is it possible to give an instance a value?

Bruno Desthuilliers bdesth.quelquechose at
Wed Mar 7 01:12:17 CET 2007

manstey a écrit :
> Hi,
> My question probably reflects my misunderstanding of python objects,
> but I would still like to know the answer.
> The question is, is it possible for an instnace to have a value (say a
> string, or integer) that can interact with other datatypes and be
> passed as an argument?

In Python, strings and integers *are* objects (instances of resp. str 
and int).

> The following code of course gives an error:
> class Test(object):
>      def __init__(self, val):
>            self.val = val
>>>>a = Test('hello')
>>>>a.val  + ' happy'
> 'hello happy'
>>>>a + 'happy'
> TypeError: unsupported operand type(s) for +: 'Test' and 'str'

No surprise so far.

> Is there a way to make a have the value a.val

Forget about the value/instance distinction - it doesn't exists in Python.

> when it is used as
> above, 

implement __add__ (and any other appropriate magic method)

> or as an argument (eg function(a, 10, 'sdf') etc)?
> The only fudge I discovered for simple addition was to add to the
> class
>    def __add__(self, obj):
>        return a.val + obj

So what's your problem exactly ?

> but this doesn't solve the problem in general. I have tried
> subclassing the string type, but as it is immutable, this is not
> flexible the way a.val is

Err... In your above example implementation, a.val actually *is* a string.

> (i.e. it can't e reassigned


a.val = "tutu"

> and remain a
> subclass).


> Any pointers, or is my question wrong-headed?

I'm afraid I don't get the point.

> btw, my motivation is wanting to mimic another oo language which
> allows this, so it allows:
> 'Sydney'
> '%String'
>>>>Person.Address = 'Canberra'
>>>>print Person.Address. Person.Address.type
> Canberra %String
> etc.

> We have had to implement Person.Address as Person.Address.val, making
> Address an instance with .val, .type, etc.

 >>> class Person(object):
...     def __init__(self):
...             self.address = "sidney"
 >>> person = Person()
 >>> person.address
 >>> type(person.address)
<type 'str'>
 >>> # this is equivalent:
 >>> person.address.__class__
<type 'str'>
 >>> # of if you want the name of the class instead
 >>> # of the class object itself:
 >>> person.address.__class__.__name__

Looks like you'd better learn the Python way instead of wasting your 
time trying to mimic some other language (BTW, is it VB or  C# ?).


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