Default selected radio button isn't assigned a value resulting in NameError

I am trying to create a GUI with radiobuttons using tkinter from python. I am having trouble retrieving the value of the selected option, even though I have specified the variable (material_category) in question to be global inside the selection function.

When I press the Enter Data button to run my code without selecting a different option other than the one appearing selected by default (i.e. Common Steel), I am greeted with this error:

"NameError: name 'material_category' is not defined"

This error appears only if the user doesn't fiddle at all with the radio-button windows and presses the Enter button button straight away. In case the user gets to press any radio button the code works fine.

I am bamboozled though, as to why the starting radio button (i.e. "Common Steel") appears selected but is not assigned a value (in other words why x is not assigned the value of 0, even though that option appears selected). Is there a way I could assign the default value of 0 to x?

What can I do to actually make this code run without errors or handling an exception? Is there a better, more compact way to code this in order to omit the material_category_selection() function that checks the selection?

Python version: 3.11.1
IDE: VSCodium

import tkinter
from tkinter import *
from tkinter import ttk
from tkinter import messagebox

def enter_data():
    print(material_category)
    window.quit()

#GUI 

window = tkinter.Tk()
window.title("Stress Data Entry")
window.resizable(0,0)

frame = tkinter.Frame(window)
frame.pack()


#MATERIAL CATEGORY

material_category_frame =tkinter.LabelFrame(frame, text="Material Category")
material_category_frame.grid(row= 0, column=1, padx=20, pady=10)

material_category_option=["Common Steel","Soldering Steel", "Carburized Steel", "Upgraded Steel", "Nitrogenous Steel"]

x=IntVar()

def material_category_selection():
    global material_category
    if(x.get()==0):
        material_category="commonsteel"
    elif(x.get()==1):
        material_category="solderingsteel"
    elif(x.get()==2):
        material_category="carburizedsteel"
    elif(x.get()==3):
        material_category="upgradedsteel"
    elif(x.get()==4):
        material_category="nitrogenoussteel"
    else:
        tkinter.messagebox.showwarning(title="Error!", message="Something went wrong!")

for index in range(len(material_category_option)):
    radiobutton = Radiobutton(material_category_frame, 
    text=material_category_option[index], 
    variable=x, 
    value=index,
    padx=20,
    pady=10,
    command=material_category_selection)

for widget in material_category_frame.winfo_children():
    widget.grid_configure(padx=10, pady=5)

button = tkinter.Button(frame, text="Enter Data", command= enter_data)
button.grid(row=3, column=1, sticky="news", padx=20, pady=10)

window.mainloop()

The error is caused by that material_category_selection() has not been executed yet so material_category is not created.

I would suggest to put the logic of material_category_selection() into enter_data() as I don't see any necessity of material_category_selection():

# removed wildcard import as it is not recommended
import tkinter
from tkinter import messagebox

def enter_data():
    global material_category
    try:
        if(x.get()==0):
            material_category="commonsteel"
        elif(x.get()==1):
            material_category="solderingsteel"
        elif(x.get()==2):
            material_category="carburizedsteel"
        elif(x.get()==3):
            material_category="upgradedsteel"
        elif(x.get()==4):
            material_category="nitrogenoussteel"
        else:
            raise Exception("Something went wrong!")
        print(material_category)
        window.destroy()
    except Exception as ex:
        messagebox.showwarning(title="Error!", message=ex)

#GUI

window = tkinter.Tk()
window.title("Stress Data Entry")
window.resizable(0, 0)

frame = tkinter.Frame(window)
frame.pack()

material_category_frame =tkinter.LabelFrame(frame, text="Material Category")
material_category_frame.grid(row= 0, column=1, padx=20, pady=10)

material_category_option=["Common Steel", "Soldering Steel", "Carburized Steel", "Upgraded Steel", "Nitrogenous Steel"]

x = tkinter.IntVar() # value 0 will be set by default
for index, text in enumerate(material_category_option):
    radiobutton = tkinter.Radiobutton(material_category_frame,
                                      text=text,
                                      variable=x,
                                      value=index,
                                      padx=20,
                                      pady=10) # removed command option
    radiobutton.grid_configure(padx=10, pady=5, sticky="w")

button = tkinter.Button(frame, text="Enter Data", command= enter_data)
button.grid(row=3, column=1, sticky="news", padx=20, pady=10)

window.mainloop()

In your case, if the user does not click a Radiobutton but clicks 'Enter Data', it calls enter_data(), but you never defined material_category, giving a NameError. So, you can use error handling:

def enter_data():
try:
print(material_category)
window.quit()
except NameError:
tkinter.messagebox.showwarning(title="Error!", message="Please provide a material!")