eventloops.rst

Integrating with GUI event loops

When the user types %gui qt, IPython integrates itself with the Qt event loop, so you can use both a GUI and an interactive prompt together. IPython supports a number of common GUI toolkits, but from IPython 3.0, it is possible to integrate other event loops without modifying IPython itself.

Supported event loops include qt4, qt5, gtk2, gtk3, wx, osx and tk. Make sure the event loop you specify matches the GUI toolkit used by your own code.

To make IPython GUI event loop integration occur automatically at every startup, set the c.InteractiveShellApp.gui configuration key in your IPython profile (see setting_config).

If the event loop you use is supported by IPython, turning on event loop integration follows the steps just described whether you use Terminal IPython or an IPython kernel.

However, the way Terminal IPython handles event loops is very different from the way IPython kernel does, so if you need to integrate with a new kind of event loop, different steps are needed to integrate with each.

Integrating with a new event loop in the terminal

5.0

There is a new API for event loop integration using prompt_toolkit.

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:

IPython.terminal.pt_inputhooks

register(name, inputhook)

Register the function inputhook as the event loop integration for the GUI name. If name='foo', then the user can enable this integration by running %gui foo.

Integrating with a new event loop 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 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 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 IPython.kernel.zmq.eventloops.