Arithmetic Arranger TypeError

I have to build a function that returns an arithmetic arrangement.
Input: arithmetic_arranger(["32 + 698", "3801 - 2", "45 + 43", "123 + 49"])
Output:

   32      3801      45      123
+ 698    -    2    + 43    +  49
-----    ------    ----    -----

If the other argument is True, arithmetic_arranger(["32 + 698", "3801 - 2", "45 + 43", "123 + 49"], True):
Output:

  32         1      9999      523
+  8    - 3801    + 9999    -  49
----    ------    ------    -----
40     -3800     19998      474

I made this one:

def arithmetic_arranger(problems, show = True):
#["1 + 3", "1 - a", "1 * 4"]
arranged_problems = []
#too many problems supplied
if len(problems)>5:
return "Error: Too many problems."
for i, ele in enumerate(problems):
sub = ele.split()
num1 = sub[0]
oper = sub[1]
num2 = sub[2]
#appropriate operators "+","-"
if sub[1] not  in "-+":
return "Error: Operator must be '+' or '-'."
#only contain digits
if sub[0].isdigit() and sub[2].isdigit() is False:
return "Error: Numbers must only contain digits."
#max 4 digits
if len(sub[0])>4 or len(sub[2])>4:
return "Error: Numbers cannot be more than four digits."
ray = "-"
maxlen_line = max(len(num1), len(num2))
ach = maxlen_line + 2
line_num1 = f"{num1:>{ach}}"
line_num2 = oper + f"{num2:>{ach-1}}"
line_rays = ray*ach
try:
arranged_problems[0] += (" "*4) + line_num1
except IndexError:
arranged_problems.append(line_num1)
try:
arranged_problems[1] += (" "*4) + line_rays
except IndexError:
arranged_problems.append(line_rays)
try:
arranged_problems[2] += (" "*4) + line_num2
except IndexError:
arranged_problems.append(line_num2)
answ = 0 
if oper == "+":
answ = int(num1) + int(num2)
else:
answ = int(num1) - int(num2)
line_answ = f"{str(answ):>{ach}}"
try:
arranged_problems[3] += (" "*4) + line_answ
except IndexError:
arranged_problems.append(line_answ)
if show == True:
arranged_problems = f"{arranged_problems[0]}\n{arranged_problems[1]}\n{arranged_problems[2]}\n{arranged_problems[3]}"
else:
arranged_problems = f"{arranged_problems[0]}\n{arranged_problems[1]}\n{arranged_problems[2]}"
return arranged_problems

And this my result:

Traceback (most recent call last):
File "/home/runner/boilerplate-arithmetic-formatter/main.py", line 7, in <module>
print(arithmetic_arranger(["32 + 698", "3801 - 2", "45 + 43", "123 + 49"]))
File "/home/runner/boilerplate-arithmetic-formatter/arithmetic_arranger.py", line 32, in arithmetic_arranger
arranged_problems[0] += (" "*4) + line_num1
TypeError: 'str' object does not support item assignment

I do not understand why because in arranged_problems[0] each argument is a str.

Thanks!

When you get to line 54-57:

    if show == True:
arranged_problems = f"{arranged_problems[0]}\n{arranged_problems[1]}\n{arranged_problems[2]}\n{arranged_problems[3]}"
else:
arranged_problems = f"{arranged_problems[0]}\n{arranged_problems[1]}\n{arranged_problems[2]}"

You are assigning arranged_problems as a string. It is no longer a list after this. During the next loop iteration, you then get to line 32:

arranged_problems[0] += (" "*4) + line_num1

which tries to reassign a value in the string's character array. As strings are immutable, this throws that error. The (" "*4) + line_num1 section can evaluate to a string fine, but once arranged_problems is a string you can't assign into it.

To fix this, you need to change the assignment at lines 54-57 so that you aren't overwriting arranged_problems with a string.