Rowell Dionicio

The previous day went over the basics of the Python List. On this day I go a little further into List Methods.

List Methods allow you to work with the items within a List. With methods such as removing and sorting. Useful methods when working with a list at any capacity. 

Removing Values

There’s going to be a time where you need to remove a value from a list. It’s possible to do it with the del Statement.

wifi = ['rssi', 'snr', 'spatial streams']
wifi
['rssi', 'snr', 'spatial streams']
del wifi[0]
wifi
['snr', 'spatial streams']

But it is possible to do it with the remove method as well:

wifi = ['rssi', 'snr', 'spatial streams']
wifi.remove('rssi')
wifi
['snr', 'spatial streams']

The difference is the remove method uses parenthesis with the actual name of the item within the list. It’s easier to call out the actual name than the index number within the list.

But what happens if the item isn’t in the list? A ValueError occurs:

wifi.remove('antenna')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: list.remove(x): x not in list

Sorting Values

Let’s say I have a list of Tweeps and I want to sort it alphabetically.

tweeps = ['françois', 'robert', 'sam', 'keith', 'alex']
tweeps
['françois', 'robert', 'sam', 'keith', 'alex']
tweeps.sort()
tweeps
['alex', 'françois', 'keith', 'robert', 'sam']

Good stuff. It also works with numbers.

ages = [54, 92, 65, 27, 3]
ages.sort()
ages
[3, 27, 54, 65, 92]

What a list is not capable of sorting is a mix of integers and strings. You end up with a TypeError:

mix = ['spam', 'rice', 3, 5, 'red']
mix.sort()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: '<' not supported between instances of 'int' and 'str'

I’m sure in networking there will be many reasons to use a list. I can just see it happening but I don’t have any specific use case as of yet. It’s still early for me to figure out what I’d be using a list for. Maybe a list of IP addresses or interfaces of a switch or router.

Lists look like they’ll be useful when I get to a point where I can write scripts. Python Lists can hold multiple values in an ordered sequence.

Within the list are items which are ordered by an index. The index starts with 0. Each item is separated by a comma and any strings must have quotes around them. 

To assign a list to a variable and retrieve an item from the list the syntax looks like:

groceries = ['oranges', 'spinach', 'eggs', 'milk']
groceries[0]
'oranges'

As I said earlier, the first value in the list starts with 0. It’s possible to go in reverse by using negative indexes:

groceries = ['oranges', 'spinach', 'eggs', 'milk']
groceries[-1]
'milk'

What if you wanted to access multiple values within a list? That’s done by slicing. A slice starts with the first integer, a colon, and the second integer is where the slice ends, but not including that item. Take a look:

groceries[0:3]
['oranges', 'spinach', 'eggs']

Modifying items within a list is possible. Let’s say you wanted to change oranges to apples in the groceries list. We need to modify index 0 within the list.

groceries[0]
'oranges'
groceries[0] = 'apples'
groceries[0]
'apples'

Removing items from a Python list is fairly simple as well by using the del statement. We must reference the index of the item we want to remove.:

groceries
['apples', 'spinach', 'eggs', 'milk']
del groceries[0]
groceries
['spinach', 'eggs', 'milk']

The items in the list move up in their index values which means spinach is now at index 0.

Adding items to the Python list is done by appending it using what’s called a method. Methods are a little different when it comes to lists because we call on the method on a value:

groceries
['spinach', 'eggs', 'milk']
groceries.append('yogurt')
groceries
['spinach', 'eggs', 'milk', 'yogurt']

By appending to the list, the new item we are adding gets added to the end of the list. 

It is possible to add items to the list at a specific index. The index value must be passed, with the new item, using the insert method:

groceries
['spinach', 'eggs', 'milk', 'yogurt']
groceries.insert(0, 'pears')
groceries
['pears', 'spinach', 'eggs', 'milk', 'yogurt']

There’s quite a lot you can do with lists. They are very flexible and can be modified with ease. That’s all I’ve done with Day 2 of my Python learning exercise. The next Day will include more Python List work. 

On to more practicing…