Example

try:
    from tkinter import Tk
    from tkinter.ttk import Style, Button, Label
except ImportError:
    from Tkinter import Tk
    from ttk import Style, Button, Label
from sys import platform
from tkfontchooser import askfont

# create main window
root = Tk()
style = Style(root)
if "win" == platform[:3]:
    style.theme_use('vista')
elif "darwin" in platform:
    style.theme_use('clam')
else:
    style.theme_use('clam')
bg = style.lookup("TLabel", "background")
root.configure(bg=bg)
label = Label(root, text='Chosen font: ')
label.pack(padx=10, pady=(10,4))

def callback():
    # open the font chooser and get the font selected by the user
    font = askfont(root)
    # font is "" if the user has cancelled
    if font:
        # spaces in the family name need to be escaped
        font['family'] = font['family'].replace(' ', '\ ')
        font_str = "%(family)s %(size)i %(weight)s %(slant)s" % font
        if font['underline']:
            font_str += ' underline'
        if font['overstrike']:
            font_str += ' overstrike'
        label.configure(font=font_str, text='Chosen font: ' + font_str.replace('\ ', ' '))

Button(root, text='Font Chooser', command=callback).pack(padx=10, pady=(4,10))
root.mainloop()