Python number-like class that remembers arithmetic operations?

Question

I\'m wondering if there exists a python module that would allow me to do something like this:

x = MagicNumber()
x.value = 3
y = 2 * (x + 2) ** 2 - 8
print y          


        

Answer 1

Isn't this called a function? This may sound like a simple answer, but I mean it sincerely.

def y(x):
    return 2 * (x + 2) ** 2 - 8

Aren't you thinking in the wrong direction with this one?

To address the clarification:

class MyParams():
    distance = 0.0
    speed = 0.0
    def __call__(self):
        return self.distance / self.speed

p = MyParams()
p.distance = 13.4            # These are properties
p.speed = 3.14               # where __get__ returns MagicNumber instances
time = p()  # 4.26
p.speed = 2.28
time = p()  # 5.88

I guess I'm more in favor of this type of a solution, although I see the benefit in the sympy module. Preference, I guess.

Answer 2

Something like this?

import operator

MAKE_BINARY  = lambda opfn : lambda self,other : BinaryOp(self, asMagicNumber(other), opfn)
MAKE_RBINARY = lambda opfn : lambda self,other : BinaryOp(asMagicNumber(other), self, opfn)

class MagicNumber(object):
    __add__  = MAKE_BINARY(operator.add)
    __sub__  = MAKE_BINARY(operator.sub)
    __mul__  = MAKE_BINARY(operator.mul)
    __radd__ = MAKE_RBINARY(operator.add)
    __rsub__ = MAKE_RBINARY(operator.sub)
    __rmul__ = MAKE_RBINARY(operator.mul)
    # __div__  = MAKE_BINARY(operator.div)
    # __rdiv__ = MAKE_RBINARY(operator.div)
    __truediv__ = MAKE_BINARY(operator.truediv)
    __rtruediv__ = MAKE_RBINARY(operator.truediv)
    __floordiv__ = MAKE_BINARY(operator.floordiv)
    __rfloordiv__ = MAKE_RBINARY(operator.floordiv)

    def __neg__(self, other):
        return UnaryOp(self, lambda x : -x)

    @property
    def value(self):
        return self.eval()

class Constant(MagicNumber):
    def __init__(self, value):
        self.value_ = value

    def eval(self):
        return self.value_

class Parameter(Constant):
    def __init__(self):
        super(Parameter, self).__init__(0.0)

    def setValue(self, v):
        self.value_ = v

    value = property(fset=setValue, fget=lambda self: self.value_)

class BinaryOp(MagicNumber):
    def __init__(self, op1, op2, operation):
        self.op1 = op1
        self.op2 = op2
        self.opn = operation

    def eval(self):
        return self.opn(self.op1.eval(), self.op2.eval())


class UnaryOp(MagicNumber):
    def __init__(self, op1, operation):
        self.op1 = op1
        self.operation = operation

    def eval(self):
        return self.opn(self.op1.eval())

asMagicNumber = lambda x : x if isinstance(x, MagicNumber) else Constant(x)

And here it is in action:

x = Parameter()
# integer division
y = 2*x*x + 3*x - x//2

# or floating division
# y = 2*x*x + 3*x - x/2

x.value = 10
print(y.value)
# prints 225

x.value = 20
print(y.value)
# prints 850

# compute a series of x-y values for the function
print([(x.value, y.value) for x.value in range(5)])
# prints [(0, 0), (1, 5), (2, 13), (3, 26), (4, 42)]

Answer 3

>>> magic = lambda x: eval('2 * (x + 2) ** 2 - 8')
>>> magic(2)
24
>>> magic(3)
42
>>> magic = lambda x: eval('x ** 4')
>>> magic(2)
16
>>> magic(3)
81

Answer 4

I think the difficulty is in "how to keep operators priority" rather than implementing a class.

I suggest to look at a different notation (like Reverse Polish Notation) that may help in getting rid of priority issues...

Answer 5

You could give sympy, a computer algebra system written in Python, give a try.

E.g.

>>> from sympy import Symbol
>>> x = Symbol('x')
>>> y = 2 * (x + 2) ** 2 - 8
>>> y
2*(x + 2)**2 - 8
>>> y.subs(x,3)
42
>>> y.subs(x,2)
24