[Tutor] Newbie question on Self, Master and others.

alan.gauld@bt.com alan.gauld@bt.com
Tue, 22 Aug 2000 12:15:29 +0100

> I'm some experiance with VB and am new to Python. In 
> reviewing some code examples I've come across the "def 
> __init__()" function with several different, what seems to me 
> are argument options. IE: self, master, master=None. Would 

init() is the Python constructor mechaniusm(which ISTR doesn't 
exist in VB!). When you create a new object the init method gets called

The self, master, etc are all variables like any other but any 
function(method) defined within a class causes Python to use the 
first parameter as a placeholder for the object.

class Foo:
	def __init__(bar):
		# do something

	def jiggle(baz,spam):
		# do some other thing

f = Foo()	# f gets a reference to the new FOO object
		# Foo.__init__(f) gets called behind the scenes
		# thus bar gets the value of f, which is the new object

f.jiggle('burble') # calls Foo.jiggle(f,'burble') in a friendly way

By *convention* the first parameter in a method is called 'self'
(because it will be populated by Python with a reference to 
the current object - the one owning the method. Thus normally 
you'd see the above example as:

class Foo:
	def __init__(self):
		# do something

	def jiggle(self, spam):
		# do some other thing

f = Foo()

The master=None bit simply says that master is a parameter, 
which if no value is provided it will take the value None.


>>> def sayHello(name='World'):
...     print 'Hello ' + name
>>> sayHello('Alan')		# I provide a value
Hello Alan
>>> sayHello()		# no value so use default
Hello World

This stuff is covered in my online tutor at:

Look under 'modules & functions' for the default args and 
under 'OOP' for __init__() stuff.


Alan G.