Pfxcalc, a prefix notation calculator.

pfxcalc.py

"""
Pfxcalc
Programmed by zyabin101
This code is hereby released into the public domain
"""

import ast

def lit_input(prompt):
    """
    Safely evaluate input. Only returns the following Python literal
    structures: strings, numbers, tuples, lists, dicts, booleans, and 'None'.

    Raises ValueError for malformed strings.
    """
    return ast.literal_eval(raw_input(prompt))

def lorr_input(prompt, result):
    inputted = raw_input(prompt)
    return result if inputted == "result" else ast.literal_eval(inputted)

# Initialization
x = None

# Print welcome message
print "Welcome to Pfxcalc. Type 'help' for more information."

# Main program loop
while True:
    # Get user input
    cmd = raw_input(">> ")
    # Process input
    if cmd == "quit":
        break
    elif cmd == "result":
        print x
    elif cmd == "help":
        helpmsg = """
        Pfxcalc is a prefix notation calculator.
        Prefix notation, aka Polish notation, is a form of notation for logic,
        arithmetic and algebra, like infix and postfix (reverse Polish)
        notation.
        
        The expression '+ 3 4' in prefix notation is as valid as '3 + 4' in
        infix notation, as well as '3 4 +' in postfix notation.
        
        Every computation starts with entering a command and the result is
        printed *and* saved in a variable that can be recalled by entering
        'result'. The variable can even be used as an argument for functions,
        using the same command.

        List of commands:
        quit - self explanatory
        result - recall the result variable
        help - print this message
        + - add two numbers or sequences
        - - subtract two numbers
        * - multiply two numbers, or a sequence by a number
        / - divide two numbers
        """
        print helpmsg
    elif cmd == "+":
        x = (lorr_input("Left side addend? ", x) +
             lorr_input("Right side addend? ", x))
        print x # Implicitly output result
    elif cmd == "-":
        x = lorr_input("Minuend? ", x) - lorr_input("Subtrahend? ", x)
        print x # Implicitly output result
    elif cmd == "*":
        x = (lorr_input("Left side factor? ", x) *
             lorr_input("Right side factor? ", x))
        print x # Implicitly output result
    elif cmd == "/":
        x = lorr_input("Dividend? ", x) / lorr_input("Divisor? ", x)
        print x # Implicitly output result
    else:
        print "Unrecognized command."