First execution in kernel takes longer and confuses jupyter-console

[fmagin]

It seems that the first snippet executed in a kernel triggers some initialization that takes up to a few seconds. This isn't a big issue in the QT console or the notebook, but when using the console via jupyter-console --kernel=kotlin and finishing the input with enter, the console will give the following warning:

/usr/lib/python3.9/site-packages/jupyter_console/ptshell.py:784: UserWarning: The kernel did not respond to an is_complete_request. Setting `use_kernel_is_complete` to False.

Afterwards hitting Enter will just start a new line, and Alt+Enter is required to finish a line. The workaround for this is executing the first command with Alt+Enter. This issue also doesn't appear when starting jupyter-console with the argument --simple-prompt.

I am not fully sure what exactly is happening here and whether the fix is moving the initialization that causes the delay into the startup itself before the first input is prompted, or if properly handling the the is_complete_request during this initialization is the better solution.

This is not really a bad bug, only affects a rare use case (console only usage) and can easily be worked around, but it is probably fairly confusing for someone not familiar with the Jupyter internals that cause this.

[ileasile]

There is a number of options in Jupyter Console:
https://jupyter-console.readthedocs.io/en/latest/config_options.html
Try to use bigger value of ZMQTerminalInteractiveShell.kernel_is_complete_timeout:

jupyter console --kernel=kotlin --ZMQTerminalInteractiveShell.kernel_is_complete_timeout=10.0

But I think it is generally a good point to pre-initialize REPL. I'll think about it.