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## Beginning programming with Python

These are notes of a talk I delivered during Software Freedom Day 2009 at Chadstone Shopping Centre, Melbourne, Australia. Slides are also available. This talk requires no previous experiences with programming. After covering some basic principles of programming, these principles are then demonstrated using the Python programming language.

What’s a program?

When you start programming, you are asserting more control over a computer than if you were using the computer. To this end, you need to have a deeper understanding of how computers work.

As a computer user, you use programs that have been written by other people. Such programs include:

• web browsers, e.g. Firefox, IE, Opera, Safari
• word processors, e.g. Abiword, OpenOffice, Word
• multimedia players, e.g. MPlayer, VLC
• document viewers, e.g. Acrobat Reader, evince, Foxit, okular
• email clients, e.g. Evolution, Outlook, Thunderbird
• webmail, e.g. Gmail, Hushmail, Windows Live, Yahoo!

You might use some of these programs throughout a working day, without having to be concerned about the their internals. Why would you want to worry about how a program work when you simply want to use it?

I have referred to various programs, but have not talked about what a program is. Think of a program as a sequence of instructions that specify how to perform a task. The instructions might be about directing a computer to carry out a mathematical calculation, position a window at a certain place on the screen, process some data, or send an email. Regardless of what the purpose of the program is, the instructions that constitute the program must be written in a format comprehensible by a computer.

Programming languages

Regardless of what the program is meant to achieve, you must write it using a programming language. At the lowest level, computers read your program as a bunch of ones and zeros, which is essentially what computers do: they process ones and zeros, or a sequence of bits. For example, “A” might be represented as 01000001, “B” as 01000010, and “C” as 01000011, so “ABC” becomes

$\text{ABC} \longrightarrow 010000010100001001000011$

But then programming using ones and zeros is next to impossible and very error-prone. To make it easier to program a computer, you can use a high-level programming language. A high-level programming language can look a lot like a natural language such as English to make it easier for people to read and understand what a program does. Examples of high-level programming languages include

• C, C++
• Java, JavaScript
• Python, Ruby
• Perl, PHP

Programming languages may differ in many ways, but they usually share a number of basic principles:

• input: Retrieve data from somewhere, e.g. the keyboard, a file, somewhere on the web, a device.
• output: Display data on the screen, send data to a file, over the web, or to a device.
• math: Perform basic mathematical operations such as addition, multiplication, subtraction, and division.
• conditional execution: Check for a condition. If it is true, then execute the sequence of instructions for that condition. If it is false, execute a different sequence of instructions.
• repetition: Perform a sequence of instructions repeatedly.

These basic principles of programming languages are what constitute a computer program. When writing a program, you mix and match them in certain ways in order that the program carry out the instructions you want it to execute.

Hello, World!

I will be using Python for all code examples from now on. Python is a programming language suitable for teaching and learning how to program. It is also used for writing very complex programs. You can get Python from http://www.python.org.

Upon loading Python from the command line, you would see something like this:

Python 2.6.2 (r262:71600, Jul 10 2009, 10:49:45)
[GCC 4.2.4 (Ubuntu 4.2.4-1ubuntu3)] on linux2
>>>


Now Python is waiting for you to enter some instructions for it to run. The three greater than signs >>> is the Python prompt, awaiting your instructions.

When learning a programming language, one of the first things that people learn is how to display the phrase, “Hello, World!”. This can be easily achieved using Python as follows:

>>> print "Hello, World!"
Hello, World!


Here, print is a statement that tells Python to display “Hello, World!”. It does so, removing the opening and closing quotation marks. The opening and closing quotation marks are used to denote the beginning and end of the text you want to display on the screen, which is why you don’t see these quotation marks in the result. You can also use Python as a calculator to do basic mathematics such as addition, multiplication, division, and subtraction.

>>> 2 + 3   # addition
5
>>> 2 * 3   # multiplication
6
>>> 12 / 4  # division
3
>>> 10 - 5  # subtraction
5


Notice that Python ignores everything after the hash symbol #. The text “# addition” is a comment. It is there to explain what the code statement “2 + 3” is doing. You don’t need to comment every line of your code. Keep in mind that months from now when you again read your code you might have forgotten what it is doing. In programming, precision is as important as thorough documentation.

Put on your problem solving hat

Programming is a creative activity mixed with logical precision. You write a set of precise instructions for a computer to carry out. Many people enjoy programming because it is a way for them to be creative. When things don’t go as intended, you need to put on your detective hat and figure out what went wrong. This process is called debugging.

Syntax error

In the text “Hello, World!”, suppose you leave out the closing quotation mark. This is what would happen if you do so:

>>> print "Hello, World!
File "", line 1
print "Hello, World!
^
SyntaxError: EOL while scanning string literal


Python is saying that

print "Hello, World!


is not a valid code statement in the Python programming language. With the text “SyntaxError“, Python is also pointing out the type of error in the above code. In this case, it is a syntax error, meaning that the code violates the syntax of the Python language. In US English, acronyms and abbreviations are spelt with full stops, e.g. U.S.A., Ms. and Mr. In Australian English, you would spell these as USA, Ms and Mr. Even if you spell them as U.S.A., Ms. and Mr. in an email to another Australian person, the person would likely understand what you mean without complaining. But Python is not so forgiving. It would complain whenever your code doesn’t follow the syntax of the Python language.

Runtime error

Another type of error that can lurk in your code is runtime error. Runtime errors are also called exceptions because they indicate that something exceptional has happened. Consider this Python code:

>>> print n
Traceback (most recent call last):
File "", line 1, in
NameError: name 'n' is not defined


Python is telling you that it doesn’t know what n is. Is n a number? Is it meant to denote the text “Hello, World!“? If n is meant to be holding some value, then it becomes something known as a variable. Think of a variable as a box for storing something so that it can be retrieved later on. You can assign something to n by using the equal symbol =.

>>> n = 23
>>> print n
23


In the above code, the variable n is assigned the number 23. That value is then displayed on the screen.

You can also assign a string to a variable. A string is a sequence of characters. Besides numbers, strings are very useful in many programming languages. You can also assign a string to a variable using the equal symbol:

>>> name = "Wendy Smith"
>>> print name
Wendy Smith


Garbage in, garbage out

Semantic errors usually occur when your program runs successfully, but the program doesn’t do what you intended it to do. This situation is often described as, “Garbage in, garbage out.” Semantic errors can be thought of as the meaning of a sequence of instructions. Do the instructions give you the result they are meant to give?

Here is an example of a semantic error:

>>> name = "123"
>>> print name
123


In this case, the code runs OK. Python doesn’t complain about anything; it has carried out the instructions as you have given. However, the result is not what was intended. The variable name should have referred to a sensible name such as “Wendy Smith“. But instead the variable refers to a number. In a large program, semantic errors can be difficult to find.

Functions

In programming, a function is a way to name a sequence of instructions. With a function, you can run that sequence of instructions over and over again by using the function name, instead of having to type the sequence of instructions every time you want to use the instructions. Suppose you want to write a function that displays the name and age of a person. Let’s give that function the name “person“:

>>> def person(name, age):
...     print "Name is", name
...     print "Age is", age
...
>>> person("Wendy Smith", 23)
Name is Wendy Smith
Age is 23


The word def tells Python that you are starting to define a function. The function name is person, whose parameters are contained within a pair of opening and closing parentheses. The parameters of a function are variables that hold values you pass to the function so that the function can use those values. The function person takes two parameters, i.e. name and age which are both variables. You can then use the function by specifying the function name, followed by the values for the parameters. In this case, the parameter name is given the string “Wendy Smith” and the parameter age is given the number 23.

Functions without parameters

A function doesn’t need to have any parameters. You can define a function that takes no parameters whatsoever and still do something useful.

>>> def recite_poem():
...     print "It's a world of laughter and a world of tears"
...     print "It's a world of hope and a world of fears"
...
>>> recite_poem()
It's a world of laughter and a world of tears
It's a world of hope and a world of fears


And you can recite the poem again and again. Let’s say you want to recite the poem five times. You can write another function that uses the function recite_poem() five times:

>>> def recite_poem():
...     print "It's a world of laughter and a world of tears"
...     print "It's a world of hope and a world of fears"
...
>>> def rehearse():
...     recite_poem()
...     recite_poem()
...     recite_poem()
...     recite_poem()
...     recite_poem()
...
>>> rehearse()
It's a world of laughter and a world of tears
It's a world of hope and a world of fears
It's a world of laughter and a world of tears
It's a world of hope and a world of fears
It's a world of laughter and a world of tears
It's a world of hope and a world of fears
It's a world of laughter and a world of tears
It's a world of hope and a world of fears
It's a world of laughter and a world of tears
It's a world of hope and a world of fears


The function rehearse() certainly does the job if you always want to rehearse the poem exactly five times. What if you want to rehearse the poem more than five times? There should be an easier way to write the function rehearse().

Repeat after me

When you need to perform a task a certain number of times, it usually indicates that you can use a loop to do the task. A loop is a way to run a sequence of instructions for some fixed number of times. One of the most common loop is the “for” loop. Like many other programming languages, in Python you can use the “for” loop to repeatedly run some instructions over and over again.

To recite the poem five times, you can do as follows:

>>> for i in range(5):
...     recite_poem()
...
It's a world of laughter and a world of tears
It's a world of hope and a world of fears
It's a world of laughter and a world of tears
It's a world of hope and a world of fears
It's a world of laughter and a world of tears
It's a world of hope and a world of fears
It's a world of laughter and a world of tears
It's a world of hope and a world of fears
It's a world of laughter and a world of tears
It's a world of hope and a world of fears


Now you can use the “for” loop to rewrite the function rehearse() so that it rehearses the poem for the number of times that you want it to.

>>> def rehearse(n):
...     for i in range(n):
...         recite_poem()
...
>>> rehearse(3)
It's a world of laughter and a world of tears
It's a world of hope and a world of fears
It's a world of laughter and a world of tears
It's a world of hope and a world of fears
It's a world of laughter and a world of tears
It's a world of hope and a world of fears


As you can see, the function rehearse() now has one parameter called n. Given any positive whole number, the function rehearse() would recite the poem that many times. Using a for loop, the function rehearse() is now much shorter and more compact than previously. It is also more general and works for a wider range of situations than before.

If this then that

There are times when you want to run a sequence of instructions only if some condition is true. Control flow allows you to separate your code into different sequences, each of which would be run if a specified condition holds true. A common type of control flow is the if statement. Here’s a simple if statement in Python:

>>> if 3 > 0:
...     print "3 is positive"
...
3 is positive


The condition here is to check that three is greater than zero. This is indeed true, so the message “3 is positive” is displayed.

If this, else that

If a condition is false, you can use the else statement to run another sequence of instructions.

>>> if -5 > 0:
...     print "-5 is positive"
... else:
...     print "-5 is negative"
...
-5 is negative


In this case, $-5$ is less than zero so the message “-5 is negative” is displayed.

Let’s write a function called check_number() that checks whether a number is positive, negative, or zero.

>>> def check_number(n):
...     if n > 0:
...         print n, "is positive"
...     if n < 0:
...         print n, "is negative"
...     if n == 0:
...         print n, "is zero"
...
>>> check_number(3)
3 is positive
>>> check_number(-5)
-5 is negative
>>> check_number(0)
0 is zero


A whole number is either positive, negative or zero. The code takes care of each of those three cases. Notice that the double-equal sign == means “is equal to”, whereas the single-equal sign = means “assign the value on the right to the variable on the left”. So if you want to test whether or not two things are equal to each other, use the double-equal sign ==.

If, else if, else

The function check_number() does what you want. But you can write it in a different way using all of the three control flow statements if, else and elif. The statement elif is short for “else if”. Using those three control flow statements, you can rewrite the function check_number() as follows:

>>> def check_number(n):
...     if n > 0:
...         print n, "is positive"
...     elif n < 0:
...         print n, "is negative"
...     else:
...         print n, "is zero"
...
>>> check_number(13)
13 is positive
>>> check_number(-7)
-7 is negative
>>> check_number(0)
0 is zero


The rewritten function works as expected and doesn’t use lots of if statements.

What’s after this?

I have talked about some basic principles that are common to many programming languages. These include input, output, mathematics, control flow, and repetition. Each of these five principles were demonstrated using the Python programming language. Here I list some resources that talk about beginning programming with Python.

1. Allen B. Downey. Think Python: How to Think Like a Computer Scientist.
2. Peter Norton, Alex Samuel, David Aitel, Eric Foster-Johnson, Leonard Richardson, Jason Diamond, Aleatha Parker, Michael Roberts. Beginning Python. Wiley Publishing, 2005.
3. Mark Pilgrim. Dive Into Python. Apress, 2004.
4. The Python Tutorial. Python Software Foundation, 2009.
1. 20 September 2009 at 2:10 pm