Python 中的冻结?

Question

我已经用Python编程一段时间了，最近在工作中开始使用Ruby。这两种语言非常相似。不过，我刚遇到一个Ruby的特性，我不知道怎么在Python中实现，那就是Ruby的freeze方法。

irb(main):001:0> a = [1,2,3]
=> [1, 2, 3]
irb(main):002:0> a[1] = 'chicken'
=> "chicken"
irb(main):003:0> a.freeze
=> [1, "chicken", 3]
irb(main):004:0> a[1] = 'tuna'
TypeError: can't modify frozen array
        from (irb):4:in `[]='
        from (irb):4

有没有办法在Python中模仿这个功能呢？

补充说明：我意识到我让人觉得这个功能只适用于列表；其实在Ruby中，freeze是一个在Object上的方法，所以你可以让任何对象变得不可变。我为造成的混淆感到抱歉。

Answer 1

>>> a = [1,2,3]
>>> a[1] = 'chicken'
>>> a
[1, 'chicken', 3]
>>> a = tuple(a)
>>> a[1] = 'tuna'
Traceback (most recent call last):
  File "<pyshell#4>", line 1, in <module>
    a[1] = 'tuna'
TypeError: 'tuple' object does not support item assignment

另外，可以对比一下 set 和 frozenset，还有 bytearray 和 bytes。

数字和字符串本身是不可改变的：

>>> a = 4
>>> id(a)
505408920
>>> a = 42        # different object
>>> id(a)
505409528

Answer 2

你可以创建一个新的类，继承自 list，然后添加一个叫做“frozen”的标志，这样就可以阻止 __setitem__ 进行任何操作了：

class freezablelist(list):
    def __init__(self,*args,**kwargs):
        list.__init__(self, *args)
        self.frozen = kwargs.get('frozen', False)

    def __setitem__(self, i, y):
        if self.frozen:
            raise TypeError("can't modify frozen list")
        return list.__setitem__(self, i, y)

    def __setslice__(self, i, j, y):
        if self.frozen:
            raise TypeError("can't modify frozen list")
        return list.__setslice__(self, i, j, y)

    def freeze(self):
        self.frozen = True

    def thaw(self):
        self.frozen = False

接下来可以试着使用它：

>>> from freeze import freezablelist as fl
>>> a = fl([1,2,3])
>>> a[1] = 'chicken'
>>> a.freeze()
>>> a[1] = 'tuna'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "freeze.py", line 10, in __setitem__
    raise TypeError("can't modify frozen list")
TypeError: can't modify frozen list
>>> a[1:1] = 'tuna'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "freeze.py", line 16, in __setslice__
    raise TypeError("can't modify frozen list")
TypeError: can't modify frozen list
>>>