利用python元类来实现ORM

Bertha 。 2022-08-07 08:38 192阅读 0赞

**利用python元类来实现ORM**

**       python元类有两种实现方式：type() 和 metaclass。**

**       下面利用metaclass方式来实现一个简单的ORM：**

#!/usr/bin/env python
    # -*- coding: utf-8 -*-
    
    ' Simple ORM using metaclass '
    
    __author__ = 'Michael Liao'
    
    class Field(object):
        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):
        def __init__(self, name):
            super(StringField, self).__init__(name, 'varchar(100)')
    
    class IntegerField(Field):
        def __init__(self, name):
            super(IntegerField, self).__init__(name, 'bigint')
    
    class ModelMetaclass(type):
    
        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.iteritems():
                if isinstance(v, Field):
                    print('Found mapping: %s ==> %s' % (k, v))
                    mappings[k] = v
            for k in mappings.iterkeys():
                attrs.pop(k)
            attrs['__mappings__'] = mappings # 保存属性和列的映射关系
            attrs['__table__'] = name # 假设表名和类名一致
            return type.__new__(cls, name, bases, attrs)
    
    class Model(dict):
        __metaclass__ = ModelMetaclass
    
        def __init__(self, **kw):
            super(Model, self).__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__.iteritems():
                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))
    
    # testing code:
    
    class User(Model):
        id = IntegerField('uid')
        name = StringField('username')
        email = StringField('email')
        password = StringField('password')
    
    u = User(id=12345, name='Michael', email='test@orm.org', password='my-pwd')
    u.save()