Python - ProcessPoolExecutor hangs when called from a mpl_connect handler

I'm using parallel processing to generate a plot of functions using complex numbers. My script allows you to zoom in on an area of the plot using the standard matplotlib controls and then regenerate the plot within the new limits to improve resolution.

This is my first foray into parallel processing and I've got as far as understanding that I need to preface with if __name__ == __main__: to allow the module to be imported properly. When running my script, the first plot is successfully generated and appears as expected. However, when the plotting function is called again from my event handler it instead hangs indefinitely. I assume that the hang is caused by some similar issue to that of requiring if __name__ == __main__:, as the parallel processes are being spawned from outside the main body of the script, but I haven't figured out anything further than this.

import numpy as np
import matplotlib.pyplot as plt
from concurrent.futures import ProcessPoolExecutor
import multiprocessing

res = [1000, 1000]
base_factor = 2.
cpuNum = multiprocessing.cpu_count()

def brot(c, depth=200):
    z = complex(0)

    for i in range(depth):
        z = pow(z, 2) + c

        if abs(z) > 2:
            return i

    return -1

def brot_gen(span):
    re_span = span[0]
    im_span = span[1]

    mset = np.zeros([len(im_span), len(re_span)])

    for re in range(len(re_span)):
        for im in range(len(im_span)):
            mset[im][re] = brot(complex(re_span[re], im_span[im]))

    return mset

def brot_gen_parallel(re_lim, im_lim):
    re_span = np.linspace(re_lim[0], re_lim[1], res[0])
    im_span = np.linspace(im_lim[0], im_lim[1], res[1])

    split_re_span = np.array_split(re_span, cpuNum)

    packages = [(sec, im_span) for sec in split_re_span]
    print("Generating set between", re_lim, "and", im_lim, "...")

    with ProcessPoolExecutor(max_workers = cpuNum) as executor:
            result = executor.map(brot_gen, packages)

    mset = np.concatenate(list(result), axis=1)
    print("Set generated")

    return mset

def handler(ax):
    def action(event):
        if event.button == 2:
            cur_re_lim = ax.get_xlim()
            cur_im_lim = ax.get_ylim()

            mset = brot_gen_parallel(cur_re_lim, cur_im_lim)

            ax.cla()
            ax.imshow(mset, extent=[cur_re_lim[0], cur_re_lim[1], cur_im_lim[0], cur_im_lim[1]], origin="lower", vmin=0, vmax=200, interpolation="bilinear")

            plt.draw()

    fig = ax.get_figure()
    fig.canvas.mpl_connect('button_release_event', action)

    return action

if __name__ == "__main__":
    re_lim = np.array([-2.5, 2.5])
    im_lim = res[1]/res[0] * re_lim

    mset = brot_gen_parallel(re_lim, im_lim)

    plt.imshow(mset, extent=[re_lim[0], re_lim[1], im_lim[0], im_lim[1]], origin="lower", vmin=0, vmax=200, interpolation="bilinear")

    ax = plt.gca()

    f = handler(ax)

    plt.show()

EDIT: I wondered if there was a bug in the code causing an exception, but that this might not be being successfully passed back to the console, however I tested this by running the same task without splitting it into parallel tasks and it completed successfully.