = []
        for k, v in self.__mappings__.items():
            fields.append(v.name)
            params.append('?')
            args.append(getattr(self, k, None))
        sql = 'insert into %s (%s) values (%s)' % (
            self.__table__, ', '.join(fields), ', '.join(params))
        print('sql: %s' % sql)
        print('args: %s' % str(args))
# ------------------


class User(Model):
    '''user'''
    id = IntegerField('id')
    name = StringField('name')
    email = StringField('email')
    password = StringField('password')


# ------------------
if __name__ == '__main__':
    USER = User(id=123, name='Michael',
                email='[email protected]', password='my-pwd')
    USER.save()

輸出結果

[Running] python "d:\git\pythonStudy\hello_world.py"
Found Model: User
Found mapping: id ==> <IntegerField:id>
Found mapping: name ==> <StringField:name>
Found mapping: email ==> <StringField:email>
Found mapping: password ==> <StringField:password>
sql: insert into User (id, name, email, password) values (?, ?, ?, ?)
args: [123, 'Michael', '[email protected]', 'my-pwd']

[Done] exited with code=0 in 0.243 seconds

原文：廖雪峯的Python教程