Printing data members of a class

[Michael Hudson]

John Fisher <jfisher at are.berkeley.edu> writes:

> Hey folks,
> 
> I'm trying to set up a class so that when instances of it are printed, a
> Python prints just a certain data member of the instance.  For example:
> 
> >>> class x:
> ... 	data = array([1, 2])

Honk! Honk! You don't actually write it like that do you? You're
setting yourself up for the famous mutable class attribute problem.

[schnipp]

> I've tried playing around with repr, and defining __repr__() in the
> class, but haven't had any luck yet.  I'd appreciate any suggestions on
> how to do this.

A point to bear in mind: __repr__ must return a string. Don't do this:

class x:
    ...
    def __repr__(self):
        print self.data

do this:

class x:
    ...
    def __repr__(self):
        return `self.data`

This trips me up every now and again.

HTH,
Michael