Lists

Lists#

If we want to store many items, we can create many variables. However, this quickly becomes tedious and error-prone. Instead, we should group our data items together in collections. Lists are the standard collections in Python.

Making Lists#

Lists can contain any python data type. We make lists by placing items in square brackets, [ ], separated by commas.

numbers = [1, 2, 3]

We can also make an empty list:

judges = []

Let’s look at the list of judges from a recent ECtHR case.

judges = ['Anne Grøstad', 'Ivana Jelić', 'Jovan Ilievski', 'Lətif Hüseynov',
          'Mārtiņš Mits', 'Stéphanie Mourou-Vikström', 'Síofra O’Leary', 'Victor Soloveytchik']

Accessing list elements#

We can access list elements by using indexes in square brackets. This is like with strings, where we can access individual characters by using indexes.

print(judges[0])

Anne Grøstad

Note

Remember that Python counts from 0, not 1.

We can also count backwards from the last element with negative indexes.

print(judges[-1])

Victor Soloveytchik

We can get a new list containing a part of the original list. This is called a list slice, and we get it by using two indexes, a from index and a to index:

selection = judges[2:4]
print(selection)

['Jovan Ilievski', 'Lətif Hüseynov']

List Operations#

Python list objects have many methods for manipulating their content. There are also built-in list functions. Here, we’ll look at a few of these methods and functions.

Length#

As with strings, we can use the function len() to get the length of a list.

print(len(judges))

Append#

We can append items to a list. Here we use examples from the joined cases C-403/08 and C-429/08 (Premier League).

defendants = ['QC Leisure']
defendants.append('SR Leisure Ltd')
defendants.append('AV Station plc')
print(defendants)

['QC Leisure', 'SR Leisure Ltd', 'AV Station plc']

Insert#

If the order of the list items is significant, we can insert an element at a specific place:

defendants.insert(0, 'Owen')
print(defendants)

['Owen', 'QC Leisure', 'SR Leisure Ltd', 'AV Station plc']

Finding items#

There are two ways to check if a list contains an item. First, we can use the in operator that we have already seen:

print('Owen' in defendants)

True

If we need to find the position of the item, we can use <listname>.index().

print('Owen is defendant number', defendants.index('Owen'))

Owen is defendant number 0

Removing items#

We can remove a given list item:

item = 'Owen'
if item in defendants:
    defendants.remove(item)
print(defendants)

['QC Leisure', 'SR Leisure Ltd', 'AV Station plc']

We can also remove an item at a given position with the method <listname>.pop(index). This method returns the element which was removed, so we can use it if necessary. The most common would be to remove the first or last element:

print(defendants)
defendant = defendants.pop(0)
print(defendant)
print(defendants.pop(-1))
print(defendants)

['QC Leisure', 'SR Leisure Ltd', 'AV Station plc']
QC Leisure
AV Station plc
['SR Leisure Ltd']

Nested Lists#

Lists can contain other lists, and these are called nested lists. For example, we could have a case where there are two groups of defendants.

defendants = [['QC Leisure', 'SR Leisure Ltd', 'AV Station plc', 'Owen'],
              ['Media Protection Services Ltd']]
print(defendants)

[['QC Leisure', 'SR Leisure Ltd', 'AV Station plc', 'Owen'], ['Media Protection Services Ltd']]

We can access the elements one step at a time, or by using multiple indexes:

defendant_group = defendants[0]
print(defendant_group[0])
# is equivalent to
print(defendants[0][0])

QC Leisure
QC Leisure

List References#

Unlike simple Python variables, list variables contain references to lists, not the lists themselves. If we assign the value of one list variable to another, we don’t get a copy of the list. Instead, both variables refer to the same list, and we can call them aliases.

defendants = ['QC Leisure', 'SR Leisure Ltd']

defendants2 = defendants
defendants.append('AV Station plc')
print('defendants: ', defendants)
print('defendants2:', defendants2)

defendants:  ['QC Leisure', 'SR Leisure Ltd', 'AV Station plc']
defendants2: ['QC Leisure', 'SR Leisure Ltd', 'AV Station plc']

Since both variables refer to the same list, when we modify one of them, the change applies to both.

Copying Lists#

We can also make a new copy of a list, by passing it to the function list(). Then, the variables refer to different lists, and we can modify them independently:

defendants2 = list(defendants)
defendants2.pop()
print('defendants: ', defendants)
print('defendants2:', defendants2)

defendants:  ['QC Leisure', 'SR Leisure Ltd', 'AV Station plc']
defendants2: ['QC Leisure', 'SR Leisure Ltd']

Characters#

Strings are made up of Unicode characters. Strings aren’t really lists but sequences of characters. However, in some ways they behave like lists. We can get a single character by using an index in brackets:

name = 'John Doe'
print(name[0])