Python decorating property setter with list

Question

globalList = []
class MyList:
    def __init__(self):
        self._myList = [1, 2, 3]

    @property
    def myList(self):
        return self._myList + globalList
    @myList.setter
    def myList(self, val):
        self._myList = val

mL1 = MyList()
print("myList: ", mL1.myList)
mL1.myList.append(4)
print("after appending a 4, myList: ", mL1.myList)
mL1.myList.extend([5,6,"eight","IX"])
print("after extend, myList: ", mL1.myList)

Result:

myList:  [1, 2, 3]
after appending a 4, myList:  [1, 2, 3]
after extend, myList:  [1, 2, 3]

The problem I am facing is that mL1.myList.append(4) and mL1.myList.extend([5,6,"eight","IX"]) do not modify the _myList attribute in the mL1 object. How could I do to resolve the problem?

Solution 1

I define a method append() and a method extend() for the class object. It respectively appends to member myList and extends member myList.

global globalList  
globalList = []
class MyList():
    def __init__(self):
        self._myList = [1, 2, 3]

    @property
    def myList(self):
        return self._myList + globalList
    @myList.setter
    def myList(self, val):
        self._myList = val

    def append(self, val):
        self.myList = self.myList + [val]
        return self.myList  

    def extend(self, val):
        return self.myList.extend(val)


mL1 = MyList()
print("myList: ", mL1.myList)
mL1.append(4)
print("after appending a 4, myList: ", mL1.myList)
mL1.myList.extend([5,6,"eight","IX"])
print("after extend, myList: ", mL1.myList)

result is

>>> 
('myList: ', [1, 2, 3])
('after appending a 4, myList: ', [1, 2, 3, 4])
('after extend, myList: ', [1, 2, 3, 4, 5, 6, 'eight', 'IX'])

Solution 2

I would subclass list and override a few methods:

import itertools

class ExtendedList(list):
    def __init__(self, other=None):
        self.other = other or []

    def __len__(self):
        return list.__len__(self) + len(self.other)

    def __iter__(self):
        return itertools.chain(list.__iter__(self), iter(self.other))

    def __getitem__(self, index):
        l = list.__len__(self)

        if index > l:
            return self.other[index - l]
        else:
            return list.__getitem__(self, index)

It should work with just about everything:

In [9]: x = ExtendedList([1, 2, 3])

In [10]: x
Out[10]: [1, 2, 3]

In [11]: x.append(9)

In [12]: x
Out[12]: [9, 1, 2, 3]

In [13]: x.extend([19, 20])

In [14]: x
Out[14]: [9, 19, 20, 1, 2, 3]

In [15]: sum(x)
Out[15]: 54

Comments

[drerD]

Hi thanks for the answer, it's interesting to see how you implement it. Hope you don't mind me asking some simple question: self.other = other or [], is short-circuit evaluation used here? so if the other parameter is not provided, then it's going to be empty list?

[Blender]

@user14042: Yep, if other is truthy (not None or an empty list in this case), it'll be returned. If not, the other side will be.

[drerD]

Thnak you for the suggestion!

[user1346466]

The code is highly broken :) MyList does not derive from object, which breaks the setter property, so after assigning to myList it will be a plain list. Once this is fixed, append() will add globalList to _mylist for every call. The extend implementation is broken too, it should extend _myList, otherwise it would only extend the temporary list returned by the property. And last, the example is calling mL1.myList.extend instead of mL1.extend.