[PYTHON] When generating a large number of graphs with matplotlib, I do not want to display the graph on the screen (jupyter environment)

In conclusion, the solution is plt.close ().

Introduction

In jupyter (Google Colab.) Environment, if you want to generate a large number of graphs and save them as an image file using matplotlib, if you do not want to display the graph in the ** execution result cell (on the screen) of the notebook **there is.

In particular, if you try to display more than 20 graphs, you will get the following warning.

/usr/local/lib/python3.6/dist-packages/ipykernel_launcher.py:8: RuntimeWarning: More than 20 figures have been opened. Figures created through the pyplot interface (matplotlib.pyplot.figure) are retained until explicitly closed and may consume too much memory. (To control this warning, see the rcParam figure.max_open_warning).

A note on avoiding this issue.

Current status

The following code is "generate 40 normal random numbers with an average of 50 and a standard deviation of 10 and draw a histogram of them and save them as a png file".

I'm looping 50 times to see the distribution of each set when I try 50 sets.

A large number of graphs are output in the execution result cell

import numpy as np
import matplotlib.pyplot as plt

#Generate 50 image files
for i in range(50) : 
  dat = (np.random.normal(50,10,size=40)).astype(np.int64)
  plt.figure()
  plt.hist(dat,bins=10,range=(0,100))
  plt.yticks(range(0,31,10))
  plt.xlim(0,100)
  plt.ylim(0,30)
  plt.savefig(f'fig{i}.png') #Save as image file

When I execute the above code, ** the warning at the beginning is displayed **, and the execution result cell is lined with a large number of graphs, which makes the notebook difficult to see. At the same time, the notebook (xxxx.ipynb) also contains the image of the execution result, so the file size will also increase.

• In a notebook environment, the graph is automatically output on the screen without specifying plt.show ().

Solution

The problem is solved by putting plt.close () after outputting the graph to a file plt.savefig (...).

Do not output the graph in the execution result cell

import numpy as np
import matplotlib.pyplot as plt

for i in range(50) : 
  dat = (np.random.normal(50,10,size=40)).astype(np.int64)
  plt.figure()
  plt.hist(dat,bins=10,range=(0,100))
  plt.yticks(range(0,31,10))
  plt.xlim(0,100)
  plt.ylim(0,30)
  plt.savefig(f'fig{i}.png')
  plt.close() #■■■ Addition ■■■

When I execute the above code, nothing is output to the execution result cell. Of course, the file output will be done without any problem.