What Do the Arguments in Tkinter Variable Trace Method Callbacks Mean?

Arguments to Tkinter Variable trace Method Callbacks

Python's Tkinter variable classes (e.g., StringVar(), BooleanVar()) provide a trace method for tracking variable changes. This method accepts four arguments within its callback function: self, n, m, x.

Understanding the Argument Sequence

• First Argument (n): Internal Variable Name
  The first argument is the internal variable name. If you assign your variable a name (e.g., StringVar(name='foo')), this will be that name. Otherwise, Tkinter will generate a name for you (e.g., PYVAR0).
• Second Argument (m): List Index or Empty String
  For list variables (unlikely in Tkinter), this argument represents the index in the list. For scalar variables, m will be an empty string.
• Third Argument (x): Triggered Operation
  This argument specifies which operation triggered the trace: "read," "write," or "unset."

Example

Consider a BooleanVar() variable. When the callback function is executed, the arguments n, m, and x might appear as '', 'PYVAR0', and 'w', respectively. Here's what they mean:

• n: '' (internal variable name)
• m: 'PYVAR0' (index for a list variable, or empty string for a scalar variable)
• x: 'w' (write operation triggered the trace)

Further Information

Tkinter wraps a Tcl/Tk interpreter. The comprehensive documentation for variable traces is available at http://tcl.tk/man/tcl8.5/TclCmd/trace.htm#M14. Note, however, that the Tkinter wrapper may occasionally modify the trace data.

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