[Tutor] Another class issue

[alan.gauld@bt.com]

> 	Since class data members/variables can be accessed 
> directly in Python i.e 
> Spam.meat = "Mechanically separated chicken", thus negating 
> the need for 
> data access methods i.e Spam.setTypeOfMeat("Mechanically separated 
> chicken"), what would be the reasoning, motivation or 
> whatever to implement 
> classes in Python? 


To do OO programming which is about encapsulation of 
data and function in a single entity. Whether you 
access the data directly or via methods is somewhat 
irrelevant. 

One of the worst types of OO design is to implement 
a class(in C++) which has a get/set pair for every 
data member - this is a huge waste of effort and 
misses the point of offering a behavioural 
interface to the class. If the behavioral interface 
is done properly there will be no need for a 
programmer to access the internal data, so 
whether the language permits it or not is academic.

To illustrate:

A car object has a simple interface to the driver:
turn right/left, accelerate, brake, park, switch 
on/off etc. The driver can *if he wishes* do these 
things by accessing the component parts of his car 
(eg bypass the ignition key and hot wire it) but 
he choooses not to because there is a simpler 
interface provided.

Data hiding is one part of OO programming that is 
heavily emphasised in C++ etc but is not universally 
adopted by OO languages - many Lisp OO dialects have 
no data hiding at all but support other aspects of 
OO more fully than C++

OO is about how you organise programs - objects 
requesting services of other objects by senbding 
messages to them rather than a rigid heirarchy of 
function calls. Other capabilities, like polymorphism,
either through inheritance structures or otherwise 
enhance the power of that metaphor by making programs 
extensible in ways that are difficult to achieve using 
procedural methods/techniques.

Alan G.






As I see it (through a maybe 
> narrow/imperfect view), 
> the classes are more akin to a C struct with some functions/methods 
> included (said methods. For exmpale, why create a class such as:
> 
> class Spam:
> 	colour1 = "red"
> 	colour2 = "brown"
> 	def colourChange(self, colour):
> 		self.colour1 = colour
> 
> when this would do:
> 
> Spam.colour1 = "yellow"
> 
> If you have followed my convoluted questioning, what I am 
> really asking is 
> this: why use classes in Python? What would be the purpose 
> (other than a 
> wrapper for a library, etc)?
> 
> Ken
>