@property decorator在Python中是如何工作的?
我想了解内置函数@property decorator在Python中是如何工作的?,python,properties,decorator,python-decorators,python-internals,Python,Properties,Decorator,Python Decorators,Python Internals,我想了解内置函数属性是如何工作的。使我困惑的是,属性也可以用作装饰器,但它仅在用作内置函数时才接受参数,而在用作装饰器时不接受参数 此示例来自: property的参数包括getx、setx、delx和一个文档字符串 在下面的代码中,属性用作装饰器。它的对象是x函数,但在上面的代码中,参数中没有对象函数的位置 class C: def __init__(self): self._x = None @property def x(self):
属性
是如何工作的。使我困惑的是,属性
也可以用作装饰器,但它仅在用作内置函数时才接受参数,而在用作装饰器时不接受参数
此示例来自:
property
的参数包括getx
、setx
、delx
和一个文档字符串
在下面的代码中,属性
用作装饰器。它的对象是x
函数,但在上面的代码中,参数中没有对象函数的位置
class C:
def __init__(self):
self._x = None
@property
def x(self):
"""I'm the 'x' property."""
return self._x
@x.setter
def x(self, value):
self._x = value
@x.deleter
def x(self):
del self._x
在这种情况下,如何创建x.setter
和x.deleter
装饰器?这只是创建只读属性的快捷方式。所以
@property
def x(self):
return self._x
相当于
def getx(self):
return self._x
x = property(getx)
第一部分很简单:
@property
def x(self): ...
与
def x(self): ...
x = property(x)
- 反过来,这是一种简化的语法,用于仅使用getter创建
属性
@x.setter
def x(self, value): ...
返回一个新属性,该属性继承旧的x
加上给定的setter的所有内容
x.deleter
的工作方式相同。函数属性()
返回一个特殊的:
>>属性()
此对象具有额外的方法:
真的和
def foo(self): return self._foo
foo = property(foo)
因此foo
函数被property(foo)
替换,我们在上面看到的是一个特殊对象。然后,当您使用@foo.setter()
时,您要做的是调用上面我向您展示的property().setter
方法,该方法返回属性的新副本,但这次用修饰的方法替换setter函数
下面的序列还通过使用这些decorator方法创建一个full-on属性
首先,我们用一个getter创建一些函数和一个属性对象:
>>> def getter(self): print('Get!')
...
>>> def setter(self, value): print('Set to {!r}!'.format(value))
...
>>> def deleter(self): print('Delete!')
...
>>> prop = property(getter)
>>> prop.fget is getter
True
>>> prop.fset is None
True
>>> prop.fdel is None
True
接下来,我们使用.setter()
方法添加一个setter:
>>> prop = prop.setter(setter)
>>> prop.fget is getter
True
>>> prop.fset is setter
True
>>> prop.fdel is None
True
最后,我们使用.deleter()
方法添加一个删除器:
>>> prop = prop.deleter(deleter)
>>> prop.fget is getter
True
>>> prop.fset is setter
True
>>> prop.fdel is deleter
True
最后但并非最不重要的一点是,属性
对象充当了一个对象,因此它具有用于连接实例属性获取、设置和删除的属性和方法:
>>> class Foo: pass
...
>>> prop.__get__(Foo(), Foo)
Get!
>>> prop.__set__(Foo(), 'bar')
Set to 'bar'!
>>> prop.__delete__(Foo())
Delete!
描述符Howto包括属性()
类型的
下面是如何实现@property
的一个简单示例:
class Thing:
def __init__(self, my_word):
self._word = my_word
@property
def word(self):
return self._word
>>> print( Thing('ok').word )
'ok'
否则,word
仍然是一个方法而不是一个属性
class Thing:
def __init__(self, my_word):
self._word = my_word
def word(self):
return self._word
>>> print( Thing('ok').word() )
'ok'
这包括:
class C(object):
def __init__(self):
self._x = None
@property
def x(self):
"""I'm the 'x' property."""
return self._x
@x.setter
def x(self, value):
self._x = value
@x.deleter
def x(self):
del self._x
同:
class C(object):
def __init__(self):
self._x = None
def _x_get(self):
return self._x
def _x_set(self, value):
self._x = value
def _x_del(self):
del self._x
x = property(_x_get, _x_set, _x_del,
"I'm the 'x' property.")
class C(object):
def __init__(self):
self._x = None
def _x_get(self):
return self._x
def _x_set(self, value):
self._x = value
def _x_del(self):
del self._x
x = property(_x_get, doc="I'm the 'x' property.")
x = x.setter(_x_set)
x = x.deleter(_x_del)
class C(object):
def __init__(self):
self._x = None
def _x_get(self):
return self._x
x = property(_x_get, doc="I'm the 'x' property.")
def _x_set(self, value):
self._x = value
x = x.setter(_x_set)
def _x_del(self):
del self._x
x = x.deleter(_x_del)
同:
class C(object):
def __init__(self):
self._x = None
def _x_get(self):
return self._x
def _x_set(self, value):
self._x = value
def _x_del(self):
del self._x
x = property(_x_get, _x_set, _x_del,
"I'm the 'x' property.")
class C(object):
def __init__(self):
self._x = None
def _x_get(self):
return self._x
def _x_set(self, value):
self._x = value
def _x_del(self):
del self._x
x = property(_x_get, doc="I'm the 'x' property.")
x = x.setter(_x_set)
x = x.deleter(_x_del)
class C(object):
def __init__(self):
self._x = None
def _x_get(self):
return self._x
x = property(_x_get, doc="I'm the 'x' property.")
def _x_set(self, value):
self._x = value
x = x.setter(_x_set)
def _x_del(self):
del self._x
x = x.deleter(_x_del)
同:
class C(object):
def __init__(self):
self._x = None
def _x_get(self):
return self._x
def _x_set(self, value):
self._x = value
def _x_del(self):
del self._x
x = property(_x_get, _x_set, _x_del,
"I'm the 'x' property.")
class C(object):
def __init__(self):
self._x = None
def _x_get(self):
return self._x
def _x_set(self, value):
self._x = value
def _x_del(self):
del self._x
x = property(_x_get, doc="I'm the 'x' property.")
x = x.setter(_x_set)
x = x.deleter(_x_del)
class C(object):
def __init__(self):
self._x = None
def _x_get(self):
return self._x
x = property(_x_get, doc="I'm the 'x' property.")
def _x_set(self, value):
self._x = value
x = x.setter(_x_set)
def _x_del(self):
del self._x
x = x.deleter(_x_del)
这与:
class C(object):
def __init__(self):
self._x = None
@property
def x(self):
"""I'm the 'x' property."""
return self._x
@x.setter
def x(self, value):
self._x = value
@x.deleter
def x(self):
del self._x
属性可以用两种方式声明
- 为属性创建getter、setter方法,然后将它们作为参数传递给属性函数
- 使用@属性装饰器
你可以看看我写的几个例子 我阅读了这里的所有帖子,意识到我们可能需要一个真实的例子。实际上,为什么我们有@property?
因此,考虑使用认证系统的烧瓶应用程序。
您可以在models.py
中声明模型用户:
class User(UserMixin, db.Model):
__tablename__ = 'users'
id = db.Column(db.Integer, primary_key=True)
email = db.Column(db.String(64), unique=True, index=True)
username = db.Column(db.String(64), unique=True, index=True)
password_hash = db.Column(db.String(128))
...
@property
def password(self):
raise AttributeError('password is not a readable attribute')
@password.setter
def password(self, password):
self.password_hash = generate_password_hash(password)
def verify_password(self, password):
return check_password_hash(self.password_hash, password)
在这段代码中,我们通过使用@property
来“隐藏”属性password
,当您试图直接访问它时,它会触发AttributeError
断言,而我们使用@property.setter来设置实际实例变量password\u hash
现在,在auth/views.py
中,我们可以使用以下命令实例化用户:
...
@auth.route('/register', methods=['GET', 'POST'])
def register():
form = RegisterForm()
if form.validate_on_submit():
user = User(email=form.email.data,
username=form.username.data,
password=form.password.data)
db.session.add(user)
db.session.commit()
...
请注意,当用户填写表单时,来自注册表的属性password
。密码确认在前端通过EqualTo('Password',message='Passwords must match')
进行(如果您想知道,但这是一个与表单相关的不同主题)
我希望这个例子会有用这里是另一个例子:
##
## Python Properties Example
##
class GetterSetterExample( object ):
## Set the default value for x ( we reference it using self.x, set a value using self.x = value )
__x = None
##
## On Class Initialization - do something... if we want..
##
def __init__( self ):
## Set a value to __x through the getter / setter... Since __x is defined above, this doesn't need to be set...
self.x = 1234
return None
##
## Define x as a property, ie a getter - All getters should have a default value arg, so I added it - it will not be passed in when setting a value, so you need to set the default here so it will be used..
##
@property
def x( self, _default = None ):
## I added an optional default value argument as all getters should have this - set it to the default value you want to return...
_value = ( self.__x, _default )[ self.__x == None ]
## Debugging - so you can see the order the calls are made...
print( '[ Test Class ] Get x = ' + str( _value ) )
## Return the value - we are a getter afterall...
return _value
##
## Define the setter function for x...
##
@x.setter
def x( self, _value = None ):
## Debugging - so you can see the order the calls are made...
print( '[ Test Class ] Set x = ' + str( _value ) )
## This is to show the setter function works.... If the value is above 0, set it to a negative value... otherwise keep it as is ( 0 is the only non-negative number, it can't be negative or positive anyway )
if ( _value > 0 ):
self.__x = -_value
else:
self.__x = _value
##
## Define the deleter function for x...
##
@x.deleter
def x( self ):
## Unload the assignment / data for x
if ( self.__x != None ):
del self.__x
##
## To String / Output Function for the class - this will show the property value for each property we add...
##
def __str__( self ):
## Output the x property data...
print( '[ x ] ' + str( self.x ) )
## Return a new line - technically we should return a string so it can be printed where we want it, instead of printed early if _data = str( C( ) ) is used....
return '\n'
##
##
##
_test = GetterSetterExample( )
print( _test )
## For some reason the deleter isn't being called...
del _test.x
基本上,与C(对象)示例相同,只是我使用了x来代替。。。我也不在uu init中初始化-。。。好。。我知道,但是它可以被删除,因为_x被定义为类的一部分
输出为:
[ Test Class ] Set x = 1234
[ Test Class ] Get x = -1234
[ x ] -1234
[ Test Class ] Get x = None
[ x ] None
如果我在init中注释掉self.x=1234,则输出为:
[ Test Class ] Set x = 1234
[ Test Class ] Get x = -1234
[ x ] -1234
[ Test Class ] Get x = None
[ x ] None
如果我在getter函数中将_default=None设置为_default=0(因为所有getter都应该有一个默认值,但它不是由我看到的属性值传入的,所以你可以在这里定义它,实际上它并不坏,因为你可以定义默认值一次,然后在任何地方都使用它),例如:def x(self,_default=0):
注意:getter逻辑只是让值被它操纵,以确保它被它操纵-打印语句也是如此
注意:我已经习惯了Lua,能够在调用单个函数时动态创建10+个helper,我为Python做了一些类似的东西,但没有使用属性,它在一定程度上可以工作,但是,即使函数是在使用之前创建的,有时在创建之前调用它们仍然存在一些问题,这很奇怪,因为它不是那样编码的。。。我更喜欢Lua元表的灵活性,并且我可以使用实际的setter/getter,而不是直接访问变量。。。不过,我很喜欢用Python构建某些东西的速度——例如gui程序。虽然我设计的一个可能没有很多额外的库是可能的-如果我在AutoHotkey编码,我可以直接访问我需要的DLL调用,同样可以在爪哇,C,C++,和更多-也许我还没有找到正确的东西,但对于这个项目,我可以从Python切换…< / P>
注意:此论坛中的代码输出已损坏-我必须在代码的第一部分添加空格才能使其正常工作-复制/粘贴时,请确保将所有空格转换为选项卡。。。。我使用Python的制表符,因为在一个文件中
class Celsius:
def __init__(self, temperature = 0):
self.set_temperature(temperature)
def to_fahrenheit(self):
return (self.get_temperature() * 1.8) + 32
def get_temperature(self):
return self._temperature
def set_temperature(self, value):
if value < -273:
raise ValueError("Temperature below -273 is not possible")
self._temperature = value
class Celsius:
def __init__(self, temperature = 0):
self.temperature = temperature
def to_fahrenheit(self):
return (self.temperature * 1.8) + 32
def get_temperature(self):
print("Getting value")
return self.temperature
def set_temperature(self, value):
if value < -273:
raise ValueError("Temperature below -273 is not possible")
print("Setting value")
self.temperature = value
temperature = property(get_temperature,set_temperature)
temperature = property(get_temperature,set_temperature)
# make empty property
temperature = property()
# assign fget
temperature = temperature.getter(get_temperature)
# assign fset
temperature = temperature.setter(set_temperature)
C = Celsius()
C.temperature
# instead of writing C.get_temperature()
class Celsius:
def __init__(self, temperature = 0):
self.temperature = temperature
def to_fahrenheit(self):
return (self.temperature * 1.8) + 32
@property
def temperature(self):
print("Getting value")
return self.temperature
@temperature.setter
def temperature(self, value):
if value < -273:
raise ValueError("Temperature below -273 is not possible")
print("Setting value")
self.temperature = value
class OurClass:
def __init__(self, a):
self.x = a
y = OurClass(10)
print(y.x)
class OurClass:
def __init__(self,x):
self.x = x
@property
def x(self):
return self.__x
@x.setter
def x(self, x):
if x < 0:
self.__x = 0
elif x > 1000:
self.__x = 1000
else:
self.__x = x
class Money:
def __init__(self, dollars, cents):
self.dollars = dollars
self.cents = cents
money = Money(27, 12)
print("I have {} dollar and {} cents.".format(money.dollars, money.cents))
# prints I have 27 dollar and 12 cents.
class Money:
def __init__(self, dollars, cents):
self.total_cents = dollars * 100 + cents
money = Money(27, 12)
print("I have {} dollar and {} cents.".format(money.dollars, money.cents))
class Money:
def __init__(self, dollars, cents):
self.total_cents = dollars * 100 + cents
# Getter and setter for dollars...
@property
def dollars(self):
return self.total_cents // 100
@dollars.setter
def dollars(self, new_dollars):
self.total_cents = 100 * new_dollars + self.cents
# And the getter and setter for cents.
@property
def cents(self):
return self.total_cents % 100
@cents.setter
def cents(self, new_cents):
self.total_cents = 100 * self.dollars + new_cents
money = Money(27, 12)
print("I have {} dollar and {} cents.".format(money.dollars, money.cents))
# prints I have 27 dollar and 12 cents.
money.dollars += 2
print("I have {} dollar and {} cents.".format(money.dollars, money.cents))
# prints I have 29 dollar and 12 cents.
money.cents += 10
print("I have {} dollar and {} cents.".format(money.dollars, money.cents))
# prints I have 29 dollar and 22 cents.
class UtilityMixin():
@property
def get_config(self):
return "This is property"
util = UtilityMixin()
print(util.get_config)
print(property) #<class 'property'>
class C:
def __init__(self):
self._x=None
@property
def x(self):
return self._x
@x.setter
def x(self, value):
self._x = value
@x.deleter
def x(self):
del self._x
c = C()
c.x="a"
print(c.x)
class C:
def __init__(self):
self._x=None
def g(self):
return self._x
def s(self, v):
self._x = v
def d(self):
del self._x
prop = property(g,s,d)
c = C()
c.x="a"
print(c.x)
prop = property(g,s,d)
prop = property(fget=g,fset=s,fdel=d)
| fget
| function to be used for getting an attribute value
| fset
| function to be used for setting an attribute value
| fdel
| function to be used for del'ing an attribute
| doc
| docstring
class C:
def __init__(self):
self._x=None
def g(self):
return self._x
def s(self, x):
self._x = x
def d(self):
del self._x
def s2(self,x):
self._x=x+x
x=property(g)
x=x.setter(s)
x=x.deleter(d)
c = C()
c.x="a"
print(c.x) # outputs "a"
C.x=property(C.g, C.s2)
C.x=C.x.deleter(C.d)
c2 = C()
c2.x="a"
print(c2.x) # outputs "aa"
function decorator(undecorated_func):
print("calling decorator func")
inner():
print("I am inside inner")
return undecorated_func
return inner
undecorated_function= decorator(undecorated_func)
class Person:
def __init__(self,name):
self._name=name
@property
def name(self):
return self._name
@name.setter
def name(self.value):
self._name=value
name=property(name) # Person.__dict__ you ll see name
name=name.setter(name)
name=property(name)
name=name.setter(name)