The range function lets us build a list of numbers.

In [12]: 
list(range(10, 20))
Out[12]:
[10, 11, 12, 13, 14, 15, 16, 17, 18, 19]

Notice anything funny?

Python uses this convention everywhere.

In [13]: 
a = list(range(10, 20))
type(a)
Out[13]:
list

Let's talk about indexing.

Indexing in Python starts at 0.

In [14]: 
a[0]
Out[14]:
10

And goes from there.

In [15]: 
a[1]
Out[15]:
11
In [16]: 
a[2]
Out[16]:
12

What do negative numbers do?

In [6]: 
a[-1]
Out[6]:
19
In [17]: 
a[-2]
Out[17]:
18

You can get a sub-list by slicing.

In [19]: 
a[3:7]
Out[19]:
[13, 14, 15, 16]

Start and end are optional.

In [20]: 
a[3:]
Out[20]:
[13, 14, 15, 16, 17, 18, 19]
In [21]: 
a[:3]
Out[21]:
[10, 11, 12]

Again, notice how the end entry is not included:

In [23]: 
print(a[:3])
print(a[3])

[10, 11, 12]
13

Slicing works on any collection type! (list, tuple, str, numpy array)

In [24]: 
a = "CS357"
a[-3:]
Out[24]:
'357'