Variable ComboBox Named Values

[MC2020]

Hello All

I feel like I'm missing something dumb here. I have code below to pick from a drop down list (combobox). There is some naming problem with my code. My expectation would be the combobox options would be "Trump", "Obama", and "Bush". Instead I get "dict_keys(['Trump'," , "'Obama'," , and "'Bush'])"

Does anyone know adjustment I can make to get the values I am looking for?

presidents = {'Trump' : {2020,2019,2018,2017} , 'Obama' : {2016,2015,2014,2013,2012,2011,2010,2009} , 'Bush' : {2008,2007,2006,2005,2004,2003,2002,2001}}

import tkinter as tk
from tkinter import ttk

app = tk.Tk() 
app.title('President')
app.geometry('500x300')

def function():
    print(combobox.get())

label = tk.Label(app, text = "Who is your favorite president?")
label.grid(column=0, row=0)

combovalues = presidents.keys()
combobox = ttk.Combobox(app, values=combovalues)
combobox.grid(column=5, row=0)

resultButton = tk.Button(app, text = 'Confirm', command=function)
resultButton.place(x=15,y=45)