Event loops in the terminal

In the terminal, IPython uses prompt_toolkit to prompt the user for input. prompt_toolkit provides hooks to integrate with an external event loop.

To integrate an event loop, define a function which runs the GUI event loop until there is input waiting for prompt_toolkit to process. There are two ways to detect this condition:

# Polling for input.
def inputhook(context):
    while not context.input_is_ready():
        # Replace this with the appropriate call for the event loop:
        iterate_loop_once()

# Using a file descriptor to notify the event loop to stop.
def inputhook2(context):
    fd = context.fileno()
    # Replace the functions below with those for the event loop.
    add_file_reader(fd, callback=stop_the_loop)
    run_the_loop()

Once you have defined this function, register it with IPython:

Event loops in the kernel

The kernel runs its own event loop, so it's simpler to integrate with others. IPython allows the other event loop to take control, but it must call :meth:`IPython.kernel.zmq.kernelbase.Kernel.do_one_iteration` periodically.

To integrate with this, write a function that takes a single argument, the IPython kernel instance, arranges for your event loop to call kernel.do_one_iteration() at least every kernel._poll_interval seconds, and starts the event loop.

Decorate this function with :func:`IPython.kernel.zmq.eventloops.register_integration`, passing in the names you wish to register it for. Here is a slightly simplified version of the Tkinter integration already included in IPython:

@register_integration('tk')
def loop_tk(kernel):
    """Start a kernel with the Tk event loop."""
    from tkinter import Tk

    # Tk uses milliseconds
    poll_interval = int(1000*kernel._poll_interval)
    # For Tkinter, we create a Tk object and call its withdraw method.
    class Timer(object):
        def __init__(self, func):
            self.app = Tk()
            self.app.withdraw()
            self.func = func

        def on_timer(self):
            self.func()
            self.app.after(poll_interval, self.on_timer)

        def start(self):
            self.on_timer()  # Call it once to get things going.
            self.app.mainloop()

    kernel.timer = Timer(kernel.do_one_iteration)
    kernel.timer.start()

Some event loops can go one better, and integrate checking for messages on the kernel's ZMQ sockets, making the kernel more responsive than plain polling. How to do this is outside the scope of this document; if you are interested, look at the integration with Qt in :mod:`IPython.kernel.zmq.eventloops`.