CODEWITHSUNDEEP
pythonlistoverriding
attaboyabhipro
attaboyabhipro

Reputation: 1648

How to override the slice functionality of list in its derived class

I make a class like below:

class MyList(list):
    def __init__(self, lst):
        self.list = lst

I want slice functionality to be overridden in MyList

Upvotes: 28

Views: 19806

Answers (3)

Yair
Yair

Reputation: 78

As both @Martijn Pieters and @jamylak said, you need to implement the __getitem__ method.

In general, if one wants to override the slicing functionality of it's super class, and keep using super's methods that already uses super's __getitem__, just split it into cases, and benefit from both worlds.

For example:

import pandas as pd

class SubFrame(pd.DataFrame):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

    def __getitem__(self, item):
        if isinstance(item, slice):
            # your implementation
            print("'My slicing'")
            return SubFrame(super().__getitem__(item))
        else:
            return super().__getitem__(item)

Now you can do:

>>> sf = SubFrame({'A': (10,20,30,40,50)})
>>> sf[1:-2]
'My slicing'
    A
1  20
2  30
>>> type(sf[1:-2])
<class '__main__.SubFrame'>

And [] will still call super's __getitem__

>>> sf['A']  # super's __getitem__
0    10
1    20
2    30
3    40
4    50
Name: A, dtype: int64
>>> type(sf['A'])
<class 'pandas.core.series.Series'>

Upvotes: 1

Martijn Pieters
Martijn Pieters

Reputation: 1122082

You need to provide custom __getitem__(), __setitem__ and __delitem__ hooks.

These are passed a slice object when slicing the list; these have start, stop and step attributes. However, these values could be None, to indicate defaults. Take into account that the defaults actually change when you use a negative stride!

However, they also have a slice.indices() method, which when given a length produces a tuple of (start, stop, step) values suitable for a range() object. This method takes care of such pesky details as slicing with a negative strides and no start or stop indices:

def __getitem__(self, key):
    if isinstance(key, slice):
        indices = range(*key.indices(len(self.list)))
        return [self.list[i] for i in indices]
    return self.list[key]

or, for your case:

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

because a list can take the slice object directly.

In Python 2, list.__getslice__ is called for slices without a stride (so only start and stop indices) if implemented, and the built-in list type implements it so you'd have to override that too; a simple delegation to your __getitem__ method should do fine:

def __getslice__(self, i, j):
    return self.__getitem__(slice(i, j))

Upvotes: 56

jamylak
jamylak

Reputation: 133564

class MyList(list):
    def __init__(self, lst):
        self.list = lst

doesn't make much sense... self is the list object itself, and it has already been created at this point, maybe you want to override __new__, however you probably don't need to touch that. Anyway you want to override __getitem__ like so:

def __getitem__(self, val):
    if isinstance( val, slice):
        # do stuff here

Upvotes: 8

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