How To capture an id into an array..

[Peter Otten]

Balaji wrote:

> Suppose if some body wants to assign an e=E() to an expression class..
> he may define then e=v+v1(where v and v1 both belong to a variable class)
> now again if we define say v2=V() and then try to multiply v2 with e then
> I want to raise an error that it is a nonlinear term..
> 
> So  what I though was if i keep track of all the instances id then I might
> raise an exception...
> 
> But the problem was i couldnt check the right side of an assigment...

I'm not sure what you mean - my guess is entirely different from Christan's
- but anyway:

I suppose you want an expression 

a operator b

to either raise an exception or return a new expression and base this
decision on both the kind of a and b. You can do this by providing a lookup
table with all allowed operations (or all forbidden ops, if that is
significantly smaller). Here's the simplest implementation I could think
of:

<expr.py>
import sets

supported = sets.Set([
    # operation, leftoperand, rightoperand
    ("+","v", "v"),
    ("+","+", "v"),
    ("+","v", "+"),
    ("+","+", "+"),
    ("*","v", "v"),
    # complete with all allowed
    # combinations of operators/values
    ])

class Base:
    """Shared implementation for Expressions and Values """
    def op(self, operator, other):
        if (operator, self.operator, other.operator) in supported:
            return Expression(operator, self, other)
        else:
            raise ValueError("unsupported operation")

    def __add__(self, other):
        return self.op("+", other)

    def __mul__(self, other):
        return self.op("*", other)

    # add __div__ etc. here

class Expression(Base):
    def __init__(self, operator, left, right):
        self.operator = operator
        self.left = left
        self.right = right

    def __str__(self):
        return str(self.left) + self.operator + str(self.right)

class Value(Base):
    operator = "v"
    def __init__(self, value):
        self.value = value
    def __str__(self):
        return "Value(%s)" % self.value

if __name__ == "__main__":
    print Value(3) * Value(4)
    print Value(1) + Value(2) + Value(3)
    print Value(1) + Value(2) * Value(3) # will raise an exception
</expr.py>


Peter