可变的@property？

如何解决可变的@property？

有没有办法在 Python 中定义 MUTABLE（不仅仅是可重新分配的！）属性？我需要一个从文件返回列表的属性，并在该列表中使用 __setitem__ 时更改该文件。

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不起作用的例子：

>>> import mmap
>>> def someClass():
    def __init__(self,fileName):
        self.fileName = fileName

    @property
    def data(self):
        """Return data at start of eil in 4 byte chunks"""
        
        with open(self.fileName,mode = 'r+b') as f:
            with mmap.mmap(f.fileno(),length=0,access=mmap.ACCESS_READ) as fmap:
                data = [fmap[i:i+4] for i in range(1024)]
        
        return data
    
    @data.setter
    def data(self,value):
        """Save data to file"""
        
        with open(self.fileName,mode = 'w+b') as f:
            with mmap.mmap(f.fileno(),access=mmap.ACCESS_WRITE) as fmap:
                for i,data in enumerate(value):
                    fmap[i:i+4] = data

>>> a = someClass('C:\test.txt')
>>> a.data[0] == a.data[1]
False
>>> a.data[0] = a.data[1]
>>> a.data[0] == a.data[1]
False # Mutation did not register to file

解决方法

我对 mmap 一无所知，所以我可能会搞砸那里的逻辑，但总体思路是一样的。
正如@jasonharper 在评论中所建议的，solotion 是创建一个定义 __setitem__ 的客户类并将其用于 someClass().data。

class FileUpdatingList:
    def __init__(self,filename):
        self.filename = filename

    def __getitem__(self,index):
        with open(self.fileName,mode = 'r+b') as f:
            with mmap.mmap(f.fileno(),length=0,access=mmap.ACCESS_READ) as fmap:
                return fmap[index:index+4]
    
    def __setitem__(self,index,data):
        with open(self.fileName,mode = 'w+b') as f:
            with mmap.mmap(f.fileno(),access=mmap.ACCESS_WRITE) as fmap:
                fmap[index:index+4] = data

现在的问题是 someClass().data = value 不再有效，您必须使用 someClass.data[:] = value。要解决此问题，您可以编写一个属性来为您执行此操作：

def someClass():
    def __init__(self,fileName):
        self.fileName = fileName
        self._data = FileUpdatingList(filename)

    @property
    def data(self):
        """Return data at start of eil in 4 byte chunks"""
        return self._data
    
    @data.setter
    def data(self,value):
        """Save data to file"""
        self._data[:] = value