Lists—Python

Lists are a sequence type object that Python provides to us for grouping data together into a single variable. Let’s consider a common application of a list before we go into details.

names = ['Bob Belcher',
         'Linda Belcher',
         'Tina Belcher',
         'Gene Belcher',
         'Louise Belcher']
for name in names:
    print (name)

This code creates a list called names and populates it with five names. Then it prints the names to the console. We could accomplish the same output by using this code.

bob = 'Bob Belcher'
linda = 'Linda Belcher'
tina = 'Tina Belcher'
gene = 'Gene Belcher'
louise = 'Louise Belcher'

print(bob)
print(linda)
print(tina)
print(gene)
print(louise)

A quick comparison shows that the first example is not only much easier to read, but it is also more maintainable. If we want to print additional names, we only need to add them to the names list in the first example. However, in the second example, we need to add new name variable and another print statement. This may not seem like a big deal with six names, but obviously 6,000 is a much different case.

It’s for this reason that almost every programming language provides some sort of a collection object. List is one such data type that Python provides out of the box. Let’s look at common list operations.

Check if item is in the list

We may want to check if a list has a certain item.

names = ['Bob Belcher',
         'Linda Belcher',
         'Tina Belcher',
         'Gene Belcher',
         'Louise Belcher']
if 'Bob Belcher' in names:
    print('Found Bob')

This code prints ‘Found Bob’ because 'Bob Belcher' in names returns True.

Check if item is not in list

We can also check if a list does not have an item.

names = ['Bob Belcher',
         'Linda Belcher',
         'Tina Belcher',
         'Gene Belcher',
         'Louise Belcher']
if 'Teddy' not in names:
    print('No Teddy here!')

This code would print ‘No Teddy here!’ because 'Teddy' not in names is True. Our names list does not have ‘Teddy’

Combine lists

We can add two lists together (called concatenation) using the + operator.

belchers = ['Bob Belcher',
            'Linda Belcher',
            'Tina Belcher',
            'Gene Belcher',
            'Louise Belcher']

pestos = ['Jimmy Pesto',
          'Jimmy Pesto Jr.',
          'Andy Pesto',
          'Ollie Pesto']

family_frackus = belchers + pestos
print(family_frackus)

This code will print all of the names to standard out when run.

Accessing items

Lists use the index operator access items

belchers = ['Bob Belcher',
            'Linda Belcher',
            'Tina Belcher',
            'Gene Belcher',
            'Louise Belcher']
print(belchers[0])
print(belchers[3])

Keep in mind that lists are 0 based, so belchers[0] is ‘Bob Belcher’ while belchers[3] is ‘Gene Belcher’

Add item to list

We can add an item to a list using the append() method.

belchers = ['Bob Belcher',
            'Linda Belcher',
            'Tina Belcher',
            'Gene Belcher',
            'Louise Belcher']
belchers.append('Teddy')
print(belchers[5])

Remove item from a list

We use the del operator to remove an item from a list

belchers = ['Bob Belcher',
            'Linda Belcher',
            'Tina Belcher',
            'Gene Belcher',
            'Louise Belcher']
del belchers[0]

Using del belchers[0] removes ‘Bob Belcher’ from the list.

Replace item in a list

We can replace an item in a list by specifying the index and assigning a new value to it.

belchers = ['Bob Belcher',
            'Linda Belcher',
            'Tina Belcher',
            'Gene Belcher',
            'Louise Belcher']
belchers[0] = 'Mort'

The code belchers[0] = 'Mort' replaces ‘Bob Belcher’ with ‘Mort’

Length of the list

We get the length of the list using len.

belchers = ['Bob Belcher',
            'Linda Belcher',
            'Tina Belcher',
            'Gene Belcher',
            'Louise Belcher']
print(str(len(belchers))) 

The print(str(len(belchers))) prints 5 to the console.

Conclusion

We can do a lot more with lists than what was discussed in this post. Make sure you check out the Python documentation for a complete list of features!