Why I can't use += to change a local variable of a function in the sub-function in it?

Question

Enviroment:

Python 3.10.3, VSCode, Windows 11

Describing:

I write a program with tkinter by Python.

def examination():
    exam = Frame()
    exam.pack()
    quesinfo = []
    quesnumb = 0

    def next():
        is_right.forget()
        nextques.forget()
        okayques.grid(column=5, row=2, columnspan=2)
        quesnumb += 1
        quesinfo.append(core())
        question.set(quesinfo[quesnumb][0])

This is just a part of my code, but the problem is here. When I run the program, the interpreter says:

Exception in Tkinter callback
Traceback (most recent call last):
  File "C:\Users\${MyName}\AppData\Local\Programs\Python\Python310\lib\tkinter\__init__.py", line 1921, in __call__
    return self.func(*args)
  File "d:\Python\Usable Programs\Auto Maths Questions UI\Main.py", line 102, in next
    quesnumb = quesnumb + 1
UnboundLocalError: local variable 'quesnumb' referenced before assignment

You know, quesnumb is a local variable, so I can't use global to declare it. But, that's joking! Look at to the source code again:

quesinfo.append(core())
question.set(quesinfo[quesnumb][0])

It's correct, But why quesnumb += 1 is not?

Fixing and Debugging

I tried my best to fix it.
For testing and dubugging, I replaced quesnumb += 1 to quesnumb = 1. It ran successfully!
I think, maybe quesnumb can just reference or assignment? So I replaced it to quesnumb = quesnumb + 1.

Exception in Tkinter callback
Traceback (most recent call last):
  File "C:\Users\${MyName}\AppData\Local\Programs\Python\Python310\lib\tkinter\__init__.py", line 1921, in __call__
    return self.func(*args)
  File "d:\Python\Usable Programs\Auto Maths Questions UI\Main.py", line 102, in next
    quesnumb = quesnumb + 1
UnboundLocalError: local variable 'quesnumb' referenced before assignment

Uh-huh... That's a real Unexpect Wrong...

I must get the answer in time, because I'll use this program on a competition at Sunday (Mar. 27). Winning the competition is all depends on this program. Could you help me to check the problem and find the root cause? Thanks so much!

It looks like the first code snippet is wrongly intended. You declare a function inside a function. — Damiaan, Mar 25 '22 at 12:06
variable in a function are read-only to inner functions. reading, re-defining (albeit as one local to the inner function) are okay. writing is a no — Tidemor, Mar 25 '22 at 12:08
@Damiaan that's intended (no pun intended), to be defined as a sub-function — Tidemor, Mar 25 '22 at 12:08
Variable `quesnumb` is not `global`. It is `nonlocal` so use that keyword — re-za, Mar 25 '22 at 12:11

Why I can't use += to change a local variable of a function in the sub-function in it?

Enviroment:

Describing:

Fixing and Debugging

0 Answers0