OO conventions

[bruno at modulix]

Blair P. Houghton wrote:

(snip)

> So Image.open(filename) seems right as a factory function that opens
> the file, figures out what it really is, constructs the appropriate
> subclass object (likely by passing a string to the constructor, e.g.,
> JPGImage(filename)), and returns the object via the superclass type.

Why "via the superclass type" ? "returns the object" is enough.

> The caller can then either check a flag in the superclass to see what
> type the subclass is,

Why the h... ? We don't care what type it is, as long at it does what we
expect it to do.

> or just assume it's the right type of image

Yes

(snip)
> (or does Python have RTTI?

Much better than 'RTTI'.

> I don't recall if
> I've seen it, yet...).

obj.__class__ is a reference to the class (which is itself an object...)


> Though if the filename doesn't match the content, someone should raise
> an exception...

Why ? filenames and extensions are nothing more than conventions.
Image.open() 's responsability is to create an object of the appropriate
class, period. If *your program* needs to ensure that the image type
matches the filename, it's your problem to check this.

> But this means that Image.open(filename) is a static method of the
> superclass, not requiring instantiation.

AFAIK, it's just a function in the Image module (which is itself an
object - instance of class module - but that's orthogonal).

>  Image(string) could easily
> default to assuming string is a filename, doing the same work as
> Image.open(filename), though it would at least partially construct an
> Image instance each time it's called, which isn't what you want.
> Image.open(filename) defined as a static method (wait...is that
> possible in Python? I hate being the newbie) 

It is (search for 'staticmethod' and 'classmethod'). But there's not
much use for 'static methods' in Python - we usually just use plain
functions ('classmethods' are another beast - much more useful than
staticmethods)


-- 
bruno desthuilliers
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