What does calling Tk() actually do?

Question

I was brushing up on Tkinter when I looked upon a minimal example from the NMT Tkinter 8.5 Reference.

#!/usr/bin/env python
import tkinter as tk

class Appli         


        

Answer 1

Bryan Oakley's answer is spot on. Creating a widget will implicitly create an instance of the tcl/tk interpreter. I would, however, like to add some pieces of code, to better understand how Tk is implicitly created.

Whenever a Widget object is created (whether it is a Frame or a Button or even a ttk-based widget), the BaseWidget class' __init__ method is called, which in turn calls the _setup method. Here is a snippet of the relevant part:

def _setup(self, master, cnf):
    """Internal function. Sets up information about children."""
    if _support_default_root:
        global _default_root
        if not master:
            if not _default_root:
                _default_root = Tk()
            master = _default_root
    self.master = master
    self.tk = master.tk

Both _support_default_root and _default_root are global variables, declared in lines 132-133 of the __init__.py file in the tkinter package. They are initialized to the following values:

_support_default_root = 1
_default_root = None

This means that, if master isn't provided, and if an interpreter wasn't already created, an instance of Tk gets created and assigned as the default root for all future widgets.

There is also something interesting when creating an instance of the Tk class. The following snippet comes from the Tk._loadtk method:

if _support_default_root and not _default_root:
    _default_root = self

Which means that, regardless of how the Tk class gets initialized, it is always set up as the default root.