A surprisingly capable RPN calculator in about 100 lines of Python code

rpn.py

# Based on:
# A surprisingly capable RPN calculator in about 100 lines of Zig code
# https://cryptocode.github.io/blog/docs/prefix-calculator

# Example session:
#
# Python RPN Calculator
# Type mc to clear memory slots, exit to quit.
# #  see if 2 + 2 is still 4
# # mc
# 2 2 +
# M0 = 4.0
# # quadratic formula, we M0, M1 and M2 instead of a, b and c.
# mc
# 5
# M0 = 5.0
# 6
# M1 = 6.0
# 1
# M2 = 1.0
# # First solution
# M1 1 ~ * M1 2 ^ 4 M0 * M2 * - √ + 2 M0 * /
# M3 = -0.2
# # Second solution
# M1 1 ~ * M1 2 ^ 4 M0 * M2 * - √ - 2 M0 * /
# M4 = -1.0
# # Volume of a sphere
# mc
# 4.8
# M0 = 4.8
# 4 3 / 𝜋 * M0 3 ^ *
# M1 = 463.2466863277364
# # Average of numbers
# 1 2 3 4 5 6 7 8 9 10 avg
# M0 = 5.5
# # done!
# exit

import math

memslots = []

def main():
    print("Python RPN Calculator")
    print("Type mc to clear memory slots, exit to quit.")

    while True:
        line = input().strip()
        # # comment
        if len(line) == 0 or line[0] == "#":
            continue
        match line:
            # exit to terminate the calculator
            case "exit":
                break
            # mc to clear the memory slots
            case "mc":
                memslots.clear()
                continue

        try:
            memslots.append(parse_line(line))
            print(f"M{len(memslots)-1} = {memslots[-1]}")
        except Exception as e:
            print(e)


def parse_line(line: str) -> float:
    """Parses input line using a stack

    Args:
        line (str): RPN input line, e.g. "2 2 3 * +"

    Raises:
        Exception: If there was an error parsing line.

    Returns:
        float: top of stack after parsing.
    """
    stack = []

    for tok in line.split(" "):
        match tok.lower():
            # * / + - %	Basic arithmetic/modulus
            case "*":
                stack.append(stack.pop() * stack.pop())
            case "/":
                val = stack.pop()
                stack.append(stack.pop() / val)
            case "+":
                stack.append(stack.pop() + stack.pop())
            case "-":
                val = stack.pop()
                stack.append(stack.pop() - val)
            case "^":
                val = stack.pop()
                stack.append(pow(stack.pop(), val))
            case "%":
                val = stack.pop()
                stack.append(stack.pop() % val)
            # ~	Negation
            case "~":
                stack.append(-stack.pop())
            # float
            case c if "0" <= c <= "9" or c == ".":
                try:
                    stack.append(float(tok))
                except Exception:
                    raise Exception(f"Invalid character in floating point number {tok}")
            # M...	Memory reference, such as M0
            case c if c[0] == "m":
                index = int(tok[1:])
                try:
                    stack.append(memslots[index])
                except Exception:
                    raise Exception(f"Invalid memory memory slot index in {tok}")
            # sin, cos, tan	Trigonometric functions
            case "sin":
                stack.append(math.sin(stack.pop()))
            case "cos":
                stack.append(math.cos(stack.pop()))
            case "tan":
                stack.append(math.tan(stack.pop()))
            # sqrt or √	Square root
            case "sqrt" | "√":
                stack.append(math.sqrt(stack.pop()))
            # pi or 𝜋	Push 𝜋 to the stack
            case "pi" | "𝜋":
                stack.append(math.pi)
            # e	Push Euler’s number to the stack
            case "e":
                stack.append(math.e)
            # avg	Pop all numbers from the stack, push the average
            case "avg":
                avg = sum(stack) / len(stack)
                stack.clear()
                stack.append(avg)
            case _:
                raise Exception(f"Invalid operation {tok}")

        # print(f"{tok=} {stack=}")

    return stack[-1]


if __name__ == "__main__":
    main()