Best practise implementation for equal by value objects

[John Krukoff]

On Wed, 2008-08-06 at 05:50 -0700, Slaunger wrote:
> Hi,
> 
> I am new here and relatively new to Python, so be gentle:
> 
> Is there a recommended generic implementation of __repr__ for objects
> equal by value to assure that eval(repr(x)) == x independet of which
> module the call is made from?
> 
> Example:
> 
> class Age:
> 
>     def __init__(self, an_age):
>         self.age = an_age
> 
>     def __eq__(self, obj):
>         self.age == obj.age
> 
>     def __repr__(self):
>         return self.__class__.__name__ + \
>                "(%r)" % self.age
> 
> age_ten = Age(10)
> print repr(age_ten)
> print eval(repr(age_ten))
> print eval(repr(age_ten)).age
> 
> Running this gives
> 
> Age(10)
> Age(10)
> 10
> 
> Exactly as I want to.
> 
> The problem arises when the Age class is iomported into another module
> in another package as then there is a package prefix and the above
> implementation of __repr__ does not work.
> 
> I have then experimented with doing somthing like
> 
>     def __repr__(self):
>         return self.__module__ + '.' + self.__class__.__name__ +
> "(%r)" % self.age
> 
> This seems to work when called from the outside, but not from the
> inside of the module. That is, if I rerun the script above the the
> module name prefixed to the representation I get the following error
> 
> Traceback (most recent call last):
>   File "valuetest.py", line 15, in <module>
>     print eval(repr(age_ten))
> __main__.Age(10)
>   File "<string>", line 1, in <module>
> NameError: name '__main__' is not defined
> 
> This is pretty annoying.
> 
> My question is: Is there a robust generic type of implementation of
> __repr__ which I can use instead?
> 
> This is something I plan to reuse for many different Value classes, so
> I would like to get it robust.
> 
> Thanks,
> Slaunger
> --
> http://mail.python.org/mailman/listinfo/python-list

Are you really sure this is what you want to do, and that a less tricky
serialization format such as that provided by the pickle module wouldn't
work for you?

-- 
John Krukoff <jkrukoff at ltgc.com>
Land Title Guarantee Company