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。
另一种方法是创建一个使实例不可变的包装器。
class Immutable(object):
def __init__(self, wrapped):
super(Immutable, self).__init__()
object.__setattr__(self, '_wrapped', wrapped)
def __getattribute__(self, item):
return object.__getattribute__(self, '_wrapped').__getattribute__(item)
def __setattr__(self, key, value):
raise ImmutableError('Object {0} is immutable.'.format(self._wrapped))
__delattr__ = __setattr__
def __iter__(self):
return object.__getattribute__(self, '_wrapped').__iter__()
def next(self):
return object.__getattribute__(self, '_wrapped').next()
def __getitem__(self, item):
return object.__getattribute__(self, '_wrapped').__getitem__(item)
immutable_instance = Immutable(my_instance)
这在只有一些实例必须是不可变的情况下很有用(比如函数调用的默认参数)。
也可以用于不可变工厂,如:
@classmethod
def immutable_factory(cls, *args, **kwargs):
return Immutable(cls.__init__(*args, **kwargs))
也保护对象。__setattr__,但由于Python的动态特性,可能会被其他技巧所绊倒。
就像字典一样
我有一个开源库,在那里我以函数的方式做事情,所以在不可变对象中移动数据是有帮助的。但是,我不希望必须转换我的数据对象以便客户机与它们交互。所以,我想到了这个-它给你一个字典一样的对象,这是不可变的+一些帮助方法。
这要归功于Sven Marnach对限制属性更新和删除的基本执行的回答。
import json
# ^^ optional - If you don't care if it prints like a dict
# then rip this and __str__ and __repr__ out
class Immutable(object):
def __init__(self, **kwargs):
"""Sets all values once given
whatever is passed in kwargs
"""
for k,v in kwargs.items():
object.__setattr__(self, k, v)
def __setattr__(self, *args):
"""Disables setting attributes via
item.prop = val or item['prop'] = val
"""
raise TypeError('Immutable objects cannot have properties set after init')
def __delattr__(self, *args):
"""Disables deleting properties"""
raise TypeError('Immutable objects cannot have properties deleted')
def __getitem__(self, item):
"""Allows for dict like access of properties
val = item['prop']
"""
return self.__dict__[item]
def __repr__(self):
"""Print to repl in a dict like fashion"""
return self.pprint()
def __str__(self):
"""Convert to a str in a dict like fashion"""
return self.pprint()
def __eq__(self, other):
"""Supports equality operator
immutable({'a': 2}) == immutable({'a': 2})"""
if other is None:
return False
return self.dict() == other.dict()
def keys(self):
"""Paired with __getitem__ supports **unpacking
new = { **item, **other }
"""
return self.__dict__.keys()
def get(self, *args, **kwargs):
"""Allows for dict like property access
item.get('prop')
"""
return self.__dict__.get(*args, **kwargs)
def pprint(self):
"""Helper method used for printing that
formats in a dict like way
"""
return json.dumps(self,
default=lambda o: o.__dict__,
sort_keys=True,
indent=4)
def dict(self):
"""Helper method for getting the raw dict value
of the immutable object"""
return self.__dict__
辅助方法
def update(obj, **kwargs):
"""Returns a new instance of the given object with
all key/val in kwargs set on it
"""
return immutable({
**obj,
**kwargs
})
def immutable(obj):
return Immutable(**obj)
例子
obj = immutable({
'alpha': 1,
'beta': 2,
'dalet': 4
})
obj.alpha # 1
obj['alpha'] # 1
obj.get('beta') # 2
del obj['alpha'] # TypeError
obj.alpha = 2 # TypeError
new_obj = update(obj, alpha=10)
new_obj is not obj # True
new_obj.get('alpha') == 10 # True
这种方式不停止对象。__setattr__从工作,但我仍然发现它有用:
class A(object):
def __new__(cls, children, *args, **kwargs):
self = super(A, cls).__new__(cls)
self._frozen = False # allow mutation from here to end of __init__
# other stuff you need to do in __new__ goes here
return self
def __init__(self, *args, **kwargs):
super(A, self).__init__()
self._frozen = True # prevent future mutation
def __setattr__(self, name, value):
# need to special case setting _frozen.
if name != '_frozen' and self._frozen:
raise TypeError('Instances are immutable.')
else:
super(A, self).__setattr__(name, value)
def __delattr__(self, name):
if self._frozen:
raise TypeError('Instances are immutable.')
else:
super(A, self).__delattr__(name)
你可能需要根据用例重写更多的东西(比如__setitem__)。
你可以创建一个@immutable装饰器,它覆盖__setattr__并将__slots__更改为一个空列表,然后用它装饰__init__方法。
编辑:正如OP所指出的,改变__slots__属性只会阻止新属性的创建,而不会阻止修改。
Edit2:下面是一个实现:
Edit3:使用__slots__会破坏这段代码,因为if会停止对象__dict__的创建。我正在寻找替代方案。
Edit4:嗯,就是这样。这是一个很粗鄙的问题,但可以作为练习:-)
class immutable(object):
def __init__(self, immutable_params):
self.immutable_params = immutable_params
def __call__(self, new):
params = self.immutable_params
def __set_if_unset__(self, name, value):
if name in self.__dict__:
raise Exception("Attribute %s has already been set" % name)
if not name in params:
raise Exception("Cannot create atribute %s" % name)
self.__dict__[name] = value;
def __new__(cls, *args, **kws):
cls.__setattr__ = __set_if_unset__
return super(cls.__class__, cls).__new__(cls, *args, **kws)
return __new__
class Point(object):
@immutable(['x', 'y'])
def __new__(): pass
def __init__(self, x, y):
self.x = x
self.y = y
p = Point(1, 2)
p.x = 3 # Exception: Attribute x has already been set
p.z = 4 # Exception: Cannot create atribute z
..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")
我通过重写__setattr__创建了不可变类,并且如果调用者是__init__,则允许该集合:
import inspect
class Immutable(object):
def __setattr__(self, name, value):
if inspect.stack()[2][3] != "__init__":
raise Exception("Can't mutate an Immutable: self.%s = %r" % (name, value))
object.__setattr__(self, name, value)
这还不够,因为它允许任何人的___init__来改变对象,但你懂的。