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More powerful version of the RPN code in development, treat as unstable expect to use most of your available program memory. Additions over the simple RPN version are; str and rcl for storing and recalling a memory value; auto for standard format numbers; fix for setting fixed decimal notation (push a number to the stack first for the number of places, i.e. 2 fix for rounding to two decimal places); eng for engineering notation (where the exponent is in powers of a thousand); si for displaying International System units (G, M, k, m, u, n, etc).

from math import *
from random import random

ops = {'+': [2,lambda x,y: x+y],
 '-': [2,lambda x,y: x-y],
 '*': [2,lambda x,y: x*y],
 '/': [2,lambda x,y: x/y],
 'pow': [2,lambda x,y: pow(x,y)],
 'sqr': [1,lambda x: x*x],
 'recip': [1,lambda x : 1/x],
 '!': [1,lambda x: fact(int(round(x)))],
 'deg': [1,lambda x: degrees(x)],
 'rad': [1,lambda x: radians(x)],
 'log': [1,lambda x: log(x)],
 'log10': [1,lambda x: log10(x)],
 'sqrt': [1,lambda x: sqrt(x)],
 'sin': [1,lambda x: sin(x)],
 'cos': [1,lambda x: cos(x)],
 'tan': [1,lambda x: tan(x)],
 'asin': [1,lambda x: asin(x)],
 'acos': [1,lambda x: acos(x)],
 'atan': [1,lambda x: atan(x)],
 'pi': [0,pi],
 'e': [0,e],
 'rnd': [0,random()]}

def evaluate(tokens, s, mem):
 for t in tokens:
  if t == 'swap':
   if len(s) >= 2:
    a = s.pop(); b = s.pop()
    s.append(a); s.append(b)
  elif t == 'roll':
   if len(s) >= 2: s.insert(0, s.pop())
  elif t == 'drop':
   if len(s) >= 1: s.pop()
  elif t == 'clr':
   s = []
  elif t == 'sto':
   if len(s) >= 1: mem = s[-1]
  elif t == 'rcl':
   s.append(mem)
  elif t in ops:
   op = ops[t][1]
   q = ops[t][0]
   if q == 0:
    s.append(op)
   elif q == 1 and len(s) >= 1:
    a = s.pop()
    try: s.append(op(a))
    except: print("Err"); s.append(a)
   elif q == 2 and len(s) >= 2:
    a = s.pop(); b = s.pop()
    try: s.append(op(b,a))
    except: print("Err"); s.append(b); s.append(a)
  elif set(t).issubset(set("0123456789.-")):
   s.append(float(t))
  else:
   print("Err %s" % t)
 return s, mem

def p(s):
 print(s, end='')

def fact(n):
 f = 1
 while n > 0:
  f *= n; n -= 1
 return f

def eng_not(n, fix):
 for exp in range(-24, 25, 3):
  if abs(n) < pow(10, exp): break
 return '{}E{}'.format(n/float(pow(10, exp-3)), exp-3)

def si_not(n, fix):
 i = 0
 for exp in range(-24, 25, 3):
  if abs(n) < pow(10, exp): break
  i += 1
 u = 'yzafpnum kMGTPEZY'[i-1]
 if u == ' ': u = ''
 return '{}{}'.format(n/float(pow(10, exp-3)), u)

s = []
mem = 0.0
fix = 3
mode = 0
while True:
 expr = input('> ')
 if expr == 'q':
  break
 elif expr == 'fix':
  if len(s) >= 1: fix = abs(int(round(s.pop()))); auto = False
 elif expr == 'auto':
  mode=0
 elif expr == 'eng':
  mode=1
 elif expr == 'si':
  mode=2
 elif expr == '?':
  print('Help:', end=' ')
  w = 0
  for o in sorted(list(ops.keys()) + ['fix','eng','si','auto','sto','rcl','?','q','clr','swap','roll','drop']):
   print(o, end=' '); w += 1
   if w > 5: print(); w = 0
  if w != 0: print()
  continue
 elif len(expr) == 0:
   continue
 else:
   s, mem = evaluate(expr.split(), s, mem)
 print('[', end='')
 for i in s:
  if mode==0: p(i)
  elif mode==1: p(eng_not(i, fix))
  elif mode==2: p(si_not(i, fix))
  else: p('{:.{p}f}'.format(i, p=fix))
  if i is not s[-1]: p(', ')
 else:
  print(']')