"list indices must be integers, not str"

[tooredrabbit]

I'm writing a calculator that takes simple input. This is the code so far.

def getlist(rawstring):
  stringclip = rawstring.replace(" ", "")
  oplist = {'+':' + ','-':' - ','*':' * ','/':' / ','(':' ( ',')':' ) ','^':' ^ '}
  ''' This replacement implementation came from http://gomputor.wordpress.com/ '''
  for i, j in oplist.iteritems():
    stringclip = stringclip.replace(i, j)
  return stringclip.split()
 
def domath(blist, op):
  for i in blist:
    if blist[i] == op:
      x = blist.pop(i - 1)
      y = blist.pop(i + 1)
      x = float(x)
      y = float(y)
      if op == '^':
        del blist[i]
        blist.inser(i, x**y)
      elif op == '*':
        del blist[i]
        blist.insert(i, x*y)
      elif op == '/':
        del blist[i]
        blist.insert(i, x/y)
      elif op == '+':
        del blist[i]
        blist.insert(i, x+y)
      else:
        del blist[i]
        blist.insert(i, x-y)
  return blist
 
fullstring = raw_input("What would you like to calculate?: ")
 
operlist = ['^','*','/','+','-']
 
for i in operlist:
  domath(getlist(fullstring), operlist[i])
 

When I run this code I get the error:

What would you like to calculate?: 8 / 4 + 2
Traceback (most recent call last):
  File "/Users/Tramp/Desktop/calc2.py", line 46, in <module>
    domath(getlist(fullstring), operlist)
  File "/Users/Tramp/Desktop/calc2.py", line 17, in domath
    if blist[i] == op:
TypeError: list indices must be integers, not str
 

So two questions

1) What am I doing wrong here? I thought that what I was doing was passing i, an integer, into a list which would get me the value stored at that place -- which according to my code should be a string which I should be able to then compare with another string. What am I misunderstanding?

2) Once this is solved, is there an easy way to catch errors and, instead of crashing the script, print something like "Input error"? I'm thinking of a situation in which someone feeds my script the input "4 + 5 - 6#$#@% - 1". The nonsense in the middle will cause my script to store #$#@% as a value in the list, but when it comes time to do a mathematical operation on it I will receive an error. The same thing would happen if someone wrote "5 ++ 6" into my code. Any help on this front is greatly appreciated. I'm being a bit preemptive.

I realize that I'm a newb, and I don;t want to waste anybody's time. If anyone has links that answer or partially answer my question, feel free to just drop them here. I don't want anyone to have to write out a long explanation. I'm sure what I'm doing is very elementary.

Thanks for the help.

[dwblas]

A simple print statement should help.

def domath(blist, op):
    for el in blist:
        print el, type(el) 

You should not use "i", "l", or "o" as variable names as they can look like numbers.

[tooredrabbit]

EDIT because I figured out a bit more.

So I was misunderstanding what the "for i in blaba" was actually returning. I get that now, it's already returning the value inside of the list. I can change my code to partially accomodate for that, but what I really wanted to do -- the spirit of what I was trying to do -- was to run the function passing in a different value each time by changing the index of the list by one until the list is done (I hope that makes sense).

I can rewrite my code to something like this, but it won't work. This is because I want to actually work with the position that a value is at in the list, and not just the value itself. I will now get an error when I try to use "i" as the integer position within the list (because it's not, it's the value at a given position IN the list). Is there a way to use my original code but write the "for" loop in such a way that what us being passed in is just the integer being changed by 1 until the max value in the list is reached?

def getlist(rawstring):
  stringclip = rawstring.replace(" ", "")
  oplist = {'+':' + ','-':' - ','*':' * ','/':' / ','(':' ( ',')':' ) ','^':' ^ '}
  ''' This replacement idea from http://gomputor.wordpress.com/ '''
  for i, j in oplist.iteritems():
    stringclip = stringclip.replace(i, j)
  return stringclip.split()
 
def doparemath(alist):
  for i in alist:
    if alist[i] == '(':
      print "temporary"
# will need to check at the end to see if it needs to run again (if there are other parentheses)
 
def domath(blist, op):
  for i in blist:
    if i == op:
      x = blist.pop(i - 1)
      y = blist.pop(i + 1)
      x = float(x)
      y = float(y)
      if op == '^':
        del blist[i]
        blist.inser(i, x**y)
      elif op == '*':
        del blist[i]
        blist.insert(i, x*y)
      elif op == '/':
        del blist[i]
        blist.insert(i, x/y)
      elif op == '+':
        del blist[i]
        blist.insert(i, x+y)
      else:
        del blist[i]
        blist.insert(i, x-y)
  return blist
 
fullstring = raw_input("What would you like to calculate?: ")
 
operlist = ['^','*','/','+','-']
 
for i in operlist:
  domath(getlist(fullstring), i)
 

[dwblas]

You can use enumerate:

def domath(blist, op):
    for ctr, el in enumerate(blist):
        print ctr, el, type(ctr), type(el) 
#
#  or range
def domath(blist, op):
    for ctr in range(len(blist)):
        print ctr, blist[ctr], type(ctr)