Someone replied with and then unfortunately deleted an extremely good suggestion.

Instead of each operation taking a variable number of numbers and then either returning a value or directly changing the state of the stack (which was also a bug in my previous gist, in pushing the result of C and AC): homogenize the types of the operations by making all of them take a stack and return a stack.

You could also wrap all of the basic arithmetic functions with a higher-level function so that, eg, `add` would still not actually have to be aware of the stack:

    import math, operator

    def stackFunction(arity, function):
        return lambda stack: stack[:-arity] + [function(*stack[-arity:])]

    class rpn_engine:
        def __init__(self):
            self.stack = []
            self.catalog = {"+":    stackFunction(2, operator.add),
                            "-":    stackFunction(2, operator.sub),
                            "*":    stackFunction(2, operator.mul),
                            "/":    stackFunction(2, operator.truediv),
                            "^2":   stackFunction(1, lambda x: x * x),
                            "SQRT": stackFunction(1, math.sqrt),
                            "C":    lambda stack: stack[:-1],
                            "AC":   lambda stack: []}

        def compute(self, operation):
            self.stack = self.catalog[operation](self.stack)

        def push(self, number):
            self.stack.append(number)