On Mon, May 18, 2015 at 12:29 PM, Joachim Durchholz <[email protected]> wrote:
> Am 18.05.2015 um 15:56 schrieb Carsten Knoll:
>>
>> I want to equip an Symbol with an additional attribute to store some
>> specific information right in place.
>>
>> For 'normal' Python classes it is no problem to dynamically create an
>> additional attribute for an already existing instance.
>>
>> However, for sympy Symbols if I try
>>
>> x = Symbol('x')
>> x.k = 0
>>
>> I get the error
>>
>> AttributeError: 'Symbol' object has no attribute 'k'
>
>
> That happens because we use __slots__ for performance reasons.
> However, storing attributes in that way is a bad idea in general, because
> you risk name conflicts with future versions of SymPy. (That's also the
> reason why using subclasses to extend a library tends to break over time.)
I'm not so sure about this.
The real reason that storing an attribute on x is a bad idea is that
SymPy implicitly assumes that it can replace one expression with the
same expression if the two are equal, so, e.g., if you had
x1 = Symbol('x')
x2 = Symbol('x')
x1.k = 0
and supposing it worked, along with x1 == x2 giving True, then SymPy
would assume that it has the freedom at any point to swap x1 with x2
and visa versa (and indeed, it does do this quite a bit with the
cache).
I think what you really want to do is create a subclass of Symbol that
stores the attributes in .args, so that they become part of the
equality of the object. Joachim's alternative 1 (store the
information separately) also sounds like a good one (and probably much
simpler to implement).
Aaron Meurer
>
> Alternative 1: You could set up a dict with a Symbol-to-attributes mapping:
> Symbols are hashable (that's a guaranteed property).
>
> Alternative 2: We could add a `userdict` attribute in Symbol, so that SymPy
> users can store additional attributes there.
>
> I'm not too happy with either alternative.
> Maybe we can give you better help if you describe your use case in some more
> detail.
>
> Regards,
> Jo
>
> --
> You received this message because you are subscribed to the Google Groups
> "sympy" group.
> To unsubscribe from this group and stop receiving emails from it, send an
> email to [email protected].
> To post to this group, send email to [email protected].
> Visit this group at http://groups.google.com/group/sympy.
> To view this discussion on the web visit
> https://groups.google.com/d/msgid/sympy/555A2182.8070309%40durchholz.org.
>
> For more options, visit https://groups.google.com/d/optout.
--
You received this message because you are subscribed to the Google Groups
"sympy" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To post to this group, send email to [email protected].
Visit this group at http://groups.google.com/group/sympy.
To view this discussion on the web visit
https://groups.google.com/d/msgid/sympy/CAKgW%3D6LDEL6XHcF-PyMz9PHXSdJ2LT0uUS8-WfPpqSU6k7tZLw%40mail.gmail.com.
For more options, visit https://groups.google.com/d/optout.
