Overloading assignment operator

[Steven D'Aprano]

On Tue, 23 Jan 2007 18:07:55 -0800, Russ wrote:

> Achim Domma wrote:
>> Hi,
>>
>> I want to use Python to script some formulas in my application. The user
>> should be able to write something like
>>
>> A = B * C
>>
>> where A,B,C are instances of some wrapper classes. Overloading * is no
>> problem but I cannot overload the assignment of A. I understand that
>> this is due to the nature of Python, but is there a trick to work around
>> this?
>> All I'm interested in is a clean syntax to script my app. Any ideas are
>> very welcome.
>>
>> regards,
>> Achim
> 
> Why do you need to overload assignment anyway? If you overloaded "*"
> properly, it should return
> the result you want, which you then "assign" to A as usual. Maybe I'm
> missing something.

One common reason for overriding assignment is so the left-hand-side of
the assignment can choose the result type. E.g. if Cheddar, Swiss and
Wensleydale are three custom classes, mutually compatible for
multiplication:

B = Cheddar()  # B is type Cheddar
C = Swiss()    # C is type Swiss
# without overloading assignment
A = B * C      # A is (possibly) Cheddar since B.__mul__ is called first
A = C * B      # A is (possibly) Swiss since C.__mul__ is called first
# with (hypothetical) assignment overloading 
A = B * C      # A is type Wensleydale since A.__assign__ is called

Except, of course, there is no assignment overloading in Python. There
can't be, because A may not exist when the assignment is performed, and
if it does exist it might be a complete different type.

Instead, you can do something like this:
 
A = Wensleydale(B) * Wensleydale(C)

or 

A = Wensleydale(B * C)




-- 
Steven D'Aprano