class ExpressionEvaluator(object):
    def __init__(self, expressions):
        self.values = {}
        for expression in expressions:
            node, right_side = expression.split(" = ")
            self.values[node] = right_side


    def evaluate(self, node, path=set()):
        expr_val = self.values[node]

        if self._expr_val_is_terminal(expr_val):
            return expr_val

        # we're seeing a node again and we weren't
        # able to evaluate it, so we have a cycle.
        if node in path:
            self.values[node] = "IMPOSSIBLE"
            return self.values[node]

        # visit the current node
        path.add(node)

        if self._expr_val_has_operator(expr_val):

            left_node, op, right_node = expr_val.split(" ")

            # try to evaluate left side
            left_val = self.evaluate(left_node, path)
            if left_val == "IMPOSSIBLE":
                self.values[node] = "IMPOSSIBLE"
                return self.values[node]

            # try to evaluate right side
            right_val = self.evaluate(right_node, path)
            if right_val == "IMPOSSIBLE":
                self.values[node] = "IMPOSSIBLE"
                return self.values[node]

            # update the value for the current node
            self.values[node] = self._calculate_value(op, left_val, right_val)
            return self.values[node]

        self.values[node] = self.evaluate(expr_val, path)
        return self.values[node]

    def _calculate_value(self, op, a, b):
        operations =  {"+": lambda a, b: a + b,
                       "-": lambda a, b: a - b}
        return str(operations[op](int(a), int(b)))

    def _expr_val_has_operator(self, expr_val):
        return "+" in expr_val or "-" in expr_val

    def _expr_val_is_terminal(self, expr_val):
        return (expr_val.isnumeric() or expr_val[0] == "-")


expressions = [
    "T1 = 1",
    "T2 = 4",
    "T3 = T1 + T4",
    "T4 = T1 - T2",
    "T5 = T6 + T1",
    "T6 = T5",
    "T7 = T4 + T3"
]
exp_evaluator = ExpressionEvaluator(expressions)
print(exp_evaluator.evaluate("T5"))