Python debuggers, as far as I know, require the program to be in a halted state - i.e., you're stepping through it. In Tcl, the program can be running its even

Python debuggers, as far as I know, require the program to be in a halted state - i.e., you're stepping through it. In Tcl, the program can be running its event loop at normal speed, and you can inject commands into that event loop while it runs.