What Are the Arguments Passed to Tkinter Variable Trace Callbacks?

Understanding the Arguments in Tkinter Variable Trace Callbacks

Tkinter, Python's GUI library, offers variable classes like StringVar() and BooleanVar() that support trace methods. These methods enable tracking variable changes and executing specified callbacks. However, the arguments passed to these callbacks often raise questions.

Self

The first argument represents the variable itself.

N

The second argument relates to the type of variable. For scalar variables, it remains empty. For list variables, it denotes an index into the list.

M

The third argument indicates the operation that triggered the callback. It can be "read," "write," or "unset."

X

In Tkinter, "x" often refers to an internal variable name that can be utilized in conjunction with the getvar and setvar methods. If the variable is named (e.g., StringVar(name="foo")), this name will be displayed here. Otherwise, Tkinter generates a name for the variable (e.g., PYVAR0).

Additional Context

Tkinter's trace callbacks serve as a bridge between Python and the underlying Tcl/Tk interpreter. The official documentation for the internal trace mechanism can be found at http://tcl.tk/man/tcl8.5/TclCmd/trace.htm#M14. However, note that Tkinter may occasionally modify the data passed to the callbacks.

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