How to inherit and extend a list object in Python?

Question

I am interested in using the python list object, but with slightly altered functionality. In particular, I would like the list to be 1-indexed instead of 0-indexed. E.g.:

>> mylist = MyList()
>> mylist.extend([1,2,3,4,5])
>> print mylist[1]

output should be: 1

But when I changed the __getitem__() and __setitem__() methods to do this, I was getting a RuntimeError: maximum recursion depth exceeded error. I tinkered around with these methods a lot but this is basically what I had in there:

class MyList(list):
    def __getitem__(self, key):
        return self[key-1]
    def __setitem__(self, key, item):
        self[key-1] = item

I guess the problem is that self[key-1] is itself calling the same method it’s defining. If so, how do I make it use the list() method instead of the MyList() method? I tried using super[key-1] instead of self[key-1] but that resulted in the complaint TypeError: 'type' object is unsubscriptable

Any ideas? Also if you could point me at a good tutorial for this that’d be great!

Thanks!

Asked By: mindthief

||

Source

Answer 1

Use the super() function to call the method of the base class, or invoke the method directly:

class MyList(list):
    def __getitem__(self, key):
        return list.__getitem__(self, key-1)

or

class MyList(list):
    def __getitem__(self, key):
        return super(MyList, self).__getitem__(key-1)

However, this will not change the behavior of other list methods. For example, index remains unchanged, which can lead to unexpected results:

numbers = MyList()
numbers.append("one")
numbers.append("two")

print numbers.index('one')
>>> 1

print numbers[numbers.index('one')]
>>> 'two'

Answered By: Gintautas Miliauskas

Answer 2

Instead, subclass integer using the same method to define all numbers to be minus one from what you set them to. Voila.

Sorry, I had to. It’s like the joke about Microsoft defining dark as the standard.

Answered By: Binary Phile

Answer 3

You can avoid violating the Liskov Substitution principle by creating a class that inherits from collections.MutableSequence, which is an abstract class. It would look something like this:

def indexing_decorator(func):
    def decorated(self, index, *args):
        if index == 0:
            raise IndexError('Indices start from 1')
        elif index > 0:
            index -= 1
        return func(self, index, *args)
    return decorated


class MyList(collections.MutableSequence):
    def __init__(self):
        self._inner_list = list()

    def __len__(self):
        return len(self._inner_list)

    @indexing_decorator
    def __delitem__(self, index):
        self._inner_list.__delitem__(index)

    @indexing_decorator
    def insert(self, index, value):
        self._inner_list.insert(index, value)

    @indexing_decorator
    def __setitem__(self, index, value):
        self._inner_list.__setitem__(index, value)

    @indexing_decorator
    def __getitem__(self, index):
        return self._inner_list.__getitem__(index)

    def append(self, value):
        self.insert(len(self) + 1, value)

Answered By: Aleksandar Jovanovic

Answer 4

class ListExt(list):
    def extendX(self, l):
        if l:
            self.extend(l)

Answered By: Moisey Oysgelt

How to inherit and extend a list object in Python?

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