from tkinter import *
root = Tk()
root.geometry("400x300")
root.resizable(0, 0)
root.config(bg="#0f5c7a")
root.title("Sistema de Classificação de Notas")

def limpar():
    Nota_entrada.delete(0, END)
    resultado.set("")
def app():
    n = Nota.get()
    if n >= 90:
        mensagem = "A"
    elif n >= 80:
        mensagem = "B"
    elif n >= 70:
        mensagem = "C"
    elif n >= 65:
        mensagem = "D"
    else:
        mensagem = "F"
    resultado.set(mensagem)


titulo = Label(text="Sistema de Classificação de Notas",
               font=("Arial", "16", "bold"),bg="#0f5c7a",fg="#30e0e3")
titulo.place(relx=0.06, rely=0.05)
texto_sub1  = Label(text="Nota:",
                    font=("Arial", "15", "bold"),bg="#0f5c7a",fg="#30e0e3")
texto_sub1.place(relx=0.3, rely=0.3)

Nota = IntVar()
Nota_entrada = Entry(textvariable=Nota,
                             font=("Arial", "12", "bold"),
                             bg="white", fg="blue", justify='center')
Nota_entrada.place(relx=0.55, rely=0.3, relwidth=0.25)

but1 = Button(text="Verificar", bd=2, bg='#107db2', fg='white',
              font=('verdana', 12, 'bold'), command=app)
but1.place(relx=0.05, rely=0.5, relwidth=0.25, relheight=0.1)
but_limpar = Button(text="Limpar", bd=2, bg='#107db2', fg='white',
                    font=('verdana', 12, 'bold'), command=limpar)
but_limpar.place(relx=0.35, rely=0.5, relwidth=0.25, relheight=0.1)

but_sair = Button(text="Sair", bd=2, bg='#107db2', fg='white',
                    font=('verdana', 12, 'bold'), command=root.destroy)
but_sair.place(relx=0.65, rely=0.5, relwidth=0.25, relheight=0.1)

resultado = StringVar()
resultado_texto = Label(textvariable=resultado,
                           font=("Arial", 12, "bold"), bg="#cfe2f3")
resultado_texto.place(relx=0.05, rely=0.7, relwidth=0.9,relheight=0.2)
root.mainloop()