Importing a whole package works in IDLE, but not in shell. The following works fine in IDLE:
import tkinter as tk
tk.filedialog.askopenfilename()
        This is an IDLE bug which I fixed for future 3.5.3 and 3.6.0a4 releases. Tracker issue.
For an existing 3.5 or 3.4 release, add the following to idlelib/run.py just before the LOCALHOST line.
for mod in ('simpledialog', 'messagebox', 'font',
            'dialog', 'filedialog', 'commondialog',
            'colorchooser'):
    delattr(tkinter, mod)
    del sys.modules['tkinter.' + mod]
I presume that this will work with earlier 3.x releases, but do not have them installed to test. For existing 3.6.0a_ releases, replace 'colorchooser' with 'ttk'.