# 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.

"""
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

# 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) +
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."


I'm interested in code style and problems the code has. What can be fixed here?

• Why not just print """....""" instead of print helpmsg? It's less assignment, and probably more efficient. – Conor O'Brien Jun 18 '16 at 19:15
• Most if not all single line comments seem superfluous. – Fatalize Jun 18 '16 at 19:16
• I assume this is python2.7, because no print parantheses? – Riker Jun 18 '16 at 19:16
• @EasterlyIrk Correct. – zyabin101 Jun 18 '16 at 19:26
• The lit_input() function is probably unnecessary, because it is defined but never used. – zyabin101 Jun 18 '16 at 19:28

### 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

e.g.

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


Turn that into:

elif cmd == "+":
x = (lorr_input("Left 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":
....


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.

• Good job! Have your 25 rep, which is 0.225 of my rep (c:). – zyabin101 Jun 18 '16 at 19:36
• @zyabin101 think it's 15, but welcome. You should wait a bit though, give other people a chance to answer. – Riker Jun 18 '16 at 19:37
• 1) It is 25 (10 for upvote, 15 for accept). 2) Got it, thanks for the tip. – zyabin101 Jun 18 '16 at 19:38
• Please do not use #!/usr/bin/env python as a shebang. Default Python versions varies across distributions and environments (e.g. Ubuntu/Debian defaults to Python 2.7, Arch Linux defaults to Python 3.x -- PEP394 only states that distros should point python to 2.x), so try to be as precise as possible. – ElementW Jun 19 '16 at 10:26
• @ElementW okay, will add to the post. – Riker Jun 19 '16 at 15:59