使用元类创建属性（类型化和有序）的类定义

Posted 2021-02-26

Like Django.models, GAE.db.model or SqlAlchemy definitions

import os
 
class _FieldBase_:
    _defines=[]
    def __init__(self,*a):
        _FieldBase_._defines.append(self)
 
class _FieldedMetaClass_(type):
    def __init__(cls, name, bases, dct):
        super(_FieldedMetaClass_, cls).__init__(name, bases, dct)
        if name!="Form":
            fields_to_name = dict([(slot,name) for name,slot in dct.items() if isinstance(slot,_FieldBase_)])
            cls.fields = [(fields_to_name[obj],obj ) for obj in _FieldBase_._defines]
            _FieldBase_._defines=[] # reset list !!!
 
class Form (object):
    __metaclass__ = _FieldedMetaClass_
 
    def __repr__(self):
        l=["Form '%s'"%self.__class__.__name__]
        l+=["* %-10s : %s"%(f,t.__class__.__name__) for f,t in self.fields]
        return os.linesep.join(l)
 
class FieldInt(_FieldBase_):
    def __init__(self,*a):
        _FieldBase_.__init__(self,a)
 
class FieldString(_FieldBase_):
    def __init__(self,*a):
        _FieldBase_.__init__(self,a)
 
class Form1(Form):
    name     = FieldString(1)
    surname  = FieldString(2)
    age      = FieldInt(3)
 
class Form2(Form):
    nb      = FieldInt(1)
    title   = FieldString(2)
 
print Form1()
print Form2()