起因于 @property 装饰器，它可以将对函数的调用伪装成对属性的调用，就是不用加括号了。
假设有这么一个类：
class Test(object):
def __init__(self, age=0):
self.age = age
---------分割线------------
if age < 0:
raise ValueError("Negative value not allowed:{}".format(age))
假设我们想要将 age 做一些限制，例如非负，那么我们只能添加上面的代码。
但是如果我们使用 . 来赋值呢，这不起作用啊。所以可以这样做：
class Test(object):
def __init__(self, age=0):
self._age = age
@propery
def age(self):
return self._age
@propery.setter
def age(self, age):
if age < 0:
raise ValueError("Negative value not allowed:{}".format(age))
@age.deleter
def age(self):
raise AttributeError("Can not delete age")
但是如果有很多属性要做这样的检查呢？那么不得不写一大堆 @propery.setter ，这样代码分离性就做的不好。
所以出现了描述符？
我现在的理解就是实现了__get__(), __set__() 和 __delete__() 三者任意一个即可。只不过只实现 __get__() 的话称为非数据描述符，意味着只可读，同时实现 __get__(), 和 __set__() 称为数据描述符，意味着可读写。
那么上面的代码就可以这样写了
class Integer(object):
def __init__(self, name):
# 这里也可以做一些检查，防止直接实例化
self.name = name
def __get__(self, instance, owner):
return instance.__dict__[self.name]
def __set__(self, instance, value):
if value < 0:
raise ValueError("Negative value not allowed")
instance.__dict__[self.name] = value
class Test(object):
age = Integer('test')
-----------分割线------------
def __init__(self, age):
self.age = age
如果对 django 熟悉的话，发现这很像它的 orm。但是这样不能直接实例化了，所以可以添加上面分割线下代码。
这样可以实现一个简单的 orm
# 需求
import numbers
class Field:
pass
class IntField(Field):
# 数据描述符
def __init__(self, db_column, min_value=None, max_value=None):
self._value = None
self.min_value = min_value
self.max_value = max_value
self.db_column = db_column
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 max_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_lenth for charfiled")
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 not 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))
pass
class User(BaseModel):
name = CharField(db_column="name", max_length=10)
age = IntField(db_column="age", min_value=1, max_value=100)
class Meta:
db_table = "user"
if __name__ == "__main__":
user = User(name="bobby", age=28)
# user.name = "bobby"
# user.age = 28
user.save()
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