python: how to have a property and with a setter function that detects all changes that happen to the value

Question

I have a two properties which holds lists. Whenever any item in this list changes, I would like the other list to update itself. This includes the statement `obj.myProp[3]=5`. Right now, this statement calls the getter function to get the whole list, gets the third item from the list, and sets that to 5. the `myProp` list is changed, but the second list never gets updated. ``` class Grid(object): def __init__(self,width=0,height=0): # Make self._rows a multi dimensional array # with it's size width * height self._rows=[[None] * height for i in xrange(width)] # Make `self._columns` a multi dimensional array # with it's size height * width self._columns=[[None] * width for i in xrange(height)] @property def rows(self): # Getting the rows of the array return self._rows @rows.setter def rows(self, value): # When the rows are changed, the columns are updated self._rows=value self._columns=self._flip(value) @property def columns(self): # Getting the columns of the array return self._columns…

Accepted Answer

Your problem is that you're not setting `foo.rows` on the offending line - you're getting it, and then modifying one of it's members. This isn't going to fire the setter. With the API you're proposing, you would need to return a list that has getters and setters attached as well.

You'd do better to not use the rows and columns properties to set entries, and add a `__getitem__` method like this:

```
class Grid(object):

def __init__(self, width=0, height=0):
        self._data = [None] * width * height;
        self.width = width
        self.height = height

def __getitem__(self, pos):
        if type(pos) != tuple or len(pos) != 2:
            raise IndexError('Index must be a tuple of length 2')
        x, y = pos
        if 0 <= x < self.width and 0 <= y < self.height:
            return self._data[x + self.width * y]
        else:
            raise IndexError('Grid index out of range')

def __setitem__(self, pos, value):
        if type(pos) != tuple or len(pos) != 2:
            raise IndexError('Index must be a tuple of length 2')
        x, y = pos
        if 0 <= x < self.width and 0 <= y < self.height:
            self._data[x + self.width * y] = value
        else:
            raise IndexError('Grid index out of range')

@property
    def columns(self):
        return [
            [self[x, y] for x in xrange(self.width)]
            for y in xrange(self.height)
        ]

@property
    def rows(self):
        return [
            [self[x, y] for y in xrange(self.height)]
            for x in xrange(self.width)
        ]
```

The broken line then becomes:

```
foo[0, 0] = 3
```

python: how to have a property and with a setter function that detects all changes that happen to the value

Solution

Problem