Good or bad use of __repr__?

[Steven D'Aprano]

On Tue, 03 Feb 2009 22:02:13 -0600, Alaric Haag wrote:

> Hello,
> 
> Is the use of __repr__ below a "really bad idea"?
> 
> class Dimension():
>     def __init__(self, setp, name):
>         ptr = setp.contents.dim
>         while ptr.contents.name != name:
>             ptr = ptr.contents.next
>         self.name = ptr.contents.name
>         self.size = ptr.contents.size
>         self.unlimited = bool(ptr.contents.unlimited)
>         self.coord = ptr.contents.coord
>     def __repr__(self):
>         return '%g' % (self.size)


As a rule of thumb, you should aim for:

eval( repr(obj) )

to recreate the obj. That's not always possible, but when possible, it is 
an ideal to aspire to. Given that, I'd recommend:

    def __repr__(self):
        return '%s(%s, %s)' % (
        self.__class__.__name__, self.ptr, self.name)
    def __str__(self):
        return "<dim=%g>" % self.size


except of course your class doesn't store ptr.


But looking at the code shown, I'm guessing you have bigger design 
problems than just what __repr__ should look like. I suggest you read 
this:

http://www.surfscranton.com/architecture/LawOfDemeter.htm


-- 
Steven