Python list comprehension plt.imread does not work

Question

I just don't understand what python is doing here:

I want to provide the math of a directory that contains images and store them directly as numpy array in a list. However, when I execute the following code everything works fine because I first extract the filename and then perform imread to create the numpy array:

def get_data(image_dir, annotations_dir):
  image_filenames = [ os.path.join(image_dir,i) for i in os.listdir(image_dir)]
  
  return image_filenames, image_filenames

IMAGE_DIR = os.path.join(os.getcwd(), "sample_data/images")
ANNOTATIONS_DIR = os.path.join(os.getcwd(), "sample_data/annotations")

print("IMAGE_DIR: {}\nANNOTATIONS_DIR: {}".format(IMAGE_DIR, ANNOTATIONS_DIR))

pet_images, pet_annotations = get_data(IMAGE_DIR, ANNOTATIONS_DIR)

print(len(pet_images))

my_img = plt.imread(pet_images[0])
plt.imshow(my_img)

Result:

IMAGE_DIR: /content/sample_data/images
ANNOTATIONS_DIR: /content/sample_data/annotations
7393

<matplotlib.image.AxesImage at 0x7feee24fdeb8>

(Image is shown perfectly fine)

BUT when I try to do create the numpy array inside directly it does not work. Here the following code:

def get_data(image_dir, annotations_dir):
  image_filenames = [ plt.imread((os.path.join(image_dir,i)) for i in os.listdir(image_dir)]
  
  return image_filenames, image_filenames

IMAGE_DIR = os.path.join(os.getcwd(), "sample_data/images")
ANNOTATIONS_DIR = os.path.join(os.getcwd(), "sample_data/annotations")

print("IMAGE_DIR: {}\nANNOTATIONS_DIR: {}".format(IMAGE_DIR, ANNOTATIONS_DIR))

pet_images, pet_annotations = get_data(IMAGE_DIR, ANNOTATIONS_DIR)



plt.imshow(pet_images[0])

I get:

/usr/local/lib/python3.6/dist-packages/PIL/Image.py in open(fp, mode)
   2860         warnings.warn(message)
   2861     raise UnidentifiedImageError(
-> 2862         "cannot identify image file %r" % (filename if filename else fp)
   2863     )
   2864 

UnidentifiedImageError: cannot identify image file '/content/sample_data/images/Abyssinian_100.mat'

Could someone please explain what is going on and why I cannot create the numpy array using imread inside the list comprehension directly

Best regards

Answer 1

I found out what the problem was: The reason the code crashes is that the dataset contains a filename.mat file which is not supported my imread.