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CHAPTER 1 
■ 
LANGUAGE AND SYNTAX 
 
11 
 
Listing 1-14. Simple Python Class 
>>> class my_object: 
...     def __init__(self, x, y): 
...         self.x = x 
...         self.y = y 
...          
...     def mult(self): 
...         print self.x * self.y 
...          
...     def add(self): 
...         print self.x + self.y 
...  
 
>>> obj1 = my_object(7, 8) 
>>> obj1.mult() 
56 
>>> obj1.add() 
15 
In this class example, we define a class named my_object. The class accepts two parameters, x and y. 
A class initializer method is named __init__(), and it is used to initialize any values that may be used in 
the class. An initializer also defines what values can be passed to a class in order to create an object. You 
can see that each method and function within the class accepts the self argument. The self argument is 
used to refer to the object itself, this is how the class shares variables and such. The self keyword is 
similar to this in Java code. The x and y variables in the example are named self.x and self.y in the 
initializer, that means that they will be available for use throughout the entire class. While working with 
code within the object, you can refer to these variables as self.x and self.y. If you create the object and 
assign a name to it such as obj1, then you can refer to these same variables as obj1.x and obj1.y. 
As you can see, the class is called by passing the values 7 and 8 to it. These values are then assigned 
to x and y within the class initializer method. We assign the class object to an identifier that we call obj1. 
The obj1 identifier now holds a reference to my_object() with the values we’ve passed it. The obj1 
identifier can now be used to call methods and functions that are defined within the class. 
For more information on classes, please see Chapter 6, which covers object orientation in Python. 
Classes are very powerful and the fundamental building blocks for making larger programs. 
Statements 
When we refer to statements, we are really referring to a line of code that contains an instruction that 
does something. A statement tells the Python interpreter to perform a task. Ultimately, programs are 
made up of a combination of expressions and statements. In this section, we will take a tour of statement 
keywords and learn how they can be used. 
Let’s start out by listing each of these different statement keywords, and then we will go into more 
detail about how to use each of them with different examples. I will not cover every statement keyword 
in this section as some of them are better left for later in the chapter or the book, but you should have a 
good idea of how to code an action which performs a task after reading through this section. While this 
section will provide implementation details about the different statements, you should refer to later 
chapters to find advanced uses of these features. 
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CHAPTER 1 
■
 LANGUAGE AND SYNTAX 
 
12 
 
Table 1-2. Statement Keywords 
if-elif-else 
for 
while 
continue 
break 
try-except-finally 
assert 
def 
print 
del 
raise 
import 
Now that we’ve taken a look at each of these keywords, it is time to look at each of them in detail. It 
is important to remember that you cannot use any of these keywords for variable names. 
if-elif-else Statement 
The if statement simply performs an evaluation on an expression and does different things depending 
on whether it is True or False. If the expression evaluates to True then one set of statements will be 
executed, and if it evaluates to False a different set of statements will be executed. If statements are quite 
often used for branching code into one direction or another based upon certain values which have been 
calculated or provided in the code.  
Pseudocode would be as follows: 
Listing 1-15. 
if : 
    perform an action 
else: 
    perform a different action 
Any number of if/else statements can be linked together in order to create a logical code branch. 
When there are multiple expressions to be evaluated in the same statement, then the elif statement can 
be used to link these expressions together. Note that each set of statements within an if-elif-else 
statement must be indented with the conditional statement out-dented and the resulting set of 
statements indented. Remember, a consistent indentation must be followed throughout the course of 
the program. The if statement is a good example of how well the consistent use of indention helps 
readability of a program. If you are coding in Java for example, you can space the code however you’d 
like as long as you use the curly braces to enclose the statement. This can lead to code that is very hard to 
read…the indentation which Python requires really shines through here. 
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CHAPTER 1 
■ 
LANGUAGE AND SYNTAX 
 
13 
 
Listing 1-16. Example of if statement 
>>> x = 3 
>>> y = 2 
>>> if x == y: 
...     print 'x is equal to y' 
... elif x > y: 
...     print 'x is greater than y' 
... else: 
...     print 'x is less than y' 
...  
x is greater than y 
While the code is simple, it demonstrates that using an if statement can result in branching code 
logic. 
print Statement 
The print statement is used to display program output onto the screen (you’ve already seen it in action 
several times). It can be used for displaying messages, which are printed from within a program, and also 
for printing values, which may have been calculated. In order to display variable values within a print 
statement, we need to learn how to use some of the formatting options that are available to Python. This 
section will cover the basics of using the print statement along with how to display values by formatting 
your strings of text. 
In the Java language, we need to make a call to the System library in order to print something to the 
command line. In Python, this can be done with the use of the print statement. The most basic use of the 
print statement is to display a line of text. In order to do so, you simply enclose the text that you want to 
display within single or double quotes. Take a look at the following example written in Java, and 
compare it to the example immediately following which is rewritten in Python. I think you’ll see why the 
print statement in Python makes life a bit easier. 
Listing 1-17. Java Print Output Example 
System.out.println("This text will be printed to the command line"); 
Listing 1-18. Python Print Output Example 
print 'This text will be printed to the command line' 
As you can see from this example, printing a line of text in Python is very straightforward. We can 
also print variable values to the screen using the print statement. 
Listing 1-19. 
>>> my_value = 'I love programming in Jython' 
>>> print my_value 
I love programming in Jython 
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