防止在__ init__ 之外创建新属性

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The proper way is to override __setattr__. That's what it's there for.

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Actually, you don't want __setattr__, you want __slots__. Add __slots__ = ('foo', 'bar', 'baz') to the class body, and Python will make sure that there's only foo, bar and baz on any instance. But read the caveats the documentation lists!

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最佳答案

I wouldn't use __dict__ directly, but you can add a function to explicitly "freeze" a instance:

class FrozenClass(object):
__isfrozen = False
def __setattr__(self, key, value):
if self.__isfrozen and not hasattr(self, key):
raise TypeError( "%r is a frozen class" % self )
object.__setattr__(self, key, value)


def _freeze(self):
self.__isfrozen = True


class Test(FrozenClass):
def __init__(self):
self.x = 42#
self.y = 2**3


self._freeze() # no new attributes after this point.


a,b = Test(), Test()
a.x = 10
b.z = 10 # fails

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If someone is interested in doing that with a decorator, here is a working solution:

from functools import wraps


def froze_it(cls):
cls.__frozen = False


def frozensetattr(self, key, value):
if self.__frozen and not hasattr(self, key):
print("Class {} is frozen. Cannot set {} = {}"
.format(cls.__name__, key, value))
else:
object.__setattr__(self, key, value)


def init_decorator(func):
@wraps(func)
def wrapper(self, *args, **kwargs):
func(self, *args, **kwargs)
self.__frozen = True
return wrapper


cls.__setattr__ = frozensetattr
cls.__init__ = init_decorator(cls.__init__)


return cls

Pretty straightforward to use:

@froze_it
class Foo(object):
def __init__(self):
self.bar = 10


foo = Foo()
foo.bar = 42
foo.foobar = "no way"

Result:

>>> Class Foo is frozen. Cannot set foobar = no way

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Here is approach i came up with that doesn't need a _frozen attribute or method to freeze() in init.

During init i just add all class attributes to the instance.

I like this because there is no _frozen, freeze(), and _frozen also does not show up in the vars(instance) output.

class MetaModel(type):
def __setattr__(self, name, value):
raise AttributeError("Model classes do not accept arbitrary attributes")


class Model(object):
__metaclass__ = MetaModel


# init will take all CLASS attributes, and add them as SELF/INSTANCE attributes
def __init__(self):
for k, v in self.__class__.__dict__.iteritems():
if not k.startswith("_"):
self.__setattr__(k, v)


# setattr, won't allow any attributes to be set on the SELF/INSTANCE that don't already exist
def __setattr__(self, name, value):
if not hasattr(self, name):
raise AttributeError("Model instances do not accept arbitrary attributes")
else:
object.__setattr__(self, name, value)




# Example using
class Dog(Model):
name = ''
kind = 'canine'


d, e = Dog(), Dog()
print vars(d)
print vars(e)
e.junk = 'stuff' # fails

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I like very much the solution that uses a decorator, because it's easy to use it for many classes across a project, with minimum additions for each class. But it doesn't work well with inheritance. So here is my version: It only overrides the __setattr__ function - if the attribute doesn't exist and the caller function is not __init__, it prints an error message.

import inspect


def froze_it(cls):


def frozensetattr(self, key, value):
if not hasattr(self, key) and inspect.stack()[1][3] != "__init__":
print("Class {} is frozen. Cannot set {} = {}"
.format(cls.__name__, key, value))
else:
self.__dict__[key] = value


cls.__setattr__ = frozensetattr
return cls


@froze_it
class A:
def __init__(self):
self._a = 0


a = A()
a._a = 1
a._b = 2 # error

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I like the "Frozen" of Jochen Ritzel. The inconvenient is that the isfrozen variable then appears when printing a Class.__dict I went around this problem this way by creating a list of authorized attributes (similar to slots):

class Frozen(object):
__List = []
def __setattr__(self, key, value):
setIsOK = False
for item in self.__List:
if key == item:
setIsOK = True


if setIsOK == True:
object.__setattr__(self, key, value)
else:
raise TypeError( "%r has no attributes %r" % (self, key) )


class Test(Frozen):
_Frozen__List = ["attr1","attr2"]
def __init__(self):
self.attr1   =  1
self.attr2   =  1

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What about this:

class A():
__allowed_attr=('_x', '_y')


def __init__(self,x=0,y=0):
self._x=x
self._y=y


def __setattr__(self,attribute,value):
if not attribute in self.__class__.__allowed_attr:
raise AttributeError
else:
super().__setattr__(attribute,value)

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The FrozenClass by Jochen Ritzel is cool, but calling _frozen() when initialing a class every time is not so cool (and you need to take the risk of forgetting it). I added a __init_slots__ function:

class FrozenClass(object):
__isfrozen = False
def _freeze(self):
self.__isfrozen = True
def __init_slots__(self, slots):
for key in slots:
object.__setattr__(self, key, None)
self._freeze()
def __setattr__(self, key, value):
if self.__isfrozen and not hasattr(self, key):
raise TypeError( "%r is a frozen class" % self )
object.__setattr__(self, key, value)
class Test(FrozenClass):
def __init__(self):
self.__init_slots__(["x", "y"])
self.x = 42#
self.y = 2**3




a,b = Test(), Test()
a.x = 10
b.z = 10 # fails

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Slots is the way to go:

The pythonic way is to use slots instead of playing around with the __setter__. While it may solve the problem, it does not give any performance improvement. The attributes of objects are stored in a dictionary "__dict__", this is the reason, why you can dynamically add attributes to objects of classes that we have created so far. Using a dictionary for attribute storage is very convenient, but it can mean a waste of space for objects, which have only a small amount of instance variables.

Slots are a nice way to work around this space consumption problem. Instead of having a dynamic dict that allows adding attributes to objects dynamically, slots provide a static structure which prohibits additions after the creation of an instance.

When we design a class, we can use slots to prevent the dynamic creation of attributes. To define slots, you have to define a list with the name __slots__. The list has to contain all the attributes, you want to use. We demonstrate this in the following class, in which the slots list contains only the name for an attribute "val".

class S(object):


__slots__ = ['val']


def __init__(self, v):
self.val = v




x = S(42)
print(x.val)


x.new = "not possible"

=> It fails to create an attribute "new":

42
Traceback (most recent call last):
File "slots_ex.py", line 12, in <module>
x.new = "not possible"
AttributeError: 'S' object has no attribute 'new'

Notes:

Since Python 3.3 the advantage optimizing the space consumption is not as impressive any more. With Python 3.3 Key-Sharing Dictionaries are used for the storage of objects. The attributes of the instances are capable of sharing part of their internal storage between each other, i.e. the part which stores the keys and their corresponding hashes. This helps to reduce the memory consumption of programs, which create many instances of non-builtin types. But still is the way to go to avoid dynamically created attributes.

Using slots come also with it's own cost. It will break serialization (e.g. pickle). It will also break multiple inheritance. A class can't inherit from more than one class that either defines slots or has an instance layout defined in C code (like list, tuple or int).

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None of the answers mention the performance impact of overriding __setattr__, which can be an issue when creating many small objects. (And __slots__ would be the performant solution but limits pickle/inheritance).

So I came up with this variant which installs our slower settatr after init:

class FrozenClass:


def freeze(self):
def frozen_setattr(self, key, value):
if not hasattr(self, key):
raise TypeError("Cannot set {}: {} is a frozen class".format(key, self))
object.__setattr__(self, key, value)
self.__setattr__ = frozen_setattr


class Foo(FrozenClass): ...

If you don't want to call freeze at the end of __init__, if inheritance is an issue, or if you don't want it in vars(), it can also be adapted: for example here is a decorator version based on the pystrict answer:

import functools
def strict(cls):
cls._x_setter = getattr(cls, "__setattr__", object.__setattr__)
cls._x_init = cls.__init__
@functools.wraps(cls.__init__)
def wrapper(self, *args, **kwargs):
cls._x_init(self, *args, **kwargs)
def frozen_setattr(self, key, value):
if not hasattr(self, key):
raise TypeError("Class %s is frozen. Cannot set '%s'." % (cls.__name__, key))
cls._x_setter(self, key, value)
cls.__setattr__ = frozen_setattr
cls.__init__ = wrapper
return cls


@strict
class Foo: ...

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I wrote pystrict as a solution to this problem. It's too large to paste all of the code in stackoverflow.

pystrict is a pypi installable decorator that can be used with classes to freeze them. Many solutions here don't properly support inheritance.

If __slots__ doesn't work for you (because of inheritance issues), this is a good alternative.

There is an example to the README that shows why a decorator like this is needed even if you have mypy and pylint running on your project:

pip install pystrict

Then just use the @strict decorator:

from pystrict import strict


@strict
class Blah
def __init__(self):
self.attr = 1