OO conventions

[Kirk McDonald]

Daniel Nogradi wrote:
> I'm relatively new to object oriented programming, so get confused
> about its usage once in a while. Suppose there is a class Image that
> has a number of methods, rotate, open, verify, read, close, etc. Then
> to use this class my natural guess would be to have something like
> 
> image = Image( )
> image.read( "myfile.jpg" )
> image.rotate( )
> image.close( )
> 
> But now it turns out that the PIL module uses this as
> 
> image = Image.open( "myfile.jpg" )
> image.verify( )
> image.rotate( )
> image.close( )
> 
> Perhaps the real Image class of PIL doesn't have these methods
> exactly, but doesn't matter, my point is the way it works. Is it
> normal that instead of first creating an instance of a class, it
> starts right away with one its methods? I of course understand that
> the PIL people simply made a choice that their module works this way,
> period, but I'm just wondering if it wouldn't have been more "logical"
> to do it along the way of my first example.
> 
> I guess it's just a matter of convention or how the programmer feels
> like, but there are no conventions of this type? Which would be more
> pythonic? Or I shouldn't worry and it's totally up to the developer?


If I were coding a class like that, I'd probably do it like this:

image = Image("myfile.jpg")
image.rotate()
image.close()

And, in addition, there would be an image.open() method. In the above, 
__init__ just calls it on its own when the filename is specified.

(Though, of course, I'm not at all familiar with the PIL module; there 
might be more stuff going on behind the scenes with the way they do it.)

-Kirk McDonald