Hi Chris!
On Tue, Nov 18, 2008 at 3:05 AM, Drake <[EMAIL PROTECTED]> wrote:
>
> I created a bit of preliminary code just to play around with this
> idea:
>
> --------------------
>
> from sympy.core.basic import Basic, S
> from sympy.core.function import Function
>
> class Not(Function):
>
> nargs = 1
>
> @classmethod
> def canonize(cls, x):
> if x.is_Number:
> if x is S.Zero: return S.One
> if x is S.One: return S.Zero
> raise ValueError('Not: X not in {0, 1}')
> return None
>
> class And2(Function):
>
> nargs = 2
>
> @classmethod
> def canonize(cls, x, y):
> if x.is_Number and y.is_Number:
> return x and y
> if x.is_Number:
> # 1 & Y <=> Y
> if x is S.One: return y
> # 0 & Y <=> 0
> if x is S.Zero: return S.Zero
> raise ValueError('And2: X not in {0, 1}')
> if y.is_Number:
> # X & 1 <=> X
> if y is S.One: return x
> # X & 0 <=> 0
> if y is S.Zero: return S.Zero
> raise ValueError('And2: Y not in {0, 1}')
> # X & X <=> X
> if x is y:
> return x
> return None
>
> # <clipped Or2 function>
> --------------------
Excellent, that looks like a promising beginning.
>
> I'm looking into the AssocOp code to implement the functions that will
> flatten a longer expression, and I have a question. Why does SymPy use
> this structure everywhere:
>
> class Basic(...):
> is_Atom = False
>
> class AssocOp(...):
> @classmethod
> def identity(cls):
> if cls is Mul: return S.One
^^ do you mean why it references the Mul class here? Or is your
question about something else?
>
> These base classes are referencing the type of their sub-classes.
> Doesn't this self-referential structure make it difficult to extend
> base classes? Every time I add a sub-class, I need to modify the base
> class code. For example, maybe it makes sense to add "is_Boolean" to
> the Basic class.
This is_* stuff is really confusing, Brian was asking the same
questions ---- the is_* stuff should imho not be used outside time
critical code in sympy/core. So you can use isinstance(a, Boolean) to
test if the "a" is a subclass of Boolean.
>
> I'm not a software engineer. Is there a good reason for this?
I think it is not, but please clarify what exactly you mean bove.
One should *not* modify Basic classes in sympy/core in order to extend
sympy. If it is not true, then it's a bug and it should be fixed.
Ondrej
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