Global variable from matplotlib event

Question

I am trying to retrieve an array as an output from a matplotlib event:

import numpy as np
import matplotlib.pyplot as plt

def onclick(event):
    global points
    try:
        points = np.append(points,[[event.xdata,event.ydata]],axis=0)
    except NameError:
        points = np.array([[event.xdata,event.ydata]])
    ax.plot(points[:,0],points[:,1],'o')
    plt.draw()

fig = plt.figure()
ax = fig.add_subplot(111)
ax.set_xlim((0,10))
ax.set_ylim((0,10))
plt.gcf().canvas.mpl_connect('button_press_event',onclick)
plt.show()

Even tough I have declared "points" to be global,

print(points)

returns a NameError. How can I retrieve "points"? Thank you for any help!!

Answer 1

You cannot just declare a variable as global, you have to create it initially. The following code should work as you expect.

import numpy as np
import matplotlib.pyplot as plt

points = []

def onclick(event):
    global points
    points.append([event.xdata, event.ydata])

    ax.plot(event.xdata, event.ydata,'o')
    plt.draw()

fig = plt.figure()
ax = fig.add_subplot(111)

ax.set_xlim((0, 10))
ax.set_ylim((0, 10))

plt.gcf().canvas.mpl_connect('button_press_event', onclick)

plt.show()

Shown below is a plot after I clicked 5 times.

EDIT

Instead of plotting a new marker every single time you add a point, you could instead modify the plot object you already have. Like below.

import matplotlib.pyplot as plt

fig = plt.figure()
ax = fig.add_subplot(111)

ax.set_xlim((0, 10))
ax.set_ylim((0, 10))

points = []

line, = ax.plot(points, 'o--')

def onclick(event):
    global points
    points.append([event.xdata, event.ydata])

    line.set_data(zip(*points))
    plt.draw()

plt.gcf().canvas.mpl_connect('button_press_event', onclick)

plt.show()

This will plot points once and then everytime the user clicks on the plot it will modify the line object and re-draw it.