assignments - want a PEP

[Bengt Richter]

On 31 Mar 2005 14:48:00 -0800, "Kay Schluehr" <kay.schluehr at gmx.net> wrote:

>>From time to time I still use my old Mathematica system. The
>Mathematica language has some nice properties. The one I like best is
>the possibility to create symbols from nothing. Translated into the
>Python realm following creations are possible:
>
>>>> a
>a
>
>That's it. Just make an 'a' as a pure symbol. The reason for those
>symbols is late evaluation. Most of the time an expression should be
>transformed and simplified before it is finally evaluated using
>concrete numbers or other stuff. I tried to mimic this in Python using
>a Symbol() class:
>
>>>> a = Symbol()
>>>> a
>a
>
>So 'a' would be a Symbol object that is represented by the name "a" not
>by the standard format information that tells us that a is some member
>of class Symbol(). Starting with Symbol() subclsses of Symbol() e.g
>Expr() could overload operators like __add__, __mul__ etc. in order to
>create new symbolic objects:
>
>>>> a,b = Expr(),Expr()
>>>> (a+b)/c
>a+b
>---
> c
>
>The availability of Symbol() would make it easy to create a Computer
>Algebra System ( CAS ) in Python, just using standard Python operators.
>
>Unfortunately it is not possible to implement Symbol() in an elegant
>way. Symbol() has to retrieve somehow it's own name from the
>environment where the name is created, but no assignment/name binding
>event is accessible to Python objects.
If you let your symbols like a live in a CAS class instance attribute namespace,
you can do what you like, pretty much. I.e.,

    cas = CAS()
    cas.a        # like your plain a, where you said "that's it"


    cas.a, cas.b = cas.Expr(), cas.Expr()
    (cas.a + cas.b)/cas.c

Etc.

>
>I want to propose making name binding accessible by introducing an
>__assign__ method which is called, whenever an object is bound to a
>name. By default the __assign__ method does nothing at all but can be
>implemented by custom classes.
>
>def __assign__(self,own_name):
>    # do somtehing with me and my name
>

>Instead of changing the behaviour of the current assignment operator
>"=" one could think about introduction of a new assignment ":=" that is
>connected to the __assign__ method of custom classes if available.
>
>What do You think?
>
I think Guido does not want bare names to work like properties, even though
some of that could be interesting ;-)

Regards,
Bengt Richter