[dmd]

One thing I love about much-maligned Tcl is its ability to connect a repl to a running program.

Not just remote debugging of a halted program - you can actually inspect and alter variables and issue commands into the executing code.

Is there a way to do that in, say, Python?

[hcarvalhoalves]

All Python debuggers allow you to execute statements, on top of navigating the stack.

[dmd]

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.