9. Sequential Data Types

By Bernd Klein. Last modified: 29 Jun 2022.

Introduction

Sequences are one of the principal built-in data types besides numerics, mappings, files, instances and exceptions. Python provides for six sequence (or sequential) data types:

strings
byte sequences
byte arrays
lists
tuples
range objects

Strings, lists, tuples, bytes and range objects may look like utterly different things, but they still have some underlying concepts in common:

The items or elements of strings, lists and tuples are ordered in a defined sequence
The elements can be accessed via indices

text = "Lists and Strings can be accessed via indices!"
print(text[0], text[10], text[-1])    

L S !

Accessing lists:

cities = ["Vienna", "London", "Paris", 
          "Berlin", "Zurich", "Hamburg"]
print(cities[0])
print(cities[2])
print(cities[-1])  

Vienna
Paris
Hamburg

Unlike other programming languages Python uses the same syntax and function names to work on sequential data types. For example, the length of a string, a list, and a tuple can be determined with a function called len():

countries = ["Germany", "Switzerland", "Austria", 
             "France", "Belgium", "Netherlands", 
             "England"]
len(countries)  # the length of the list, i.e. the number of objects

7

fib = [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]
len(fib)

10

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Bytes

The byte object is a sequence of small integers. The elements of a byte object are in the range 0 to 255, corresponding to ASCII characters and they are printed as such.

s = "Glückliche Fügung"
s_bytes = s.encode('utf-8') 
s_bytes

b'Gl\xc3\xbcckliche F\xc3\xbcgung'

Python Lists

So far we had already used some lists, and here comes a proper introduction. Lists are related to arrays of programming languages like C, C++ or Java, but Python lists are by far more flexible and powerful than "classical" arrays. For example, not all the items in a list need to have the same type. Furthermore, lists can grow in a program run, while in C the size of an array has to be fixed at compile time.

Generally speaking a list is a collection of objects. To be more precise: A list in Python is an ordered group of items or elements. It's important to notice that these list elements don't have to be of the same type. It can be an arbitrary mixture of elements like numbers, strings, other lists and so on. The list type is essential for Python scripts and programs, in other words, you will hardly find any serious Python code without a list.

The main properties of Python lists:

• They are ordered
• The contain arbitrary objects
• Elements of a list can be accessed by an index
• They are arbitrarily nestable, i.e. they can contain other lists as sublists
• Variable size
• They are mutable, i.e. the elements of a list can be changed

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List Notation and Examples

List objects are enclosed by square brackets and separated by commas. The following table contains some examples of lists:

Accessing List elements

Assigning a list to a variable:

languages = ["Python", "C", "C++", "Java", "Perl"]

There are different ways of accessing the elements of a list. Most probably the easiest way for C programmers will be through indices, i.e. the numbers of the lists are enumerated starting with 0:

languages = ["Python", "C", "C++", "Java", "Perl"]
print(languages[0] + " and " + languages[1] + " are quite different!") 
print("Accessing the last element of the list: " + languages[-1]) 

Python and C are quite different!
Accessing the last element of the list: Perl

The previous example of a list has been a list with elements of equal data types. But as we saw before, lists can have various data types, as shown in the following example:

group = ["Bob", 23, "George", 72, "Myriam", 29]

Sublists

Lists can have sublists as elements. These sublists may contain sublists as well, i.e. lists can be recursively constructed by sublist structures.

person = [["Marc", "Mayer"], 
          ["17, Oxford Str", "12345", "London"], 
          "07876-7876"]
name = person[0]
print(name)

['Marc', 'Mayer']

first_name = person[0][0]
print(first_name)

Marc

last_name = person[0][1]
print(last_name)

Mayer

address = person[1]
print(address)

['17, Oxford Str', '12345', 'London']

street = person[1][0]
print(street)

17, Oxford Str

We could have used our variable address as well:

street = address[0]
print(street)