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C++ programmers use 'operator overloading' to apply built-in operators to user defined classes. Thus, a complex number class may have an addition operator which makes it possible for us to use two objects of type 'complex' in an arithmetic expression in the same way we use integers or floating point numbers. The Python programming language provides much of the same functionality in a simple and elegant manner - using special method attributes. This is a quick introduction to some of the special methods through code snippets which we wrote while trying to digest the Python language reference. The code has been tested on Python ver 1.5.1.

Classes and objects in Python

Let us look at a simple class definition in Python:

class foo:
	def __init__(self, n):
		print 'Constructor called'
		self.n = n
		
	def hello(self):
		print 'Hello world'
		
	def change(self, n):
		print 'Changing self.n'
		self.n = n

f = foo(10)  # create an instance of class foo with a data field 'n' whose value is 10.
print f.n    # prints 10. Python does not support member access control the way C++ does.
f.m = 20     # you can even add new fields!
f.hello()    # prints 'Hello world'
foo.change(f, 40)
print f.n    # prints 40

We see that all member functions have a parameter called 'self'. Now, when we call f.hello(), we are actually calling a method belonging to class foo with the object 'f' as the first parameter. This parameter is usually called 'self'. Note that it can be given any other name, though you are encouraged to name it that way by convention. It is even possible to call f.hello() as foo.hello(f). C++ programmers will find some relation between 'self' and the keyword 'this'(in C++) through which the hidden first parameter to member function invocations can be accessed.

The special method __add__

class foo:
	def __init__(self, n):
		self.n = n
	def __add__(self, right):
		t = foo(0) 
		t.n = self.n + right.n
		return t
		

f1 = foo(10)  # f1.n is 10
f2 = foo(20)  # f2.n is 20
f3 = f1 + f2  
print f3.n    # prints 30

Another flavor of __add__

class foo:
	def __init__(self, n):
		self.n = n
	def __radd__(self, left):
		t = foo(0) 
		t.n = self.n + left.n
		print 'left.n is', left.n
		return t
	
f1 = foo(10)
f2 = foo(20)
f3 = f1 + f2  # prints 'left.n is 10'

How objects print themselves - the __str__ special method

class foo:
	def __init__(self, n1, n2):
		self.n1 = n1
		self.n2 = n2
	
	def __str__(self):
		return 'foo instance:'+'n1='+`self.n1`+','+'n2='+`self.n2`
		

f1 = foo(10,20)
print f1    # prints 'foo instance: n1=10,n2=20'

Truth value testing with __nonzero__

__nonzero__ is called to implement truth value testing. It should return 0 or 1. When this method is not defined, __len__ is called, if it(__len__) is defined. If a class defines neither __len__ nor __nonzero__, all its instances are considered true. This is how the language reference defines __nonzero__.

Let us put this to test.

class foo:
	def __nonzero__(self):
		return 0
		
class baz:
	def __len__(self):
		return 0
		
class abc:
	pass
	

f1 = foo()
f2 = baz()
f3 = abc()
		
if (not f1): print 'foo: false'  # prints 'foo: false'
if (not f2): print 'baz: false'  # prints 'baz: false'
if (not f3): print 'abc: false'  # does not print anything

The magic of __getitem__

class foo:
	def __init__(self, limit):
		self.limit = limit
		
	def __getitem__(self, key):
		if ((key > self.limit-1) or (key < -(self.limit-1))):
			raise IndexError
		else:
			return 2*key
			
f = foo(10)       # f acts like a 20 element array 
print f[0], f[1]  # prints 0, 2
print f[-3]       # prints -6
print f[10]       # generates IndexError

Attribute access with __getattr__

class foo:
	def __getattr__(self, name):
		return 'no such attribute'
		
f = foo()
f.n = 100
print f.n     # prints 100
print f.m     # prints 'no such attribute'

class Boss:
	def __init__(self, delegate):
		self.d = delegate
		
	def credits(self):
		print 'I am the great boss, I did all that amazing stuff'
		
	def __getattr__(self, name):
		return getattr(self.d, name)
		
	
class Worker:
	def work(self):
		print 'Sigh, I am the worker, and I get to do all the work'
		
	
w = Worker()	
b = Boss(w)
b.credits()  # prints 'I am the great boss, I did all that amazing stuff'
b.work()     # prints 'Sigh, I am the worker, and I get to do all the work'

Further Reading

Copyright © 2000, Pramode C E
Published in Issue 54 of Linux Gazette, June 2000