Always lowercase entry in tkinter

[ReturnName]

Hi, I'd like to have my entry always in lowercase and with only letters allowed. This is the code I've come up with:

def lowercase_letter_entry(P):
   if P.isalpha() or P == "":
     if P.islower():
        return True
     else:
      if not P.islower():
       P.get()lower()
       return True
   else:
    messagebox.showwarning("Warning","Only letters are allowed")
    return False

Right now the "check if entry are letters or not" is working but self.world.get() isn't in lowercase. this is how I'm using it

self.world = StringVar(self)
        self.world.set('')
        self.world=Entry(self,textvariable = self.world, width=280, bg='WHITE')  
        self.world.place(relx=0.19, rely=0.24, height=25, width=160) e
        callback_world = self.register(lowercase_letter_entry) 
        self.world.configure(validate="key", validatecommand=(callback_world, "%P"))
        print(self.world.get())

Can somebody help me? Thank you