[Tutor] When to use classes

[Ni hung]

Thank you Alan, Dave and Cameron (and folks managing this email group)!
 Your replies were very helpful.

Regards
ni



On Wed, Apr 9, 2014 at 4:25 AM, Cameron Simpson <cs at zip.com.au> wrote:

> On 08Apr2014 22:58, Ni hung <niihung at gmail.com> wrote:
> > I am learning programming using python. I think of solving a problem
> using
> > functions and for this reason all/most of my code consists of functions
> and
> > no classes.  I have some understanding of classes/Object Oriented
> > Programming. I can write simple classes but I do not understand when to
> use
> > classes.
>
> Loosely speaking, you can usefully make a class when there is a
> particular type of object on which you are make an assortedment of
> operations.
>
> For example, if you have several functions looking like this:
>
>   def modify_thing1(obj1, ...):
>
>   def add_something(obj1, ...):
>
> and so on, where obj1 is always the same kind of thing, representing
> some well define idea you have when writing the code.
>
> In that circumstance you might make a class looking like this:
>
>   class ObjName(object):
>
>     def __init__(self):
>       # "self" here is an "obj1" as used above
>       # set whatever attributes an "obj1" should have to start with
>       # you can pass in some of those values as parameters to the __init__
> function
>
>     def modify(self, ...):
>       # this is the same as "modify_thing1(obj1, ...)" earlier
>       # using "self" as "obj1"
>
>     def add(self, ...):
>       # this is the same as "add_something(obj1, ...)" earlier
>
> Then your code looks like:
>
>   obj1 = ObjName()
>   ...
>   obj1.modify(5)
>   obj1.add(6)
>
> or what have you.
>
> This has several advantages:
>
>   - all the code that affects this kind of object is in one place
>
>   - you can remove all the specifics of how things are done from the main
>     code and keep them inside the class methods.
>     This makes the main code more readable (if you pick good method names)
>     and shorter (also more readable, usually).
>
>   - later, if need be, you can change the inner workings of how
>     ObjNames work without changing the main code, or at least not
>     much - sometimes not at all
>
> Anyway, that's an ok rule of thumb for when to make a class: when
> you have a bunch of operations to do to one type of thing.
>
> Cheers,
> --
> Cameron Simpson <cs at zip.com.au>
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