Lists ¶

Lists represent the simplest, most versatile, and most used ordered collections type. They can story more than one type of data, they have a specific order, and the contents can change. For example, we can create a list of things I have in my pocket. We instruct Python to create a list by surrounding data with square brackets: [ ] .

In [1]:

in_my_pocket = [21, False, None, "apple", 3.14]
print(in_my_pocket)

in_my_pocket = [21, False, None, "apple", 3.14] print(in_my_pocket)

[21, False, None, 'apple', 3.14]

We do not necessarily have to start with data inside. We can create an empty list as well with just [ ] .

In [2]:

in_my_other_pocket = []
print(in_my_other_pocket)

in_my_other_pocket = [] print(in_my_other_pocket)

[]

Indexing ¶

One of the most fundamental things you use lists for is to get individual values. Because the order of the items in the list matter, I can refer to an item using it's position in the list. This position in programming languages is called the index ; with the first index being 0 , then 1 , then 2 , and so on.

Let's print the first value.

In [3]:

print(in_my_pocket[0])

print(in_my_pocket[0])

21

You may have noticed that we are using [] to both create the list and index it. This was designed on purpose to be more readable, and the main difference is if [] is directly next to a variable like in our in_my_pocket[0] example. If we did

in_my_pocket [0]

we would get an error.

While we can count from start to end of the list, we can also count backwards. To get the last element of a list, we can use the index -1 .

In [4]:

print(in_my_pocket[-1])

print(in_my_pocket[-1])

3.14

A table of the available elements are shown below.

Element	Index from start	Index from end
21	0	-5
False	1	-4
None	2	-3
"apple"	3	-2
3.14	4	-1

Either of these indices would get you the same element.

Length ¶

Another important thing is that you cannot use an index that is outside the bounds of the list. For example we cannot ask for the 6th element of in_my_pocket since there are only five items. We can get the length of a list by using the len function.

In [5]:

print(len(in_my_pocket))

print(len(in_my_pocket))

5

This means that there are 5 elements and the largest (positive) index I can use is minus one of that, so 4 .

Slicing ¶

While accessing individual list elements is useful, often we want to extract a subsection, or slice, of a list's items. This is accomplished in Python using slicing. Slicing allows copying a portion of a list—either from one or both sides. This enables cleanly breaking large lists into usable parts.

In Python, the colon : is used for slicing with the general syntax of start:stop:step .

• start : The index at which the slice begins (inclusive). If omitted or None , it defaults to the beginning of the sequence.
• stop : The index at which the slice ends (exclusive). If omitted or None , it defaults to the end of the sequence.
• step : The step size or the number of indices between each slice. If omitted or None , it defaults to 1.

First, let us print the full list like we normally have been doing.

In [6]:

print(in_my_pocket)

print(in_my_pocket)

[21, False, None, 'apple', 3.14]

We can get the same view if we slice in_my_pocket without specifying start , stop , or step .

In [7]:

print(in_my_pocket[::])

print(in_my_pocket[::])

[21, False, None, 'apple', 3.14]

Remember that None is often used to specify the absence of a value, so it should give us the same as [::] .

In [8]:

print(in_my_pocket[None:None:None])

print(in_my_pocket[None:None:None])

[21, False, None, 'apple', 3.14]

Yup! This gives us the same slice of the list in_my_pocket .

Now, if I want only the first two elements of the list, I want to tell Python to stop slicing (and not include) at the index of 2 .

In [9]:

print(in_my_pocket[:2])

print(in_my_pocket[:2])

[21, False]

I can also tell Python I want the elements at index 2 and 3 .

In [10]:

print(in_my_pocket[2:4])

print(in_my_pocket[2:4])

[None, 'apple']

Mutating ¶

The contents of list are mutable , meaning that the contents of the collection can be changed.

Replace ¶

For example, I can replace None in my pocket with my "wallet" .

In [11]:

in_my_pocket = [21, False, None, "apple", 3.14]
print(in_my_pocket)
in_my_pocket[2] = "wallet"
print(in_my_pocket)

in_my_pocket = [21, False, None, "apple", 3.14] print(in_my_pocket) in_my_pocket[2] = "wallet" print(in_my_pocket)

[21, False, None, 'apple', 3.14]
[21, False, 'wallet', 'apple', 3.14]

This works the same way as memory allocation for variables . Python will go to where the third element is located in memory, throw out None , and put "wallet" there instead.

Append ¶

However, what if I want to add an element to my list and not remove anything? We can do this with append . For example, let's say I keep everything in my pocket that is already there, but then I want to add my keys.

In [12]:

print(in_my_pocket)
print(len(in_my_pocket))
in_my_pocket.append("keys")
print(in_my_pocket)
print(len(in_my_pocket))

print(in_my_pocket) print(len(in_my_pocket)) in_my_pocket.append("keys") print(in_my_pocket) print(len(in_my_pocket))

[21, False, 'wallet', 'apple', 3.14]
5
[21, False, 'wallet', 'apple', 3.14, 'keys']
6

Now our list has an additional element and a new length of 6 .

Delete ¶

Just like we can add things to a list, we can also remove them with the del keyword in Python

In [13]:

print(in_my_pocket)
print(len(in_my_pocket))
del in_my_pocket[3]
print(in_my_pocket)
print(len(in_my_pocket))

print(in_my_pocket) print(len(in_my_pocket)) del in_my_pocket[3] print(in_my_pocket) print(len(in_my_pocket))

[21, False, 'wallet', 'apple', 3.14, 'keys']
6
[21, False, 'wallet', 3.14, 'keys']
5