Python: How to 'anchor' text to be always visible and set_text method Matplotlib

I have this code:

from pylab import *
from mpl_toolkits.axes_grid.anchored_artists import AnchoredText
import numpy as np
import matplotlib.pylab as plt


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

ann = AnchoredText('If you zoom in or out, i stay here\nbut can you update this text?',
                  prop=dict(size=8), frameon=True,
                  loc=2,
                  )
ann.patch.set_boxstyle("round,pad=0.,rounding_size=0.2")
ax.add_artist(ann) 

delta = 0.025
x = np.arange(-3.0, 3.0, delta)
y = np.arange(-2.0, 2.0, delta)
X, Y = np.meshgrid(x, y)
Z1 = bivariate_normal(X, Y, 1.0, 1.0, 0.0, 0.0)
Z2 = bivariate_normal(X, Y, 1.5, 0.5, 1, 1)
Z = (Z1 - Z2) * 10

plt.contourf(X, Y, Z)

plt.show()

Is there a way to anchor text to be always visible but to also be able to use a set_text method like in text objects to update the containing text? Here i used AnchoredText which is anchored perfectly but i cannot find a method to change the text it contains. In AnchoredText Matplotlib documentation i cannot find such a method available. If it cannot be done with AnchoredText can it be done with a simple text object?

EDIT

I accepted the solution from David Zwicker and here is a working example for anyone in need of it:

from pylab import *
from mpl_toolkits.axes_grid.anchored_artists import AnchoredText
import numpy as np
import matplotlib.pylab as plt
import random


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

ann = AnchoredText('If you zoom in or out, i stay here\nbut can you update this text?$4.1f$ km s$^{-1}$',
                  prop=dict(size=15), frameon=True,
                  loc=2,
                  )
ann.patch.set_boxstyle("round,pad=0.,rounding_size=0.2")
ax.add_artist(ann) 

delta = 0.025
x = np.arange(-3.0, 3.0, delta)
y = np.arange(-2.0, 2.0, delta)
X, Y = np.meshgrid(x, y)
Z1 = bivariate_normal(X, Y, 1.0, 1.0, 0.0, 0.0)
Z2 = bivariate_normal(X, Y, 1.5, 0.5, 1, 1)
Z = (Z1 - Z2) * 10

plt.contourf(X, Y, Z)

a = ["hhhhh", "qqqqq", "aaaaaaa","ttttt","yyyyy","oooooooo"]

def setanntext(self):
         ann.txt.set_text(random.choice(a))
         plt.draw()

buttonax = plt.axes([0.7, 0.05, 0.1, 0.1])
button = Button(buttonax, 'set text',color='0.85', hovercolor='0.95')
button.on_clicked(setanntext)

plt.show()

Solution

You can access the attribute txt. You can update the anchored text using

ann.txt.set_text('new text')
plt.draw()

The plt.draw() is necessary to update the plot and show the new text.