How to generate a Captcha verification code image with Python?

from captcha.image import ImageCaptcha
import matplotlib . pyplot as plt
import random
import string

characters = string.digits + string.ascii_uppercase
generator = ImageCaptcha()

# Generate a random 4 character string
random_str = ''.join([random.choice(characters) for j in range(4)])

# Generate verification code
img = generator.generate_image(random_str)

# Display verification code picture and verification code title
plt.imshow(img)
plt.title(random_str)
plt.show()

# Save verification code
file_name = 'images/'+random_str+'.jpg'
img.save(file_name)

# show verification code
img.show()