tkinter questions: behavior of StringVar, etc

[Alan G Isaac]

I'm a complete newbie to GUI.
I have a couple questions about tkinter.

1. Where is the list of changes
    in Python 3's tkinter?

2. What exactly is the role of the root object,
    traditionally created as ``root=tk.Tk()``?
    What is an example where one should create this
    before creating a Frame instance (which will
    otherwise implicitly create one as its master)?

2. Suppose I ``import tkinter as tk`` and
    then try ``s1=tk.StringVar()``.  This fails
    because no "master" is set. Why does a
    Variable need a master?

3. Now suppose I set ``root = tk.TK()`` and
    then try ``s1=tk.StringVar()``.  This
    works fine but now seems a bit magical:
    how has the value of the "master" been
    set?

4. Another bit of magic:
    Suppose I ``import tkinter as tk`` and
    then try ``f1=tk.Frame()``.  This works
    fine: apparently calling Frame also
    leads to implicit creation of a "master".
    Why is what is good for the gander (i.e.,
    implicit master creation for a Frame) not
    good for the goose (i.e., a Variable)?
    (Here I assume that there has been an
    answer to 2. above.)

5. Reading around a bit,
    it seems common to recommend setting
    the values of Variables rather than initializing
    them.  Why?  I cannot see the reason to avoid
    ``s1=tk.StringVar(value="this works fine")``
    and it looks like ``tk.StringVar(()`` is in any
    case initialized (to an empty string).

6. Why is str(s1) not its value?  More generally,
    why does a StringVar not behave more like a string?

Thanks for any insights,
Alan Isaac