@tacaswell would this flag name suits you ?

@takluyver Should we also check for os.environ["TERM"] == "dumb"

I like 'simple-prompt'.

As #9487 was merged, we also need to make this switch affect the debugger. It can probably just change debugger_cls to the parent class from IPython.core.debugger, which is equivalently plain.

As #9487 was merged, we also need to make this switch affect the debugger.

Yes.

It can probably just change debugger_cls to the parent class from IPython.core.debugger, which is equivalently plain.

Unsure what you mean exactly, I'm confused by the english construct, but gave it a try:

+    @observe('simple_prompt')
+    def _simple_prompt_changed(self, changes):
+        if changes['new'] == True:
+            self.debugger_cls = Pdb
+        else:
+            self.debugger_cls = TerminalPdb

That's basically what I meant, but I'd just change debugger_cls inside the if clause - simple_prompt can only take effect on startup anyway, so there's no need for traitlets' dynamism.

Since you won't be awake for several hours, I'm going to change it locally and merge.

I realised that init_prompt_toolkit_cli would be too late to change debugger_cls effectively, but I found another way to do it that I like more, using a property.