Path error while viewing loading training and test dataset

[take2rohit]

for i in range(len(train_image_names)):
    img = plt.imread(TRAIN_IMG_FOLDER + train_image_names[i])
    training_tensor[i,:] = np.array(img, dtype='float64').flatten()
    plt.subplot(5,5,1+i)
    plt.imshow(img, cmap='gray')
    plt.tick_params(labelleft='off', labelbottom='off', bottom='off',top='off',right='off',left='off', which='both')
plt.show()

print('Test Images:')
test_image_names = os.listdir(TEST_IMG_FOLDER)#[i for i in dataset_dir if i not in train_image_names]
testing_tensor   = np.ndarray(shape=(len(test_image_names), height*width), dtype=np.float64)

for i in range(len(test_image_names)):
    img = imread(TEST_IMG_FOLDER + test_image_names[i])
    testing_tensor[i,:] = np.array(img, dtype='float64').flatten()
    plt.subplot(8,5,1+i)
    plt.imshow(img, cmap='gray')
    plt.subplots_adjust(right=1.2, top=1.2)
    plt.tick_params(labelleft='off', labelbottom='off', bottom='off',top='off',right='off',left='off', which='both')
plt.show()

Errors in line
img = plt.imread(TRAIN_IMG_FOLDER + train_image_names[i])
img = imread(TEST_IMG_FOLDER + test_image_names[i])

To correct this error just replace these lines with

img = plt.imread(os.path.join(TRAIN_IMG_FOLDER, train_image_names[i]))
img = imread(os.path.join(TEST_IMG_FOLDER, test_image_names[i]))