tkinter - global variable not working

[albry]

Hello!

I am using Tkinter and doing GUI so that there is a 'save', 'save as', and 'open' button, along with a text box.
I have got everything working except the 'save' button. I Have tried to set the name from filedialog.askopenfilename(), to a global variable to use in my 'save' function.

I have written my save function so that if the file name already exists, save over that, if not do the save_as function. It works fine when I need to save to an existing file, but when I enter straight into the text box and hit 'save' (NOT save as), it does not run the save_as function, but instead tells me that 'filename' is not defined ?

Is it saying that there is no filename because I didn't open anything ?
Is there a different want to do this that I haven't thought about ?

I cant see what I am doing wrong ? Please help!

def open_file():
    global filename
    filename = filedialog.askopenfilename()
    ask_file= open(filename)
    text.insert(INSERT, ask_file.read())
    return filename

def save_as_file():
    save_file = filedialog.asksaveasfile( mode = "w", defaultextension = '.txt')
    save_f = text.get(0.0, END)
    try:
        save_file.write(save_f.rstrip())
    except:
        showerror(title = "Save Error")

def save_file():
    if os.path.exists(filename):
        save_text = text.get(0.0, END)
        file_save = open(filename, 'w')
        file_save.write(save_text)
        file_save.close()
    else:
        save_as_file()