Python || metaclass - SeanWu1977/Machine-Learning GitHub Wiki

# 把所有 '類別變數，函式' 進行處理
# 與instance變數(self.xxx)無關 
class TestMetaclass(type):
    def __new__(cls, name, bases, attrs):
        # cls : 當前的類別
        # name : 當前類別的名稱
        # bases : 當前類別的父類別
        # attrs : 產生instance當下的所有 '類別' 變數 與 方法
        mappings = dict()
        for k, v in attrs.items():
            ## 也可用hasattr(v, '__call__')或callable(v) 來過濾是否為function，如是function會回傳True
            if isinstance(v, <some of object type>):
                mappings[k] = v

        # 將所有類別變數移除，並用 __mappings__ 這個變數指到變數字典(ex.{'x1':'1','x2':'2'})
        # 之後在處理時，就loop __mappings__.items() 取key 跟 value
        for k in mappings.keys():
            attrs.pop(k)
        attrs['__mappings__'] = mappings 
        attrs['__table__'] = name 

        return type.__new__(cls, name, bases, attrs)

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.items():
            if isinstance(v, Field):
                print('Found mapping: %s ==> %s' % (k, v))
                mappings[k] = v
        #print(attrs)
        for k in mappings.keys():
            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
        #print(attrs)
        return type.__new__(cls, name, bases, attrs)

class Model(dict, metaclass=ModelMetaclass):

    def __init__(self, **kw):
        super(Model, self).__init__(**kw)
        # 因繼承dict，所以用(k=v,k=v)會轉成{'k':v,'k':v}

    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
        print('{}:{}'.format(key,value))

    def save(self):
        fields = []
        params = []
        args = []
        # 用定義好的欄位跟輸入的值比較，只取有用的
        for k, v in self.__mappings__.items():
            fields.append(v.name)
            params.append('?')
            args.append(getattr(self, k, None)) # 比較定義值與__setattr__(self, key, value) 產生出來的值

        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):
    # 定义类的属性到列的映射：
    # 即定義有用的欄位
    id = IntegerField('id')
    name = StringField('username')
    email = StringField('email')
    password = StringField('password')


u = User(id=12345, name='Michael', email='[email protected]', password='my-pwd')
# 因繼承dict，所以用(k=v,k=v)會轉成{'k':v,'k':v}
# 這些初始值設定跟User定義的id,name,email,password無關