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<title>An Introduction to Object-Oriented Programming in Python LG #56</title>
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<H4 ALIGN="center">
"Linux Gazette...<I>making Linux just a little more fun!</I>"
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<H1><font color="maroon">An Introduction to Object-Oriented Programming in Python</font></H1>
<H4>By <a href="mailto:mso@mso.oz.net">Michael Orr</a></H4>
</center>
<P> <HR> <P>
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<P> Somebody asked Michael Williams if he could do Python and Java versions of
his article
<A HREF="../issue55/williams.html">An Introduction to Object-Oriented
Programming in C++</A>. Here's a Python version of the code. I'll comment
on the differences between C++ and Python. Perhaps somebody else can
write a Java version?
<P> I am assuming you know the basics of Python. If not, see the
excellent
<A HREF="http://www.python.org/doc/current/tut/tut.html">Tutorial</A>
and the other documentation at
<A HREF="http://www.python.org/doc/">http://www.python.org/doc/</A>.
<H2>Houses and more houses</H2>
<P> To represent Michael's house (in section <STRONG>Classy!</STRONG> in the
C++ article), we can use the following code:
(<A HREF="misc/orr/house.py.txt">text version</A>)
<BLOCKQUOTE><PRE>
#! /usr/bin/python
"""house.py -- A house program.
This is a documentation string surrounded by triple quotes.
"""
class House:
pass
my_house = House()
my_house.number = 40
my_house.rooms = 8
my_house.garden = 1
print "My house is number", my_house.number
print "It has", my_house.rooms, "rooms"
if my_house.garden:
garden_text = "has"
else:
garden_text = "does not have"
print "It", garden_text, "a garden"
</PRE></BLOCKQUOTE>
If we run it, it prints:
<BLOCKQUOTE><PRE>
My house is number 40
It has 8 rooms
It has a garden
</PRE></BLOCKQUOTE>
<P> What does this program do? First, we define what a generic house is in the
<CODE>class</CODE> block. <CODE>pass</CODE> means "do nothing" and is required
if the block would otherwise be empty. Then we create an instance (that is, a
particular house) by calling the class name as if it were a function. The
house is then stored in the variable <CODE>my_house</CODE>.
<P> This house initially has no attributes--if we were to query
<CODE>my_house.number</CODE> before setting it, we'd get an AttributeError.
The next three lines set <EM>and create</EM> the attributes. This is a
difference between the languages: Java instances start out with certain
attributes which can never change (although their values can change), but
Python instances start out with no attributes, and you can add or delete
attributes (or change their type) later. This allows Python to be more flexible
in certain dynamic situations.
<P> We can initialize the instance at creation time by including a special
<CODE>__init__</CODE> method. (A method is a function which "belongs" to a
class.) This program:
(<A HREF="misc/orr/house2.py.txt">text version</A>)
<BLOCKQUOTE><PRE>
#! /usr/bin/python
"""house2.py -- Another house.
"""
class House:
def __init__(self, number, rooms, garden):
self.number = number
self.rooms = rooms
self.garden = garden
my_house = House(20, 1, 0)
print "My house is number", my_house.number
print "It has", my_house.rooms, "rooms"
if my_house.garden:
garden_text = "has"
else:
garden_text = "does not have"
print "It", garden_text, "a garden"
</PRE></BLOCKQUOTE>
prints:
<BLOCKQUOTE><PRE>
My house is number 20
It has 1 rooms
It does not have a garden
</PRE></BLOCKQUOTE>
Because the class has an <CODE>__init__</CODE> method, it's automatically
called when an instance is created. The arguments to <CODE>House</CODE> are
really the arguments to <CODE>__init__</CODE>. Although most programs don't,
you can also call <CODE>__init__</CODE> yourself as many times as you want:
<CODE>my_house.__init__(55, 14, 1)</CODE>. This tells the object to
"reinitialize itself".
<P> Note that <CODE>__init__</CODE> is defined with an extra first argument,
<CODE>self</CODE>. But we <EM>don't</EM> specify
<CODE>self</CODE> when we call the method. All Python methods work like
this.
<CODE>self</CODE> is in fact the instance itself, and Python supplies it
behind the scenes. You need <CODE>self</CODE> because it's the only way the
method can access the instance's attributes and other methods. Inside the
method,
<CODE>self.rooms</CODE> means the instance's attribute <CODE>rooms</CODE>, but
<CODE>rooms</CODE> means the
<STRONG>local variable</STRONG> <CODE>rooms</CODE>.
Local variables, of course, vanish when the method ends. Python's use of
<CODE>self</CODE> is parallelled in Perl and other OO languages as well.
<P> Michael didn't tell you, but C++ has a <CODE>this</CODE> pointer which
works like Python's <CODE>self</CODE>. However, in C++ you don't have to type
<CODE>this->house</CODE> if there is no local variable <CODE>house</CODE>,
and you never type <CODE>this</CODE> on a method definition line. In other
words, C++ (and Java) do the same thing as Python and Perl; they just hide it
from the programmer.
<P> In fact, <CODE>self</CODE> in Python is just a conventional name. You can
call it <CODE>this</CODE> or <CODE>me</CODE> instead if you like. I actually
like <CODE>me</CODE> better. However, I stick with <CODE>self</CODE> so that
if somebody else has to maintain my work later, it will be easier for them to
read. In contrast, C++'s variable <CODE>this</CODE> is magic and cannot be
renamed.
<P> In the C++ program, <CODE>garden</CODE> is a boolean attribute. Python
doesn't have boolean attributes, so we use an integer instead. The expression
<CODE>my_house.garden</CODE> is true if the attribute is 1 (or any
non-zero, non-empty value).
<H2>Don't be square</H2>
<P> This section corresponds to the "Member Functions" section in Williams'
article. I prefer the term "method" over "member function", as Pythoneers
usually do. Michael's <CODE>square.c</CODE> program would look like this:
(<A HREF="misc/orr/square.py.txt">text version</A>)
<BLOCKQUOTE><PRE>
#! /usr/bin/python
"""square.py -- Make some noise about a square.
"""
class Square:
def __init__(self, length, width):
self.length = length
self.width = width
def area(self):
return self.length * self.width
my_square = Square(5, 2)
print my_square.area()
</PRE></BLOCKQUOTE>
prints
<BLOCKQUOTE><PRE>
10
</PRE></BLOCKQUOTE>
<P> <CODE>area</CODE> should be self explanatory because it works exactly like
<CODE>__init__</CODE> above. To reiterate, all the <CODE>self</CODE>s in
square.py are required. I have chosen to give Square an <CODE>__init__</CODE>
method rather than setting the attributes later, because that's what most
Python programmers would do.
<H2>Function definitions outside the class definition</H2>
<P> Nothing to say here. Python does not allow methods to be defined outside
the class. Of course, this doesn't apply to ordinary (non-class) functions.
<H2>Public or private?</H2>
<P> Not much to say here either. All Python attributes and methods are public.
You <EM>can</EM> emulate private attributes and methods via the
<A HREF="http://www.python.org/doc/current/tut/node11.html#SECTION0011600000000000000000">double-underscore hack</A>, but most Python programmers don't.
Instead, they count on the programmer not to abuse the class's API.
<H2>Class constructors</H2>
The <CODE>__init__</CODE> method <EM>is</EM> the constructor.
<H2>Arrays and classes</H2>
Williams' array example can't be coded literally because of differences between
the languages, but one equivalent is:
(<A HREF="misc/orr/person.py.txt">text version</A>)
<BLOCKQUOTE><PRE>
#! /usr/bin/python
"""person.py -- A person example.
"""
class Person:
def __init__(self, age, house_number):
self.age = age
self.house_number = house_number
alex = []
for i in range(5):
obj = Person(i, i)
alex.append(obj)
print "Alex[3] age is", alex[3].age
print
for alexsub in alex:
print "Age is", alexsub.age
print "House number is", alexsub.house_number
</PRE></BLOCKQUOTE>
This prints:
<BLOCKQUOTE><PRE>
Alex[3] age is 3
Age is 0
House number is 0
Age is 1
House number is 1
Age is 2
House number is 2
Age is 3
House number is 3
Age is 4
House number is 4
</PRE></BLOCKQUOTE>
<P> Python has no equivalent to <CODE>person alex[5]</CODE> in the C++
program, which creates an array of five empty instances all at once. Instead,
we create an empty list and then use a <CODE>for</CODE> loop (which sets
<CODE>i</CODE> to 0, 1, 2, 3 and 4 respectively) to populate it. The example
shows a loop subscripting a list by index number, another loop which gets each
element in the list directly, and a <CODE>print</CODE> statement which access
an element by index number.
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Copyright © 2000, Michael Orr<BR>
Published in Issue 56 of <i>Linux Gazette</i>, August 2000</H5>
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