A prefix notation calculator that asks about everything step by step

I made 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.

The calculator asks for one value per step, and asks about everything step by step.

```
"""
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 - p

Use a shebang at the beginning

This is something like #!/usr/bin/env python2, and helps interpreters know how to run it.

Remove some of the unnecessary comments

Comments are good. Excessive comments are just hard to read.

e.g.

elif cmd == "+":
    x = (lorr_input("Left side addend? ", x) +
         lorr_input("Right side addend? ", x))
    print x # Implicitly output result

Turn that into:

elif cmd == "+":
    x = (lorr_input("Left side addend? ", x) +
         lorr_input("Right side addend? ", x))
    print x

And:

# Get user input
cmd = raw_input(">> ")
# Process input
if cmd == "quit":
    ....

Into:

cmd = raw_input(">> ")
# Process user input
if cmd == "quit":
    ....

Empty lines for readability

Using the previous example:

cmd = raw_input(">> ")
# Process user input
if cmd == "quit":
    ....

Into:

cmd = raw_input(">> ")

# Process user input
if cmd == "quit":
    ....

if __name__ == "__main__" check

This just checks if you are running this directly, or from a module import/something else.

Maybe put the while loop into a function called start() and use:

if __name__ == "__main__":
    start()

Don't assign unnecessary variables

The helpmsg var is pretty much useless, just print the plain string.

The lit_input function is not used

Either use it, or remove it. Maybe make a variation of the docstring for the lorr_input?

Other notes:

-
Nice job with the ast safe eval, but a method that doesn't involve eval would also be nice. This is a lot of work, probably unnecessary.

-
I would set x to 0 at the beginning, but that's personal preference.

-
I'm not sure, but you appear to have limited to 87 chars per line (or under). That is a PEP guideline, good job.

StackExchange Code Review Q#132394, answer score: 8