ctrl-z doesn't suspend to background on linux

[nonagonal]

I'm using prompt_toolkit v1.0.13 (via IPython 5.2.2) on Ubuntu, and ctrl-z is inserting '^Z' instead of suspending the active process.

I experimented with the prompt_toolkit code a bit, and it seems that in key_binding/bindings/basic.py:

1. load_basic_system_bindings() is called
2. suspend_to_background_supported() does return True
   but
3. for some reason the Control-Z event handler registered in load_basic_system_bindings() is never called (instead the one in registered in load_basic_bindings() is always called).

[jonathanslenders]

Hi @nonagonal,

Can you try to run this example: https://github.com/jonathanslenders/python-prompt-toolkit/blob/master/examples/system-prompt.py ?
I think this shortcut is disabled by default, and passing enable_system_bindings=True helps.

It's also always possible to add a new key binding that handles Control-Z.

Maybe it could be enabled by default in a future version if the UI is attached to stdin/stdout. Would that make sense?

[nonagonal]

Thanks for the quick response. I tried your example and ctrl-z successfully suspends the process. I also tried with enable_system_bindings=False and it still suspends the process.

The problem I'm having is in the context of IPython. I've been using IPython for a long time but only looked at the source for the first time today. So I'm not sure what else about IPython is different (compared to your example program).

I can reproduce this easily in IPython, so is there any additional info I can get using something like print() statements from IPython that would be helpful?

It was confusing because the code to register the handler in load_basic_system_bindings() was definitely executing, but the event handler didn't end up getting called (maybe it was filtered or replaced somehow at a later point in time?).

[takluyver]

For reference, IPython does not pass enable_system_bindings, but does set a specific handler for Ctrl-Z:

    supports_suspend = Condition(lambda cli: hasattr(signal, 'SIGTSTP'))
    registry.add_binding(Keys.ControlZ, filter=supports_suspend
                        )(suspend_to_bg)

[nonagonal]

Aha that's helpful. So all of the key bindings look like they're set up correctly (i.e. the filter condition returns True and the code is executed). Is it possible that different registries are being used?

When I actually press ctrl-z from within ipdb, only the basic handler which echoes ^Z is called. I'd be happy to look at the internal state of prompt_toolkit when this happens, I'm just not sure where to look.

[takluyver]

Ah, right, I forgot this was in the debugger. We do that for the main shell interface in IPython, but we don't currently configure any custom shortcuts in the debugger, so that's probably why.

[nonagonal]

It seems to me that load_basic_system_bindings should be handling it (i.e. prompt_toolkit has ctrl-z suspend to background by default on systems that support it), but I wasn't sure where to look to determine why the event handler it registers is not called.

[takluyver]

It sounds like that's only used if you pass enable_system_bindings, though, which currently we don't. Maybe we should - I haven't looked at all the implications of what that does.

[nonagonal]

Aha I was confused because the bindings are filtered after being added, so indeed prompt_toolkit is working fine. I'll close this bug and sorry for the confusion on my part.

I'll open a separate request on IPython to enable system bindings in the debugger.