6.6 Python/Tkinter for Tcl/Tk Converts

At the start of this chapter, I mentioned that Tkinter is Python's interface to the Tk GUI library, originally written for the Tcl language. To help readers migrating from Tcl to Python, and to summarize some of the main topics we met in this chapter, this section contrasts Python's Tk interface with Tcl's. This mapping also helps make Tk references written for other languages more useful to Python developers.

In general terms, Tcl's command-string view of the world differs widely from Python's object-based approach to programming. In terms of Tk programming, though, the syntactic differences are fairly small. Here are some of the main distinctions in Python's Tkinter interface:

Creation

Widgets are created as class instance objects by calling a widget class.

Masters (parents)

Parents are previously created objects, passed to widget-class constructors.

Widget options

Options are constructor or config keyword arguments, or indexed keys.

Operations

Widget operations (actions) become Tkinter widget class object methods.

Callbacks

Callback handlers are any callable objects: function, method, lambda, etc.

Extension

Widgets are extended using Python class inheritance mechanisms.

Composition

Interfaces are constructed by attaching objects, not concatenating names.

Linked variables (next chapter)

Variables associated with widgets are Tkinter class objects with methods.

In Python, widget creation commands (e.g., button) are Python class names that start with an uppercase letter (e.g., Button), two-word widget operations (e.g., add command) become a single method name with an underscore (e.g., add_command), and the "configure" method can be abbreviated as "config" as in Tcl. In Chapter 7, we will also see that Tkinter "variables" associated with widgets take the form of class instance objects (e.g., StringVar, IntVar) with get and set methods, not simple Python or Tcl variable names. Table 6-2 shows some of the primary language mappings in more concrete terms.

Table 6-2. Tk to Tkinter Mappings

Operation	Tcl/Tk	Python/Tkinter
Creation	frame .panel	panel = Frame()
Masters	button .panel.quit	quit = Button(panel)
Options	button .panel.go -fg black	go = Button(panel, fg='black')
Configure	.panel.go config -bg red	go.config(bg='red') go['bg'] = 'red'
Actions	.popup invoke	popup.invoke()
Packing	pack .panel -side left -fill x	panel.pack(side=LEFT, fill=X)

Some of these differences are more than just syntactic, of course. For instance, Python builds an internal widget object tree based on parent arguments passed to widget constructors, without ever requiring concatenated widget pathname strings. Once you've made a widget object, you can use it directly by reference. Tcl coders can hide some dotted pathnames by manually storing them in variables, but that's not quite the same as Python's purely object-based model.

Once you've written a few Python/Tkinter scripts, though, the coding distinctions in the Python object world will probably seem trivial. At the same time, Python's support for OO techniques adds an entirely new component to Tk development; you get the same widgets, plus Python's support for code structure and reuse.