Why can't I access a list by its key in Python?

Question

I am creating a list, and then accessing an element like this:

list = []
list.insert(42, "foo") 
list.insert(43, "bar") 
list.insert(44, "baz")

print(list[43])

And I have the folowing error:

print(list[43]) IndexError: list index out of range

What is wrong ? Do I have to use a dictionary to do this ?

Answer 1

There are no "keys" in a list, there are just indices.

The reason your code doesn't work the way you expect is that list.insert(index, obj) does not pad the list with "blank" entries when index is past the end of the list; it simply appends obj to the list.

You could use a dictionary for this:

In [14]: d = {}
In [15]: d[42] = "foo"
In [16]: d[43] = "bar"
In [17]: d[44] = "baz"
In [18]: print(d[43])
bar

Alternatively, you could pre-initialize your list with a sufficiently large number of entries:

In [19]: l = [None] * 50
In [20]: l[42] = "foo"
In [21]: l[43] = "bar"
In [22]: l[44] = "bar"
In [23]: print(l[43])
bar

P.S. I recommend that you don't call your variable list as it shadows the list() builtin.