Overwriting Existing Python Plots with New Function Call - python-2.7

Question

I would like to overwrite an existing plot I made in python with a new function call. I would like to produce a plot, look at it, then call the same function again with different arguments to produce another plot. I would like the second plot to replace the first plot. That is, I don't want two figure windows open; just the original window overwritten.
I have tried using interactive mode when plotting (ion()), placing plot() and show() calls in different places, and clearing figures. The problems I have are that: 1. I can never overwrite and existing window, I always open more 2. show() blocks the code from continuing and I am unable to perform the 2nd function call 3. I use interactive mode and the window appears for a second before going away
What I'm trying to do seems simple enough, but I'm having great difficulty. Any help is appreciated.

Answer 1

Easiest solution
There are many ways to do this, the easiest of which is to reset the plot's Line2D using its set_ydata(...) method and pyplot.pause. There are versions of matplotlib (<0.9, I believe) which don't have pyplot.pause, so you may need to update yours. Here's a simple minimal working example:
import numpy as np
from matplotlib import pyplot as plt
ph, = plt.plot(np.random.rand(100))
def change_plot():
ph.set_ydata(np.random.rand(100))
plt.pause(1)
while True:
change_plot()
Other approaches
Using pyplot.ion and pyplot.ioff, as detailed here. I tend to use these when I'm doing exploratory data analysis with a Python shell.
Using the matplotlib.animation package, as detailed in this very comprehensible example. This is a much more robust approach than the easy solution above, and permits all kinds of useful/fun options, such as outputting the animation to a video file, etc.
Instead of using the set_ydata method of the Lines object, you can always clear the axes (pyplot.cla()) and call the plotting command again. For example, if you are using pyplot.contour, the returned QuadContourSet has no set_zdata method, but this will work:
import numpy as np
from matplotlib import pyplot as plt
X,Y = np.meshgrid(np.arange(100),np.arange(100))
def change_plot():
Z = np.random.random(X.shape)
plt.cla()
ph = plt.contour(X,Y,Z)
plt.pause(1)
while True:
change_plot()

Answer 2

write your plotting function like
def my_plotter(ax, data, style):
ax.cla()
# ax.whatever for the rest of your plotting
return artists_added
and then call it like
data = some_function()
arts = my_plotter(plt.gca(), data, ...)
or do
fig, ax = plt.subplots()
and then call your plotting function like
arts = my_plotter(ax, data, ...)

Answer 3

I had almost the same issue and I solved it by assigning a name for each plot.
def acc(train_acc, test_acc, savename):
plt.figure(savename) # If you remove this line, the plots will be added to the same plot. But, when you assign a figure, each plot will appear in a separate plot.
ep = np.arange(len(train_acc)) + 1
plt.plot(ep, train_acc, color="blue", linewidth=1, linestyle="-", label="Train")
plt.plot(ep, test_acc, color="red", linewidth=1, linestyle="-", label="Test")
plt.title("NDCG")
plt.xlabel("Iteration")
plt.ylabel("NDCG#K")
plt.legend(loc='lower right')
plt.savefig(savename)