Unsupported operand types RE: function code challenge

# add_list([1, 2, 3]) should return 6
# summarize([1, 2, 3]) should return "The sum of [1, 2, 3] is 6."
# Note: both functions will only take *one* argument each.

item_list = [1,2,3]
new_list = []
global sentence

def add_list(item):
  global tot
  i = item  
  sentence2 = str()

  if i == 1: #if block totals list
    tot = 0 
  else:
    pass
  tot = tot + i 
  sentence2 = summarize(i) #passes string from summarize function

  if i == 3: #if block stops function prints sentence string and returns total
    print (sentence2)
    num = tot
    return num

def summarize(x):
  total = 0
  sentence = str()
  new_list.append(x)

  if x == 3: #stops the function, sums list values, creates sentence string
    for z in range(len(new_list)):
      y = new_list[z]
      total = total + y
    sentence = "The sum of "+str(new_list)+" is "+str(total)+"."
    return sentence

for item in item_list: #loop passes one value at a time for list
  if item == 3:
    t = add_list(item)
    print (t) #prints total returned from function
  else:
    add_list(item)