class Term:
    def __init__(self, variable=None, const=None):
        self.variable = variable
        self.const = const
        self.tree = []

    @classmethod
    def const(cls, const):
        term = cls(None, const)
        return term

    @classmethod
    def var(cls, variable):
        term = cls(variable, None)
        return term

    def __call__(self, **kwargs):
        return Term.calculate(Term.exchange(self.tree, **kwargs))

    def __add__(self, other):
        term = Term()
        term.tree.append("+")
        term.tree.append(self.variable)
        if len(other.tree) is not 0:
            term.tree.append(other.tree)
        else:
            term.tree.append(other.variable)
        return term

    def __sub__(self, other):
        term = Term()
        term.tree.append("-")
        term.tree.append(self.variable)
        if len(other.tree) is not 0:
            term.tree.append(other.tree)
        else:
            term.tree.append(other.variable)
        return term

    def __mul__(self, other):
        term = Term()
        term.tree.append("*")
        term.tree.append(self.variable)
        if len(other.tree) is not 0:
            term.tree.append(other.tree)
        else:
            term.tree.append(other.variable)
        return term

    def __str__(self):
        return Term.to_string(self.tree)

    @staticmethod
    def exchange(tree, **kwargs):
        if isinstance(tree[1], str) or isinstance(tree[2], str):
            if isinstance(tree[1], str):
                for key, value in kwargs.items():
                    if tree[1] == str(key):
                        tree[1] = value
            if isinstance(tree[2], str):
                for key, value in kwargs.items():
                    if tree[2] == str(key):
                        tree[2] = value
            return (tree[0], tree[1], tree[2])
        elif isinstance(tree[2], tuple):
            return Term.exchange((tree[0], tree[1], Term.exchange(tree[2])))

    @staticmethod
    def calculate(tree):
        if isinstance(tree[2], int):
            if tree[0] == "+":
                return tree[1] + tree[2]
            elif tree[0] == "-":
                return tree[1] - tree[2]
            elif tree[0] == "*":
                return tree[1] * tree[2]
        else:
            return Term.calculate((tree[0], tree[1], Term.calculate(tree[2])))

    @staticmethod
    def to_string(tree):
        if not isinstance(tree[2], list):
            return "({0} {1} {2})".format(tree[1], tree[0], tree[2])
        else:
            return Term.to_string((tree[0], tree[1], Term.to_string(tree[2])))


if __name__ == "__main__":
    x = Term.var("x")
    y = Term.var("y")
    z = Term.var("z")
    d = x + y
    print(d(x=1, y=2))