标签归档:readonly

Python只读属性

问题:Python只读属性

我不知道何时属性应该是私有的,是否应该使用属性。

我最近读到,setter和getters不是pythonic,我应该使用属性装饰器。没关系。

但是,如果我有属性,该属性不能从类外部设置,而是可以读取的(只读属性)。这个属性应该是私有的self._x吗?我所说的私有是指下划线吗?如果是,那么不使用getter怎么读?我现在知道的唯一方法是写

@property
def x(self):
    return self._x

这样我就可以读取属性,obj.x但是我无法设置它,obj.x = 1所以很好。

但是,我真的应该在乎设置不应该设置的对象吗?也许我应该离开它。但是话又说回来,我不能使用下划线,因为阅读obj._x对于用户来说很奇怪,所以我应该使用下划线obj.x,然后用户又一次不知道他一定不能设置该属性。

您的看法和做法是什么?

I don’t know when attribute should be private and if I should use property.

I read recently that setters and getters are not pythonic and I should use property decorator. It’s ok.

But what if I have attribute, that mustn’t be set from outside of class but can be read (read-only attribute). Should this attribute be private, and by private I mean with underscore, like that self._x? If yes then how can I read it without using getter? Only method I know right now is to write

@property
def x(self):
    return self._x

That way I can read attribute by obj.x but I can’t set it obj.x = 1 so it’s fine.

But should I really care about setting object that mustn’t be set? Maybe I should just leave it. But then again I can’t use underscore because reading obj._x is odd for user, so I should use obj.x and then again user doesn’t know that he mustn’t set this attribute.

What’s your opinion and practics?


回答 0

通常,在编写Python程序时应假定所有用户都同意成年人,因此他们有责任自己正确使用事物。但是,在极少数情况下,无法设置属性(例如派生值或从某个静态数据源读取的值)就没有意义,仅使用吸气剂的属性通常是首选模式。

Generally, Python programs should be written with the assumption that all users are consenting adults, and thus are responsible for using things correctly themselves. However, in the rare instance where it just does not make sense for an attribute to be settable (such as a derived value, or a value read from some static datasource), the getter-only property is generally the preferred pattern.


回答 1

西拉斯·雷Silas Ray)只是我的两分钱,走在正确的轨道上,但是我觉得自己想举个例子。;-)

Python是一种类型不安全的语言,因此,您始终必须信任代码的用户才能像合理的(明智的)人员一样使用代码。

根据PEP 8

仅对非公共方法和实例变量使用前导下划线。

要在类中具有“只读”属性,您可以使用@property修饰,您需要在继承object时使用新样式的类来进行继承。

例:

>>> class A(object):
...     def __init__(self, a):
...         self._a = a
...
...     @property
...     def a(self):
...         return self._a
... 
>>> a = A('test')
>>> a.a
'test'
>>> a.a = 'pleh'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: can't set attribute

Just my two cents, Silas Ray is on the right track, however I felt like adding an example. ;-)

Python is a type-unsafe language and thus you’ll always have to trust the users of your code to use the code like a reasonable (sensible) person.

Per PEP 8:

Use one leading underscore only for non-public methods and instance variables.

To have a ‘read-only’ property in a class you can make use of the @property decoration, you’ll need to inherit from object when you do so to make use of the new-style classes.

Example:

>>> class A(object):
...     def __init__(self, a):
...         self._a = a
...
...     @property
...     def a(self):
...         return self._a
... 
>>> a = A('test')
>>> a.a
'test'
>>> a.a = 'pleh'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: can't set attribute

回答 2

这是一种避免假设的方法

所有使用者都是成年人,因此有责任自行正确使用事物。

请在下面查看我的更新

使用@property,非常冗长,例如:

   class AClassWithManyAttributes:
        '''refactored to properties'''
        def __init__(a, b, c, d, e ...)
             self._a = a
             self._b = b
             self._c = c
             self.d = d
             self.e = e

        @property
        def a(self):
            return self._a
        @property
        def b(self):
            return self._b
        @property
        def c(self):
            return self._c
        # you get this ... it's long

使用

没有下划线:这是一个公共变量。
一个下划线:这是一个受保护的变量。
有两个下划线:这是一个私有变量。

除了最后一个,这是一个约定。如果您确实努力尝试,仍然可以使用双下划线访问变量。

那么我们该怎么办?我们是否放弃使用Python中的只读属性?

看哪!read_only_properties装潢抢救!

@read_only_properties('readonly', 'forbidden')
class MyClass(object):
    def __init__(self, a, b, c):
        self.readonly = a
        self.forbidden = b
        self.ok = c

m = MyClass(1, 2, 3)
m.ok = 4
# we can re-assign a value to m.ok
# read only access to m.readonly is OK 
print(m.ok, m.readonly) 
print("This worked...")
# this will explode, and raise AttributeError
m.forbidden = 4

你问:

哪里read_only_properties来的?

很高兴您询问,这是read_only_properties的来源:

def read_only_properties(*attrs):

    def class_rebuilder(cls):
        "The class decorator"

        class NewClass(cls):
            "This is the overwritten class"
            def __setattr__(self, name, value):
                if name not in attrs:
                    pass
                elif name not in self.__dict__:
                    pass
                else:
                    raise AttributeError("Can't modify {}".format(name))

                super().__setattr__(name, value)
        return NewClass
    return class_rebuilder

更新

我没想到这个答案会引起如此多的关注。令人惊讶的是。这鼓励我创建一个可以使用的软件包。

$ pip install read-only-properties

在您的python shell中:

In [1]: from rop import read_only_properties

In [2]: @read_only_properties('a')
   ...: class Foo:
   ...:     def __init__(self, a, b):
   ...:         self.a = a
   ...:         self.b = b
   ...:         

In [3]: f=Foo('explodes', 'ok-to-overwrite')

In [4]: f.b = 5

In [5]: f.a = 'boom'
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-5-a5226072b3b4> in <module>()
----> 1 f.a = 'boom'

/home/oznt/.virtualenvs/tracker/lib/python3.5/site-packages/rop.py in __setattr__(self, name, value)
    116                     pass
    117                 else:
--> 118                     raise AttributeError("Can't touch {}".format(name))
    119 
    120                 super().__setattr__(name, value)

AttributeError: Can't touch a

Here is a way to avoid the assumption that

all users are consenting adults, and thus are responsible for using things correctly themselves.

please see my update below

Using @property, is very verbose e.g.:

   class AClassWithManyAttributes:
        '''refactored to properties'''
        def __init__(a, b, c, d, e ...)
             self._a = a
             self._b = b
             self._c = c
             self.d = d
             self.e = e

        @property
        def a(self):
            return self._a
        @property
        def b(self):
            return self._b
        @property
        def c(self):
            return self._c
        # you get this ... it's long

Using

No underscore: it’s a public variable.
One underscore: it’s a protected variable.
Two underscores: it’s a private variable.

Except the last one, it’s a convention. You can still, if you really try hard, access variables with double underscore.

So what do we do? Do we give up on having read only properties in Python?

Behold! read_only_properties decorator to the rescue!

@read_only_properties('readonly', 'forbidden')
class MyClass(object):
    def __init__(self, a, b, c):
        self.readonly = a
        self.forbidden = b
        self.ok = c

m = MyClass(1, 2, 3)
m.ok = 4
# we can re-assign a value to m.ok
# read only access to m.readonly is OK 
print(m.ok, m.readonly) 
print("This worked...")
# this will explode, and raise AttributeError
m.forbidden = 4

You ask:

Where is read_only_properties coming from?

Glad you asked, here is the source for read_only_properties:

def read_only_properties(*attrs):

    def class_rebuilder(cls):
        "The class decorator"

        class NewClass(cls):
            "This is the overwritten class"
            def __setattr__(self, name, value):
                if name not in attrs:
                    pass
                elif name not in self.__dict__:
                    pass
                else:
                    raise AttributeError("Can't modify {}".format(name))

                super().__setattr__(name, value)
        return NewClass
    return class_rebuilder

update

I never expected this answer will get so much attention. Surprisingly it does. This encouraged me to create a package you can use.

$ pip install read-only-properties

in your python shell:

In [1]: from rop import read_only_properties

In [2]: @read_only_properties('a')
   ...: class Foo:
   ...:     def __init__(self, a, b):
   ...:         self.a = a
   ...:         self.b = b
   ...:         

In [3]: f=Foo('explodes', 'ok-to-overwrite')

In [4]: f.b = 5

In [5]: f.a = 'boom'
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-5-a5226072b3b4> in <module>()
----> 1 f.a = 'boom'

/home/oznt/.virtualenvs/tracker/lib/python3.5/site-packages/rop.py in __setattr__(self, name, value)
    116                     pass
    117                 else:
--> 118                     raise AttributeError("Can't touch {}".format(name))
    119 
    120                 super().__setattr__(name, value)

AttributeError: Can't touch a

回答 3

这是一种对只读属性略有不同的方法,由于必须对它们进行初始化,因此应该将它们称为一次写入属性,不是吗?对于那些担心直接通过访问对象字典来修改属性的偏执狂,我引入了“极端”名称处理:

from uuid import uuid4

class Read_Only_Property:
    def __init__(self, name):
        self.name = name
        self.dict_name = uuid4().hex
        self.initialized = False

    def __get__(self, instance, cls):
        if instance is None:
            return self
        else:
            return instance.__dict__[self.dict_name]

    def __set__(self, instance, value):
        if self.initialized:
            raise AttributeError("Attempt to modify read-only property '%s'." % self.name)
        instance.__dict__[self.dict_name] = value
        self.initialized = True

class Point:
    x = Read_Only_Property('x')
    y = Read_Only_Property('y')
    def __init__(self, x, y):
        self.x = x
        self.y = y

if __name__ == '__main__':
    try:
        p = Point(2, 3)
        print(p.x, p.y)
        p.x = 9
    except Exception as e:
        print(e)

Here is a slightly different approach to read-only properties, which perhaps should be called write-once properties since they do have to get initialized, don’t they? For the paranoid among us who worry about being able to modify properties by accessing the object’s dictionary directly, I’ve introduced “extreme” name mangling:

from uuid import uuid4

class Read_Only_Property:
    def __init__(self, name):
        self.name = name
        self.dict_name = uuid4().hex
        self.initialized = False

    def __get__(self, instance, cls):
        if instance is None:
            return self
        else:
            return instance.__dict__[self.dict_name]

    def __set__(self, instance, value):
        if self.initialized:
            raise AttributeError("Attempt to modify read-only property '%s'." % self.name)
        instance.__dict__[self.dict_name] = value
        self.initialized = True

class Point:
    x = Read_Only_Property('x')
    y = Read_Only_Property('y')
    def __init__(self, x, y):
        self.x = x
        self.y = y

if __name__ == '__main__':
    try:
        p = Point(2, 3)
        print(p.x, p.y)
        p.x = 9
    except Exception as e:
        print(e)

回答 4

我对创建只读属性的前两个答案不满意,因为第一个解决方案允许删除readonly属性,然后进行设置,并且不会阻止__dict__。第二种解决方案可以与测试一起解决-找到等于您将其设置为2的值并最终进行更改。

现在,获取代码。

def final(cls):
    clss = cls
    @classmethod
    def __init_subclass__(cls, **kwargs):
        raise TypeError("type '{}' is not an acceptable base type".format(clss.__name__))
    cls.__init_subclass__ = __init_subclass__
    return cls


def methoddefiner(cls, method_name):
    for clss in cls.mro():
        try:
            getattr(clss, method_name)
            return clss
        except(AttributeError):
            pass
    return None


def readonlyattributes(*attrs):
    """Method to create readonly attributes in a class

    Use as a decorator for a class. This function takes in unlimited 
    string arguments for names of readonly attributes and returns a
    function to make the readonly attributes readonly. 

    The original class's __getattribute__, __setattr__, and __delattr__ methods
    are redefined so avoid defining those methods in the decorated class

    You may create setters and deleters for readonly attributes, however
    if they are overwritten by the subclass, they lose access to the readonly
    attributes. 

    Any method which sets or deletes a readonly attribute within
    the class loses access if overwritten by the subclass besides the __new__
    or __init__ constructors.

    This decorator doesn't support subclassing of these classes
    """
    def classrebuilder(cls):
        def __getattribute__(self, name):
            if name == '__dict__':
                    from types import MappingProxyType
                    return MappingProxyType(super(cls, self).__getattribute__('__dict__'))
            return super(cls, self).__getattribute__(name)
        def __setattr__(self, name, value): 
                if name == '__dict__' or name in attrs:
                    import inspect
                    stack = inspect.stack()
                    try:
                        the_class = stack[1][0].f_locals['self'].__class__
                    except(KeyError):
                        the_class = None
                    the_method = stack[1][0].f_code.co_name
                    if the_class != cls: 
                         if methoddefiner(type(self), the_method) != cls:
                            raise AttributeError("Cannot set readonly attribute '{}'".format(name))                        
                return super(cls, self).__setattr__(name, value)
        def __delattr__(self, name):                
                if name == '__dict__' or name in attrs:
                    import inspect
                    stack = inspect.stack()
                    try:
                        the_class = stack[1][0].f_locals['self'].__class__
                    except(KeyError):
                        the_class = None
                    the_method = stack[1][0].f_code.co_name
                    if the_class != cls:
                        if methoddefiner(type(self), the_method) != cls:
                            raise AttributeError("Cannot delete readonly attribute '{}'".format(name))                        
                return super(cls, self).__delattr__(name)
        clss = cls
        cls.__getattribute__ = __getattribute__
        cls.__setattr__ = __setattr__
        cls.__delattr__ = __delattr__
        #This line will be moved when this algorithm will be compatible with inheritance
        cls = final(cls)
        return cls
    return classrebuilder

def setreadonlyattributes(cls, *readonlyattrs):
    return readonlyattributes(*readonlyattrs)(cls)


if __name__ == '__main__':
    #test readonlyattributes only as an indpendent module
    @readonlyattributes('readonlyfield')
    class ReadonlyFieldClass(object):
        def __init__(self, a, b):
            #Prevent initalization of the internal, unmodified PrivateFieldClass
            #External PrivateFieldClass can be initalized
            self.readonlyfield = a
            self.publicfield = b


    attr = None
    def main():
        global attr
        pfi = ReadonlyFieldClass('forbidden', 'changable')
        ###---test publicfield, ensure its mutable---###
        try:
            #get publicfield
            print(pfi.publicfield)
            print('__getattribute__ works')
            #set publicfield
            pfi.publicfield = 'mutable'
            print('__setattr__ seems to work')
            #get previously set publicfield
            print(pfi.publicfield)
            print('__setattr__ definitely works')
            #delete publicfield
            del pfi.publicfield 
            print('__delattr__ seems to work')
            #get publicfield which was supposed to be deleted therefore should raise AttributeError
            print(pfi.publlicfield)
            #publicfield wasn't deleted, raise RuntimeError
            raise RuntimeError('__delattr__ doesn\'t work')
        except(AttributeError):
            print('__delattr__ works')


        try:
            ###---test readonly, make sure its readonly---###
            #get readonlyfield
            print(pfi.readonlyfield)
            print('__getattribute__ works')
            #set readonlyfield, should raise AttributeError
            pfi.readonlyfield = 'readonly'
            #apparently readonlyfield was set, notify user
            raise RuntimeError('__setattr__ doesn\'t work')
        except(AttributeError):
            print('__setattr__ seems to work')
            try:
                #ensure readonlyfield wasn't set
                print(pfi.readonlyfield)
                print('__setattr__ works')
                #delete readonlyfield
                del pfi.readonlyfield
                #readonlyfield was deleted, raise RuntimeError
                raise RuntimeError('__delattr__ doesn\'t work')
            except(AttributeError):
                print('__delattr__ works')
        try:
            print("Dict testing")
            print(pfi.__dict__, type(pfi.__dict__))
            attr = pfi.readonlyfield
            print(attr)
            print("__getattribute__ works")
            if pfi.readonlyfield != 'forbidden':
                print(pfi.readonlyfield)
                raise RuntimeError("__getattr__ doesn't work")
            try:
                pfi.__dict__ = {}
                raise RuntimeError("__setattr__ doesn't work")
            except(AttributeError):
                print("__setattr__ works")
            del pfi.__dict__
            raise RuntimeError("__delattr__ doesn't work")
        except(AttributeError):
            print(pfi.__dict__)
            print("__delattr__ works")
            print("Basic things work")


main()

除非您编写库代码时使用只读属性,否则将这些属性设置为只读代码是为了增强他们的程序而将代码分发给其他人使用,而不是用于其他目的(例如应用程序开发)的代码。解决了__dict__问题,因为__dict__现在是不可变的类型。MappingProxyType,因此无法通过__dict__更改属性。设置或删除__dict__也被阻止。更改只读属性的唯一方法是更改​​类本身的方法。

尽管我认为我的解决方案比前两个解决方案要好,但可以改进。这些是此代码的弱点:

a)不允许在子类中添加设置或删除只读属性的方法。即使调用了超类的方法,也会自动禁止子类中定义的方法访问只读属性。

b)可以更改类的只读方法以克服只读限制。

但是,没有办法不编辑类来设置或删除只读属性。这不依赖于命名约定,这很好,因为Python与命名约定不太一致。这提供了一种方法,使只读属性无法通过隐藏的漏洞进行更改,而无需编辑类本身。只需在将装饰器作为参数调用时列出要只读的属性即可,它们将变为只读。

归功于Brice的回答:如何在python中另一个类的函数中获取调用方类名称?获取调用方的类和方法。

I am dissatisfied with the previous two answers to create read only properties because the first solution allows the readonly attribute to be deleted and then set and doesn’t block the __dict__. The second solution could be worked around with testing – finding the value that equals what you set it two and changing it eventually.

Now, for the code.

def final(cls):
    clss = cls
    @classmethod
    def __init_subclass__(cls, **kwargs):
        raise TypeError("type '{}' is not an acceptable base type".format(clss.__name__))
    cls.__init_subclass__ = __init_subclass__
    return cls


def methoddefiner(cls, method_name):
    for clss in cls.mro():
        try:
            getattr(clss, method_name)
            return clss
        except(AttributeError):
            pass
    return None


def readonlyattributes(*attrs):
    """Method to create readonly attributes in a class

    Use as a decorator for a class. This function takes in unlimited 
    string arguments for names of readonly attributes and returns a
    function to make the readonly attributes readonly. 

    The original class's __getattribute__, __setattr__, and __delattr__ methods
    are redefined so avoid defining those methods in the decorated class

    You may create setters and deleters for readonly attributes, however
    if they are overwritten by the subclass, they lose access to the readonly
    attributes. 

    Any method which sets or deletes a readonly attribute within
    the class loses access if overwritten by the subclass besides the __new__
    or __init__ constructors.

    This decorator doesn't support subclassing of these classes
    """
    def classrebuilder(cls):
        def __getattribute__(self, name):
            if name == '__dict__':
                    from types import MappingProxyType
                    return MappingProxyType(super(cls, self).__getattribute__('__dict__'))
            return super(cls, self).__getattribute__(name)
        def __setattr__(self, name, value): 
                if name == '__dict__' or name in attrs:
                    import inspect
                    stack = inspect.stack()
                    try:
                        the_class = stack[1][0].f_locals['self'].__class__
                    except(KeyError):
                        the_class = None
                    the_method = stack[1][0].f_code.co_name
                    if the_class != cls: 
                         if methoddefiner(type(self), the_method) != cls:
                            raise AttributeError("Cannot set readonly attribute '{}'".format(name))                        
                return super(cls, self).__setattr__(name, value)
        def __delattr__(self, name):                
                if name == '__dict__' or name in attrs:
                    import inspect
                    stack = inspect.stack()
                    try:
                        the_class = stack[1][0].f_locals['self'].__class__
                    except(KeyError):
                        the_class = None
                    the_method = stack[1][0].f_code.co_name
                    if the_class != cls:
                        if methoddefiner(type(self), the_method) != cls:
                            raise AttributeError("Cannot delete readonly attribute '{}'".format(name))                        
                return super(cls, self).__delattr__(name)
        clss = cls
        cls.__getattribute__ = __getattribute__
        cls.__setattr__ = __setattr__
        cls.__delattr__ = __delattr__
        #This line will be moved when this algorithm will be compatible with inheritance
        cls = final(cls)
        return cls
    return classrebuilder

def setreadonlyattributes(cls, *readonlyattrs):
    return readonlyattributes(*readonlyattrs)(cls)


if __name__ == '__main__':
    #test readonlyattributes only as an indpendent module
    @readonlyattributes('readonlyfield')
    class ReadonlyFieldClass(object):
        def __init__(self, a, b):
            #Prevent initalization of the internal, unmodified PrivateFieldClass
            #External PrivateFieldClass can be initalized
            self.readonlyfield = a
            self.publicfield = b


    attr = None
    def main():
        global attr
        pfi = ReadonlyFieldClass('forbidden', 'changable')
        ###---test publicfield, ensure its mutable---###
        try:
            #get publicfield
            print(pfi.publicfield)
            print('__getattribute__ works')
            #set publicfield
            pfi.publicfield = 'mutable'
            print('__setattr__ seems to work')
            #get previously set publicfield
            print(pfi.publicfield)
            print('__setattr__ definitely works')
            #delete publicfield
            del pfi.publicfield 
            print('__delattr__ seems to work')
            #get publicfield which was supposed to be deleted therefore should raise AttributeError
            print(pfi.publlicfield)
            #publicfield wasn't deleted, raise RuntimeError
            raise RuntimeError('__delattr__ doesn\'t work')
        except(AttributeError):
            print('__delattr__ works')


        try:
            ###---test readonly, make sure its readonly---###
            #get readonlyfield
            print(pfi.readonlyfield)
            print('__getattribute__ works')
            #set readonlyfield, should raise AttributeError
            pfi.readonlyfield = 'readonly'
            #apparently readonlyfield was set, notify user
            raise RuntimeError('__setattr__ doesn\'t work')
        except(AttributeError):
            print('__setattr__ seems to work')
            try:
                #ensure readonlyfield wasn't set
                print(pfi.readonlyfield)
                print('__setattr__ works')
                #delete readonlyfield
                del pfi.readonlyfield
                #readonlyfield was deleted, raise RuntimeError
                raise RuntimeError('__delattr__ doesn\'t work')
            except(AttributeError):
                print('__delattr__ works')
        try:
            print("Dict testing")
            print(pfi.__dict__, type(pfi.__dict__))
            attr = pfi.readonlyfield
            print(attr)
            print("__getattribute__ works")
            if pfi.readonlyfield != 'forbidden':
                print(pfi.readonlyfield)
                raise RuntimeError("__getattr__ doesn't work")
            try:
                pfi.__dict__ = {}
                raise RuntimeError("__setattr__ doesn't work")
            except(AttributeError):
                print("__setattr__ works")
            del pfi.__dict__
            raise RuntimeError("__delattr__ doesn't work")
        except(AttributeError):
            print(pfi.__dict__)
            print("__delattr__ works")
            print("Basic things work")


main()

There is no point to making read only attributes except when your writing library code, code which is being distributed to others as code to use in order to enhance their programs, not code for any other purpose, like app development. The __dict__ problem is solved, because the __dict__ is now of the immutable types.MappingProxyType, so attributes cannot be changed through the __dict__. Setting or deleting __dict__ is also blocked. The only way to change read only properties is through changing the methods of the class itself.

Though I believe my solution is better than of the previous two, it could be improved. These are this code’s weaknesses:

a) Doesn’t allow adding to a method in a subclass which sets or deletes a readonly attribute. A method defined in a subclass is automatically barred from accessing a readonly attribute, even by calling the superclass’ version of the method.

b) The class’ readonly methods can be changed to defeat the read only restrictions.

However, there is not way without editing the class to set or delete a read only attribute. This isn’t dependent on naming conventions, which is good because Python isn’t so consistent with naming conventions. This provides a way to make read only attributes that cannot be changed with hidden loopholes without editing the class itself. Simply list the attributes to be read only when calling the decorator as arguments and they will become read only.

Credit to Brice’s answer in How to get the caller class name inside a function of another class in python? for getting the caller classes and methods.


回答 5

注意,实例方法也是(类的)属性,如果您确实想成为坏蛋,则可以在类或实例级别设置它们。或者,您可以设置一个类变量(这也是该类的一个属性),在该变量中,方便的只读属性将无法立即使用。我要说的是,“只读属性”问题实际上比通常认为的要普遍得多。幸运的是,人们对工作的传统期望是如此强烈,以至于使我们在其他情况下视而不见(毕竟,几乎所有东西都是python中的某种属性)。

基于这些期望,我认为最通用,最轻便的方法是采用以下约定:“公开”(无前导下划线)属性是只读的,除非明确记录为可写。这包含了通常的期望,即不会对方法进行修补,而指示实例默认值的类变量则更不用说了。如果您真的对某些特殊属性感到偏执,请使用只读描述符作为最后的资源度量。

Notice that instance methods are also attributes (of the class) and that you could set them at the class or instance level if you really wanted to be a badass. Or that you may set a class variable (which is also an attribute of the class), where handy readonly properties won’t work neatly out of the box. What I’m trying to say is that the “readonly attribute” problem is in fact more general than it’s usually perceived to be. Fortunately there are conventional expectations at work that are so strong as to blind us wrt these other cases (after all, almost everything is an attribute of some sort in python).

Building upon these expectations I think the most general and lightweight approach is to adopt the convention that “public” (no leading underscore) attributes are readonly except when explicitly documented as writeable. This subsumes the usual expectation that methods won’t be patched and class variables indicating instance defaults are better let alone. If you feel really paranoid about some special attribute, use a readonly descriptor as a last resource measure.


回答 6

尽管我喜欢Oz123的类装饰器,但是您也可以执行以下操作,该操作使用显式类包装器和__new__以及类Factory方法,以在闭包内返回类:

class B(object):
    def __new__(cls, val):
        return cls.factory(val)

@classmethod
def factory(cls, val):
    private = {'var': 'test'}

    class InnerB(object):
        def __init__(self):
            self.variable = val
            pass

        @property
        def var(self):
            return private['var']

    return InnerB()

While I like the class decorator from Oz123, you could also do the following, which uses an explicit class wrapper and __new__ with a class Factory method returning the class within a closure:

class B(object):
    def __new__(cls, val):
        return cls.factory(val)

@classmethod
def factory(cls, val):
    private = {'var': 'test'}

    class InnerB(object):
        def __init__(self):
            self.variable = val
            pass

        @property
        def var(self):
            return private['var']

    return InnerB()

回答 7

那是我的解决方法。

@property
def language(self):
    return self._language
@language.setter
def language(self, value):
    # WORKAROUND to get a "getter-only" behavior
    # set the value only if the attribute does not exist
    try:
        if self.language == value:
            pass
        print("WARNING: Cannot set attribute \'language\'.")
    except AttributeError:
        self._language = value

That’s my workaround.

@property
def language(self):
    return self._language
@language.setter
def language(self, value):
    # WORKAROUND to get a "getter-only" behavior
    # set the value only if the attribute does not exist
    try:
        if self.language == value:
            pass
        print("WARNING: Cannot set attribute \'language\'.")
    except AttributeError:
        self._language = value

回答 8

有人提到使用代理对象,但我没有看到这样的示例,所以我最终尝试了一下,[可怜]。

/!\如果可能,请更喜欢类定义和类构造函数

这段代码可以有效地重写class.__new__(类构造函数),但在各个方面都更糟。减轻痛苦,如果可以,请不要使用此模式。

def attr_proxy(obj):
    """ Use dynamic class definition to bind obj and proxy_attrs.
        If you can extend the target class constructor that is 
        cleaner, but its not always trivial to do so.
    """
    proxy_attrs = dict()

    class MyObjAttrProxy():
        def __getattr__(self, name):
            if name in proxy_attrs:
                return proxy_attrs[name]  # overloaded

            return getattr(obj, name)  # proxy

        def __setattr__(self, name, value):
            """ note, self is not bound when overloading methods
            """
            proxy_attrs[name] = value

    return MyObjAttrProxy()


myobj = attr_proxy(Object())
setattr(myobj, 'foo_str', 'foo')

def func_bind_obj_as_self(func, self):
    def _method(*args, **kwargs):
        return func(self, *args, **kwargs)
    return _method

def mymethod(self, foo_ct):
    """ self is not bound because we aren't using object __new__
        you can write the __setattr__ method to bind a self 
        argument, or declare your functions dynamically to bind in 
        a static object reference.
    """
    return self.foo_str + foo_ct

setattr(myobj, 'foo', func_bind_obj_as_self(mymethod, myobj))

someone mentioned using a proxy object, I didn’t see an example of that so I ended up trying it out, [poorly].

/!\ Please prefer class definitions and class constructors if possible

this code is effectively re-writing class.__new__ (class constructor) except worse in every way. Save yourself the pain and do not use this pattern if you can.

def attr_proxy(obj):
    """ Use dynamic class definition to bind obj and proxy_attrs.
        If you can extend the target class constructor that is 
        cleaner, but its not always trivial to do so.
    """
    proxy_attrs = dict()

    class MyObjAttrProxy():
        def __getattr__(self, name):
            if name in proxy_attrs:
                return proxy_attrs[name]  # overloaded

            return getattr(obj, name)  # proxy

        def __setattr__(self, name, value):
            """ note, self is not bound when overloading methods
            """
            proxy_attrs[name] = value

    return MyObjAttrProxy()


myobj = attr_proxy(Object())
setattr(myobj, 'foo_str', 'foo')

def func_bind_obj_as_self(func, self):
    def _method(*args, **kwargs):
        return func(self, *args, **kwargs)
    return _method

def mymethod(self, foo_ct):
    """ self is not bound because we aren't using object __new__
        you can write the __setattr__ method to bind a self 
        argument, or declare your functions dynamically to bind in 
        a static object reference.
    """
    return self.foo_str + foo_ct

setattr(myobj, 'foo', func_bind_obj_as_self(mymethod, myobj))

回答 9

我知道我从头开始带回了这个线程,但是我正在研究如何使属性变为只读,并且在找到该主题之后,我对已经共享的解决方案不满意。

因此,如果您从以下代码开始,请回到最初的问题:

@property
def x(self):
    return self._x

并且您想将X设为只读,只需添加:

@x.setter
def x(self, value):
    raise Exception("Member readonly")

然后,如果您运行以下命令:

print (x) # Will print whatever X value is
x = 3 # Will raise exception "Member readonly"

I know i’m bringing back from the dead this thread, but I was looking at how to make a property read only and after finding this topic, I wasn’t satisfied with the solutions already shared.

So, going back to the initial question, if you start with this code:

@property
def x(self):
    return self._x

And you want to make X readonly, you can just add:

@x.setter
def x(self, value):
    raise Exception("Member readonly")

Then, if you run the following:

print (x) # Will print whatever X value is
x = 3 # Will raise exception "Member readonly"