How to implement a secondary custom method for object slicing, other than __getitem__ in Python

I am looking to implement a custom method in my class which helps users slice based on index. The primary slicing will be based on dictionary key. I want to implement it similar to how Pandas does it, using df.iloc[n]

here’s my code:

class Vector:
    def __init__(self, map_object: dict):
        self.dictionary = map_object
        
    def __getitem__(self, key):
        data = self.dictionary[key]
        return data

    def iloc(self, n):
        key = list(self.dictionary)[n]
        return self.dictionary[key]

However, if then write object.iloc[3] after creating the object, I get an error saying 'method' object is not subscriptable. So how can I implement this?

>Solution :

The [ ] syntax requires a proper object with a __getitem__ method. In order to have a "slice method", use a property that returns a helper which supports slicing.

The helper simply holds a reference to the actual parent object, and defines a __getitem__ with the desired behaviour:

class VectorIloc:
    def __init__(self, parent):
        self.parent = parent

    # custom logic for desired "iloc" behaviour
    def __getitem__(self, item):
        key = list(self.parent.dictionary)[item]
        return self.parent[key]

On the actual class, merely define the desired "method" as a property that returns the helper or as an attribute:

class Vector:
    def __init__(self, map_object: dict):
        self.dictionary = map_object
        # if .iloc is used often
        # self.iloc = VectorIloc(self)

    def __getitem__(self, key):
        return self.dictionary[key]

    # if .iloc is used rarely
    @property
    def iloc(self):
        return VectorIloc(self)

Whether to use a property or an attribute is an optimisation that trades memory for performance. The property is advantageous when the helper is used rarely.

Now, when calling vector.iloc[3], the vector.iloc part provides the helper and the [3] part invoces the helper’s __getitem__.

>>> vector = Vector({0:0, 1: 1, 2: 2, "three": 3})
>>> vector.iloc[3]
3