defaultlist implementation

[sbordeyne]

The collections module already has a defaultdict implementation, but no defaultlist. It's sometimes useful to have a list with a default factory, so that it never throws an IndexError

Here's such an implementation :

class defaultlist(list):
    def __init__(self, *args, factory=None, **kwargs):
        super().__init__(*args, **kwargs)
        self.factory = factory

    def __getitem__(self, index):
        if index >= len(self):
            diff = index - len(self) + 1
            for i in range(diff):
                self.append(self.factory())
        return super().__getitem__(index)

With the accompanying test :

#! /usr/bin/env python
# -*- coding: UTF-8 -*-

import reusables

from .common_test_data import *


class TestDefaultList(BaseTestClass):

    def test_defaultlist_init(self):
        test = reusables.defaultlist(factory=int)

    def test_defaultlist_length(self):
        test = reusables.defaultlist(factory=int)
        self.assertEqual(len(test), 0)
        x = test[5]
        self.assertEqual(x, 0)
        self.assertIsInstance(x, int)
        self.assertEqual(len(test), 6)

    def test_defaultlist_list_factory(self):
        test = reusables.defaultlist(factory=list)
        test[2].append(10)
        self.assertEqual(test, [[], [], [10]])

[cdgriffith]

Great idea, I have run into these cases and usually code around them, so having it handy would be cool!

I may even add this to the Box project as well.

Self-note: check that factory is callable before calling.

[cdgriffith]

Added in 0.9.6