i Have $10$ Homework files$(.$ipynb$).$ so i need to write a python program when user enters a number between $1$ and $10$ the homework file assosiated to that number should run and excutes the output of that file

the below is the program i have written:

import os

homework$\_$files $=\{$

$1$: "homework$1.$ipynb",

$2$: "homework$2.$ipynb",

$3$: "homework$3.$ipynb",

$4$: "homework$4.$ipynb",

$5$: "homework$5.$ipynb",

$6$: "homework$6.$ipynb",

$7$: "homework$7.$ipynb",

$8$: $["$homework$8-1.$ipynb", "homework$8-2.$ipynb"$],$

$9$: $["$homework$9-1.$ipynb", "homework$9-2.$ipynb"$],$

$10$: "homework$10.$ipynb",

$\}$

def run$\_$homework$($number$)$:

filename $=$ homework$\_$files.get$($number$)$

if filename:

if os$.$path.exists$($filename$)$:

exec$($open$($filename$).$read$())$

else:

print$("$Homework file does not exist."$)$

else:

print$("$Homework does not exist."$)$

def main$()$:

try:

number $=$ int$($input$("$Enter a number between $1$ and $10$: $"))$

if $1<=$ number $<=10$:

run$\_$homework$($number$)$

else:

print$("$Number must be between $1$ and $10.")$

except ValueError:

print$("$Invalid input. Please enter a number."$)$

if $\_\_$name$\_\_=="\_\_$main$\_\_"$:

main$()$

but i am facing with the issue below:

Enter a number between $1$ and $10$: $1$

$---------------------------------------------------------------------------$

NameError Traceback $($most recent call last$)$

Cell In$[17],$ line $37$

$34$ print$("$Invalid input. Please enter a number."$)$

$36$ if $\_\_$name$\_\_=="\_\_$main$\_\_"$:

$--->37$ main$()$

Cell In$[17],$ line $30,$ in main$()$

$28$ number $=$ int$($input$("$Enter a number between $1$ and $10$: $"))$

$29$ if $1<=$ number $<=10$:

$--->30$ run$\_$homework$($number$)$

$31$ else:

$32$ print$("$Number must be between $1$ and $10.")$

Cell In$[17],$ line $20,$ in run$\_$homework$($number$)$

$18$ if filename:

$19$ if os$.$path.exists$($filename$)$:

$--->20$ exec$($open$($filename$).$read$())$

$21$ else:

$22$ print$("$Homework file does not exist."$)$

File :$36$

NameError: name 'null' is not defined