虽然我从来都不需要这样做,但我突然意识到用Python创建一个不可变对象可能有点棘手。你不能只是覆盖__setattr__,因为这样你甚至不能在__init__中设置属性。子类化一个元组是一个有效的技巧:
class Immutable(tuple):
def __new__(cls, a, b):
return tuple.__new__(cls, (a, b))
@property
def a(self):
return self[0]
@property
def b(self):
return self[1]
def __str__(self):
return "<Immutable {0}, {1}>".format(self.a, self.b)
def __setattr__(self, *ignored):
raise NotImplementedError
def __delattr__(self, *ignored):
raise NotImplementedError
但是你可以通过self[0]和self[1]访问a和b变量,这很烦人。
这在Pure Python中可行吗?如果不是,我该如何用C扩展来做呢?
(只能在python3中工作的答案是可以接受的)。
更新:
从Python 3.7开始,要使用的方法是使用@dataclass装饰器,参见最新接受的答案。
..如何在C中“正确地”做这件事?
你可以使用Cython为Python创建一个扩展类型:
cdef class Immutable:
cdef readonly object a, b
cdef object __weakref__ # enable weak referencing support
def __init__(self, a, b):
self.a, self.b = a, b
它既适用于Python 2。X和3。
测试
# compile on-the-fly
import pyximport; pyximport.install() # $ pip install cython
from immutable import Immutable
o = Immutable(1, 2)
assert o.a == 1, str(o.a)
assert o.b == 2
try: o.a = 3
except AttributeError:
pass
else:
assert 0, 'attribute must be readonly'
try: o[1]
except TypeError:
pass
else:
assert 0, 'indexing must not be supported'
try: o.c = 1
except AttributeError:
pass
else:
assert 0, 'no new attributes are allowed'
o = Immutable('a', [])
assert o.a == 'a'
assert o.b == []
o.b.append(3) # attribute may contain mutable object
assert o.b == [3]
try: o.c
except AttributeError:
pass
else:
assert 0, 'no c attribute'
o = Immutable(b=3,a=1)
assert o.a == 1 and o.b == 3
try: del o.b
except AttributeError:
pass
else:
assert 0, "can't delete attribute"
d = dict(b=3, a=1)
o = Immutable(**d)
assert o.a == d['a'] and o.b == d['b']
o = Immutable(1,b=3)
assert o.a == 1 and o.b == 3
try: object.__setattr__(o, 'a', 1)
except AttributeError:
pass
else:
assert 0, 'attributes are readonly'
try: object.__setattr__(o, 'c', 1)
except AttributeError:
pass
else:
assert 0, 'no new attributes'
try: Immutable(1,c=3)
except TypeError:
pass
else:
assert 0, 'accept only a,b keywords'
for kwd in [dict(a=1), dict(b=2)]:
try: Immutable(**kwd)
except TypeError:
pass
else:
assert 0, 'Immutable requires exactly 2 arguments'
如果你不介意索引支持,那么@Sven Marnach建议的collections.namedtuple是更可取的:
Immutable = collections.namedtuple("Immutable", "a b")
这里没有包括的是完全不可变性……不仅仅是父对象,还有所有的子对象。例如,元组/frozensets可能是不可变的,但它所属的对象可能不是。下面是一个小的(不完整的)版本,它在执行不变性方面做得很好:
# Initialize lists
a = [1,2,3]
b = [4,5,6]
c = [7,8,9]
l = [a,b]
# We can reassign in a list
l[0] = c
# But not a tuple
t = (a,b)
#t[0] = c -> Throws exception
# But elements can be modified
t[0][1] = 4
t
([1, 4, 3], [4, 5, 6])
# Fix it back
t[0][1] = 2
li = ImmutableObject(l)
li
[[1, 2, 3], [4, 5, 6]]
# Can't assign
#li[0] = c will fail
# Can reference
li[0]
[1, 2, 3]
# But immutability conferred on returned object too
#li[0][1] = 4 will throw an exception
# Full solution should wrap all the comparison e.g. decorators.
# Also, you'd usually want to add a hash function, i didn't put
# an interface for that.
class ImmutableObject(object):
def __init__(self, inobj):
self._inited = False
self._inobj = inobj
self._inited = True
def __repr__(self):
return self._inobj.__repr__()
def __str__(self):
return self._inobj.__str__()
def __getitem__(self, key):
return ImmutableObject(self._inobj.__getitem__(key))
def __iter__(self):
return self._inobj.__iter__()
def __setitem__(self, key, value):
raise AttributeError, 'Object is read-only'
def __getattr__(self, key):
x = getattr(self._inobj, key)
if callable(x):
return x
else:
return ImmutableObject(x)
def __hash__(self):
return self._inobj.__hash__()
def __eq__(self, second):
return self._inobj.__eq__(second)
def __setattr__(self, attr, value):
if attr not in ['_inobj', '_inited'] and self._inited == True:
raise AttributeError, 'Object is read-only'
object.__setattr__(self, attr, value)
除了其他优秀的答案之外,我喜欢为python 3.4(或者可能是3.3)添加一个方法。这个答案建立在之前对这个问题的几个答案的基础上。
在python 3.4中,可以使用不带设置符的属性来创建不可修改的类成员。(在早期版本中,可以不使用setter为属性赋值。)
class A:
__slots__=['_A__a']
def __init__(self, aValue):
self.__a=aValue
@property
def a(self):
return self.__a
你可以这样使用它:
instance=A("constant")
print (instance.a)
它会输出constant
而是调用实例。A =10会导致:
AttributeError: can't set attribute
解释:不带设置符的属性是python 3.4(我认为是3.3)的最新特性。如果您尝试给这样的属性赋值,则会引发Error。
使用插槽,我将成员变量限制为__A_a(即__a)。
问题:赋值给_aa仍然是可能的(instance. _aa =2)。但是如果你给一个私有变量赋值,那是你自己的错…
然而,这个答案不鼓励使用__slots__。使用其他方法来阻止属性创建可能更可取。
如果您对具有行为的对象感兴趣,那么namedtuple几乎是您的解决方案。
正如namedtuple文档底部所描述的,您可以从namedtuple派生自己的类;然后,你可以添加你想要的行为。
例如(代码直接取自文档):
class Point(namedtuple('Point', 'x y')):
__slots__ = ()
@property
def hypot(self):
return (self.x ** 2 + self.y ** 2) ** 0.5
def __str__(self):
return 'Point: x=%6.3f y=%6.3f hypot=%6.3f' % (self.x, self.y, self.hypot)
for p in Point(3, 4), Point(14, 5/7):
print(p)
这将导致:
Point: x= 3.000 y= 4.000 hypot= 5.000
Point: x=14.000 y= 0.714 hypot=14.018
这种方法适用于Python 3和Python 2.7(在IronPython上也进行了测试)。
唯一的缺点是继承树有点奇怪;但这不是你经常玩的东西。
我已经创建了一个小型类装饰器decorator,以使类不可变(除了在__init__内部)。作为https://github.com/google/etils的一部分。
from etils import epy
@epy.frozen
class A:
def __init__(self):
self.x = 123 # Inside `__init__`, attribute can be assigned
a = A()
a.x = 456 # AttributeError
这也支持继承。
实现:
_Cls = TypeVar('_Cls')
def frozen(cls: _Cls) -> _Cls:
"""Class decorator which prevent mutating attributes after `__init__`."""
if not isinstance(cls, type):
raise TypeError(f'{cls.__name__} is not a class.')
cls.__init__ = _wrap_init(cls.__init__)
cls.__setattr__ = _wrap_setattr(cls.__setattr__)
return cls
def _wrap_init(init_fn):
"""`__init__` wrapper."""
@functools.wraps(init_fn)
def new_init(self, *args, **kwargs):
if hasattr(self, '_epy_is_init_done'):
# `_epy_is_init_done` already created, so it means we're
# a `super().__init__` call.
return init_fn(self, *args, **kwargs)
object.__setattr__(self, '_epy_is_init_done', False)
init_fn(self, *args, **kwargs)
object.__setattr__(self, '_epy_is_init_done', True)
return new_init
def _wrap_setattr(setattr_fn):
"""`__setattr__` wrapper."""
@functools.wraps(setattr_fn)
def new_setattr(self, name, value):
if not hasattr(self, '_epy_is_init_done'):
raise ValueError(
'Child of `@epy.frozen` class should be `@epy.frozen` too. (Error'
f' raised by {type(self)})'
)
if not self._epy_is_init_done: # pylint: disable=protected-access
return setattr_fn(self, name, value)
else:
raise AttributeError(
f'Cannot assign {name!r} in `@epy.frozen` class {type(self)}'
)
return new_setattr