Data structures

Lists

Also known as arrays in other languages, a list in Python begins and ends with square brackets [] and individual items are separated with a comma ,

Spaces don't matter here.

a = [ 1, 2, 3, 4 ]

Do I need to specify the size of the list in advance

No. Since Python is interpreted and garbage collected there's no need to explicitly allocate memory, free memory, or declare the size of a list in advance.

You can store any type of data in a list

a = [ 'Hello', 'World!' ]

You can even store multiple types of data, including other lists

a = [
  1.0,
  'Hello, World!',
  [ 5.0, True, None ]
]

Split up long lines

In the example below, you'll see a list printed with each item on a separate line. It's considered good practice to split lines at 80 characters for readability.

Indexing

Retrieving items from a list is called indexing or subscripting. Let's revisit the list from earlier

a = [
  1.0,
  'Hello, World!',
  [ 5.0, True, None ]
]

The indexes are as follows

index	value	type
0	1.0	float
1	'Hello, World!'	str
2	[ 5.0, True, None ]	list

In Python, list indexes start at 0 and increment to the length of the list minus 1. Let's retrieve a few items

a[0]
a[1]
a[2]

Insert

Python has a few different ways to insert new items into an existing list. More commonly, you will use the .insert() method. This method accepts two arguments. The first argument is a list index and the second argument is the value you want to insert at that index

a.insert(0, 'foobar')

This will insert the string foobar at index 0, pushing all subsequent items forward.

Update

Use the assignment operator = to update an item at a specific index

a[1] = 999

This will replace the value at index 1 with the integer value 999.

Delete

Use the del keyword to delete an item from a list

del a[1]

This will delete the value at index 1 and shift all subsequent items back.

Read the documentation

Refer to the official documentation here for all available list operations.

Length

To see the length of a list, pass it to the built-in len() function

len(a)

This function will return an int.

Exists

To check whether or not a specific value exists within a given list, use the in keyword

20 in a

This statement will return a bool.

Slicing

Slicing is a powerful feature built into Python. If you've ever used C or Matlab, the syntax should be familiar. Let's define a list containing the numbers 0 through 20 using the built-in range() function

For range() to return all numbers from 1 to 20, you have to specify that you want numbers from 1 to 21.

a = list(range(1, 21))

Slice syntax is defined using start:end[:step] where step is optional. If you want to retrieve the first 10 items, you would specify the following slice

a[0:10]

If you omit the start index, it will default to 0

a[:10]

Add a colon : and specify a step value of 2 to get every other item

a[:20:2]

If you don't know the length of the list, omit the end index to default to the maximum length

a[::2]

For more advanced information, visit the official documentation here