Getting empty tick labels before showing a plot in Matplotlib

Question

I'm experiencing a similar issue to the one reported here. I don't understand why the tick label text is an empty string:

import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0,2*np.pi,100)
y = np.sin(x)**2

fig, ax = plt.subplots()
ax.plot(x,y)

labels = ax.get_xticklabels()
for label in labels:
    print(label)

plt.show()

Output:

Text(0,0,'')
Text(0,0,'')
Text(0,0,'')
Text(0,0,'')
Text(0,0,'')
Text(0,0,'')
Text(0,0,'')
Text(0,0,'')

I get the same result with ax.xaxis.get_ticklabels() but the plotted graph shows eight labelled ticks on the x-axis when saved or shown. However, if I ask for the labels after I show the plot, then the labels list is properly populated. Of course, it's a bit late to do anything about changing them then.

fig, ax = plt.subplots()
ax.plot(x,y)
plt.show()

labels = ax.get_xticklabels()
for label in labels:
    print(label)

Output:

Text(0,0,'0')
Text(1,0,'1')
Text(2,0,'2')
Text(3,0,'3')
Text(4,0,'4')
Text(5,0,'5')
Text(6,0,'6')
Text(7,0,'7')

Why does this happen (Mac OS X Yosemite, Matplotlib 1.5.1) and how can I get my labels before I show or save my plot?

Solution

You've correctly identified the problem: before you call plt.show() nothing is explicitly set. This is because matplotlib avoids static positioning of the ticks unless it has to, because you're likely to want to interact with it: if you can ax.set_xlim() for example.

In your case, you can draw the figure canvas with fig.canvas.draw() to trigger tick positioning, so you can retrieve their value.

Alternatively, you can explicity set the xticks that will in turn set the the axis to FixedFormatter and FixedLocator and achieve the same result.

import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0,2*np.pi,100)
y = np.sin(x)**2

fig, ax = plt.subplots()

ax.plot(x,y)
ax.set_xlim(0,6)

# Must draw the canvas to position the ticks
fig.canvas.draw()
# Or Alternatively
#ax.set_xticklabels(ax.get_xticks())

labels = ax.get_xticklabels()
for label in labels:
    print(label.get_text())

plt.show()

Out:
0
1
2
3
4
5
6