What Do the First Two Arguments in a Tkinter Variable Trace Callback Represent?

Understanding the Arguments in Tkinter Variable Trace Callbacks

Python Tkinter offers a range of variable types, such as StringVar() and BooleanVar(), that come equipped with the trace method. This method, when called with a mode and a callback function, allows you to monitor changes to the variable.

However, the callback function passed to the trace method is called with four arguments: self, n, m, x. While x appears to indicate the mode that triggered the callback, the first two arguments, n and m, often remain enigmatic.

Unveiling the First Argument

The first argument to the callback function is the internal variable name. This name is typically the variable's user-assigned name if provided (e.g., StringVar(name='foo')). Otherwise, Tkinter generates a name like 'PYVAR0' for you. This name can be useful in conjunction with the tkinter methods getvar and setvar.

Exploring the Second Argument

For scalar variables, the second argument is always an empty string. However, if the variable is a list variable, m represents an index into that list. This is rather uncommon in Tkinter.

The Significance of the Third Argument

The third argument, x, signifies the operation that triggered the callback and can take the values "read," "write," or "unset." This information is helpful if you're employing the same method for various operations (reading, writing, or deleting).

It's important to note that Tkinter is a Python wrapper around a Tcl/Tk interpreter. The comprehensive documentation for variable traces is available here:

http://tcl.tk/man/tcl8.5/TclCmd/trace.htm#M14

Referencing this documentation can provide additional insights into the inner workings of trace callbacks.

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