Coins classifier Neural Network: Head or Tail?

Home of this article.

The global objective of these articles is to build a coin classifier, capable of scanning your pocket change and find rare / valuable coins. This is a second article in a series, so let me remind you what happened earlier.

During previous step we got a rather large dataset composed of pairs of images, loaded from an online coins site meshok.ru. Those images were uploaded to the Internet by people we do not know, and though they are supposed to contain coin's head in one image and tail in the other, we can not rule out a situation when we have two heads and no tail and vice versa. Also at the moment we have no idea which image contains head and which contains tail: this might be important when we feed data to our final classifier.

So let's write a program to distinguish heads from tails. It is a rather simple task, involving a convolutional neural network that is using transfer learning.

Same way as before, we are going to use Google Colab environment, taking the advantage of a free video card they grant us an access to. We will store data on a Google Drive, so first thing we need is to allow Colab to access the Drive:

from google.colab import drive
drive.mount("/content/drive/", force_remount=True)

Next step, we are going to install the Efficient Net. This is the pretrained network (remember I spoke about transfer learning?) that we use as a starting point, rather than training a network from scratch.

!pip install -q efficientnet 
import efficientnet.tfkeras as efn

Next, i usually have a large "include" section, please note that some files may be included that are not really used: feel free to delete them:

import numpy as np
import pandas as pd
import tensorflow as tf
from tensorflow import keras
from tensorflow.keras import layers

import sys
import random

import os
from os import listdir
from os.path import isfile, join

from tensorflow.keras import regularizers
from tensorflow.keras.optimizers import Adamax
from tensorflow.keras.preprocessing.image import ImageDataGenerator
from tensorflow.keras.preprocessing.image import array_to_img, img_to_array
from tensorflow.keras import backend as K
from tensorflow.keras.applications.vgg16 import VGG16,preprocess_input
from tensorflow.keras.applications import InceptionResNetV2, Xception, NASNetLarge

from mpl_toolkits.mplot3d import Axes3D
from sklearn.manifold import TSNE

from tensorflow.keras.layers import Input, Conv2D, MaxPooling2D, Dense, Activation, Dropout, Flatten, Lambda, concatenate, BatchNormalization, GlobalAveragePooling2D
from tensorflow.keras.callbacks import LambdaCallback
from tensorflow.keras.callbacks import ModelCheckpoint
from tensorflow.keras.models import Sequential
from sklearn.neighbors import NearestNeighbors

import seaborn as sns
import cv2
from tensorflow.python.keras.utils.data_utils import Sequence

import re	

Let's see which version of Tensorflow is used. This step is important, as Google is known for suddenly changing (increasing) versions:

import tensorflow as tf
print(tf.__version__)
tf.test.gpu_device_name()

The output in my case was:

2.4.0
'/device:GPU:0'

Then we do some additional initializations. Setting directories where our project is, and some subfolders for weight stored during training:

working_path = "/content/drive/My Drive/02_avers_or_revers/"

best_weights_filepath = working_path + "models/01_avers_or_revers.h5"
last_weights_filepath = working_path + "models/01_avers_or_revers.h5"	

We only train once, why would we do it every time, right? So we are going to use the boolean flag, if false, it means that training was already done, weights are stored in files, and instead of re-training, we can simply load those weights:

bDoTraining = True

We are going to scale down images to 256x256, use batch size 8 during training, and so on: here are constants we will need. Names are self-explainatory. We are also going to break our data to training images (used to tune network's weights), validation images used to calculate performance on data the net never saw) and the rest (testing data, used to test the result).

IMAGE_SIZE = 256
input_shape=(IMAGE_SIZE, IMAGE_SIZE, 3)

BATCH_SIZE = 8

embedding_model = 0
alpha = 0.4

TRAINING_IMAGES_PERCENT = 0.6
VALIDATION_IMAGES_PERCENT = 0.2

IMAGE_ROTATION_ANGLE = 180

We have two classes for our classifier to distinguish between:

# Class name corresponds to a folder. 
# Image path is "images" + class name + image name
arrClasses = ["head", "tail"]    	

Let's load data by reading the "head" and "tail" folders' content:

if(bDoTraining):
  pdLabels = pd.get_dummies(arrClasses)
  arrLabeledData = []

  for cls in arrClasses:
    arrImageNames = [f for f in listdir(working_path + "images/" + cls) if isfile(join(working_path, "images/", cls, f))]
    arrLabeledData.append(
    {
      'class':cls,
      'image_names':arrImageNames
    })

Function to load images:

def loadImage(path):
    img=cv2.imread(str(path))
    #img = rotate_bound(img, angle)
    img = cv2.resize(img, (IMAGE_SIZE, IMAGE_SIZE))
    img = cv2.cvtColor(img, cv2.COLOR_BGR2RGB)
    img = img.astype(np.float32)/255.
    img = img.reshape(input_shape)
    
    return img	

For an array of image info (file names and so on), get max indexes of training, validation and testing subsets:

def getClassMinMax(cls, bIsTrain):
  nLen = len(cls['image_names'])
  if(bIsTrain):
    nMinIdx = 0
    nMaxIdx = nLen * TRAINING_IMAGES_PERCENT
  else:
    nMinIdx = nLen * TRAINING_IMAGES_PERCENT + 1
    nMaxIdx = nLen * (TRAINING_IMAGES_PERCENT + VALIDATION_IMAGES_PERCENT)  
  
  return int(nMinIdx), int(nMaxIdx)

It is always a good idea to make sure everything works as intended, so let's test image loading:

if(bDoTraining):
  nClassIdx = np.random.randint(len(arrLabeledData))
  cls = arrLabeledData[nClassIdx]

  nMinIdx, nMaxIdx = getClassMinMax(cls, False)
  nImageIdx = random.randint(nMinIdx, nMaxIdx)

  arrLabeledData[0]['class']
  img = loadImage(join(working_path, "images/", cls['class'], cls['image_names'][nImageIdx]))#, 0)
  #img = img.reshape((IMAGE_SIZE, IMAGE_SIZE))
  print(cls['class'])
  plt.imshow(img)
  plt.show()

To make our dataset more diverse (augmentation), we might want to add noise to images:

def add_noise(img):
    '''Add random noise to an image'''
    VARIABILITY = 40
    deviation = VARIABILITY*random.random() / 255.
    noise = np.random.normal(0, deviation, img.shape)
    img += noise
    np.clip(img, 0., 1.)
    return img	

We will need the ImageDataGenerator to produce augmented images:

if(bDoTraining):
  datagen = ImageDataGenerator(
    samplewise_center=True,
    rotation_range=IMAGE_ROTATION_ANGLE,
    width_shift_range=0.1,
    height_shift_range=0.1,
    zoom_range=0.1 #[1, 1.2],
    #preprocessing_function=add_noise
  )	

The following function is used to get an image by index from data we loaded earlier, using image data generator we just created:

def getImage(cClass, nImageIdx, datagen):
  image_name = cClass['image_names'][nImageIdx]
  
  #angle = random.randint(-180, 180)
  img = loadImage(join(working_path, "images/", cClass['class'], cClass['image_names'][nImageIdx]))#, angle)
  
  arrImg = img_to_array(img)
  arrImg = datagen.random_transform(arrImg) # augmentation
  arrImg = add_noise(arrImg)
  
  return np.array(arrImg, dtype="float32")

Again, we need to make sure everything works, so let's see what this function returns:

if(bDoTraining):
  nClassIdx = np.random.randint(len(arrLabeledData))
  cls = arrLabeledData[nClassIdx]

  img = getImage(cls, 0, datagen)

  print(cls['class'])
  plt.imshow(img) #, cmap='gray')
  plt.show()

If we do training and for some reason want to start it over, we need to delete network we saved by that time:

def deleteSavedNet(best_weights_filepath):
    if(os.path.isfile(best_weights_filepath)):
        os.remove(best_weights_filepath)
        print("deleteSavedNet():File removed")
    else:
        print("deleteSavedNet():No file to remove") 	

As we train our network, it accumulates "history". It is a good idea to be able to show it as a chart, this way we can often see if training can be improved:

def plotHistory(history, strParam1, strParam2):
    plt.plot(history.history[strParam1], label=strParam1)
    plt.plot(history.history[strParam2], label=strParam2)
    #plt.title('strParam1')
    #plt.ylabel('Y')
    #plt.xlabel('Epoch')
    plt.legend(loc="best")
    plt.show()
    
def plotFullHistory(history):
    arrHistory = []
    for i,his in enumerate(history.history):
        arrHistory.append(his)
    plotHistory(history, arrHistory[0], arrHistory[2])    
    plotHistory(history, arrHistory[1], arrHistory[3]) 

Now a function that creates a model. It loads the EfficientNet, removes its last layers (the classifier) and attaches our own classifier, one we are going to train:

def createModel(nL2, optimizer):
  global embedding_model

  inputs = keras.Input(shape=(IMAGE_SIZE, IMAGE_SIZE, 3))
 
  model_b0 = efn.EfficientNetB0(weights='imagenet', include_top=False)(inputs)
  model_b0.trainable = False

  model_concat = model_b0 #layers.concatenate([model_b0, model_vgg16]) #, model_x]) #model_b0
  
  model_classifier = layers.Flatten(name="Flatten")(model_concat)
  
  model_classifier = layers.Dense(32, kernel_regularizer=regularizers.l2(nL2), activation='relu', name="Dense128")(model_classifier)
  model_classifier = layers.LeakyReLU(alpha=0.1, name="LeakyReLU")(model_classifier)
  model_classifier = layers.Dropout(0.4, name="Dropout")(model_classifier)
  base_model = layers.Dense(len(arrClasses), activation="softmax", kernel_regularizer=regularizers.l2(nL2), name="DenseEmbedding")(model_classifier)
               
  embedding_model = keras.Model(inputs=inputs, outputs=base_model, name="embedding_model")
  
  embedding_model.compile(loss=keras.losses.CategoricalCrossentropy(), optimizer=optimizer, metrics=["accuracy"])
 
  return embedding_model		

The following class is used to produce batches of images (and labels) that are used during training. Sequence class that is used as a parent is a new standard of Keras (if you don't want to use tfdata), it is highly paralelizeable and convenient:

from skimage.io import imread
from skimage.transform import resize
import numpy as np

# Here, `x_set` is list of path to the images
# and `y_set` are the associated classes.    

class MyImageDataGenerator(Sequence):    
  def __init__(self, bIsTrain):
    self.batch_size = BATCH_SIZE
    self.bIsTrain = bIsTrain

    nNumOfTrainSamples = 10000
    for cls in arrLabeledData:
      nMin, nMax = getClassMinMax(cls, True)
      nNumOfTrainSamples = min(nNumOfTrainSamples, nMax - nMin)

    if(self.bIsTrain):
      self.STEP_SIZE = nNumOfTrainSamples // BATCH_SIZE
    else:
      nNumOfValidSamples = int(nNumOfTrainSamples * VALIDATION_IMAGES_PERCENT / TRAINING_IMAGES_PERCENT)
      self.STEP_SIZE = nNumOfValidSamples // BATCH_SIZE
    
    if(self.STEP_SIZE < 100):
      self.STEP_SIZE = 100

    print("STEP_SIZE: ", self.STEP_SIZE, " (bIsTrain: ", bIsTrain, ")")

  def __len__(self):
    return self.STEP_SIZE

  def __getitem__(self, idx):
    arrBatchImages = []
    arrBatchLabels = []

    for i in range(self.batch_size):
      arrClassIdx = np.random.randint(len(arrLabeledData))
      cls = arrLabeledData[arrClassIdx]

      nMinIdx, nMaxIdx = getClassMinMax(cls, self.bIsTrain)
      nImageIdx = random.randint(nMinIdx, nMaxIdx)

      img = getImage(cls, nImageIdx, datagen)
      strLabel = cls['class']

      arrBatchImages.append(img)
      arrBatchLabels.append(pdLabels[strLabel].to_list())
  
    return np.array(arrBatchImages), np.array(arrBatchLabels)

We will need two objects of this class, one for training and one for validation:

if(bDoTraining):
  gen_train = MyImageDataGenerator(True)
  gen_valid = MyImageDataGenerator(False)

As usual, we need a function to show image obtained this way:

def ShowImg(img, label):
  
  print(label)
  
  fig = plt.figure()
  fig.add_subplot(1, 1, 1)
  plt.imshow(img) #, cmap='gray')
  plt.show()
  plt.close()	

And (again, as usual) we want to test the result:

if(bDoTraining):
  (images, labels) = gen_valid.__getitem__(0) #next(gen_train)

  for i, img in enumerate(images):
    ShowImg(img, labels[i])
    break	

We want to be able to stop training any time and later start from where we left, so we need to save weights at the end of each epoch. To do it, we create a list of callbacks and use it during training.

def getCallbacks(monitor, mode):
	checkpoint = ModelCheckpoint(best_weights_filepath, monitor=monitor, save_best_only=True, save_weights_only=True, mode=mode, verbose=1)

	save_model_at_epoch_end_callback = LambdaCallback(on_epoch_end=lambda epoch, logs: embedding_model.save_weights(last_weights_filepath))  

	callbacks_list = [checkpoint, save_model_at_epoch_end_callback]  # , early]

	return callbacks_list

Also, we need to be able to load the model (to continue training or to do testing):

def loadModel(embedding_model, bBest):
  if(bBest):
    path = best_weights_filepath
    strMessage = "load best model"
  else:
    path = last_weights_filepath
    strMessage = "load last model"

  if(os.path.isfile(path)):
    embedding_model.load_weights(path)
    print(strMessage, ": File loaded")
  else:
    print(strMessage, ": No file to load")

  return embedding_model

The following function does actual training:

def trainNetwork(EPOCHS, nL2, optimizer, bCumulativeLearning = False):
  global embedding_model
  global history
  global arrImages
  global arrLabels

  if(bCumulativeLearning == False):
    deleteSavedNet(best_weights_filepath)

  random.seed(7)
  
  embedding_model = createModel(nL2, optimizer)
  print("Model created")
  
  callbacks_list = getCallbacks("val_accuracy", 'max')  
      
  if(bCumulativeLearning == True):
    loadModel(embedding_model, False)

  nNumOfTrainSamples = 10000
  for cls in arrLabeledData:
    nMin, nMax = getClassMinMax(cls, True)
    nNumOfTrainSamples = min(nNumOfTrainSamples, nMax - nMin)

  STEP_SIZE_TRAIN = nNumOfTrainSamples // BATCH_SIZE
  if(STEP_SIZE_TRAIN < 100):
    STEP_SIZE_TRAIN = 100

  nNumOfValidSamples = int(nNumOfTrainSamples * VALIDATION_IMAGES_PERCENT / TRAINING_IMAGES_PERCENT)
  STEP_SIZE_VALID = nNumOfValidSamples // BATCH_SIZE
  if(STEP_SIZE_VALID < 100):
    STEP_SIZE_VALID = 100

  print(STEP_SIZE_TRAIN, STEP_SIZE_VALID)
  print("Available metrics: ", embedding_model.metrics_names)

  history = embedding_model.fit(gen_train, 
    validation_data=gen_valid, verbose=0,
    epochs=EPOCHS, steps_per_epoch=STEP_SIZE_TRAIN, 
    validation_steps=STEP_SIZE_VALID, callbacks=callbacks_list)

  print(nL2)
  plotFullHistory(history)
  
  # TBD: here, return best model, not last one
  return embedding_model

As you can see, it does some initializations, and then calls Keras's "fit" function.

Another data generator. This one reads images that we use AFTER network was trained. We don't care about labels here, as we deal with test set (or pretend it is test data).

def data_generator_simple(arrAllImageNames, arrAllImageClasses):
  i = 0
  arrImages = []
  arrImageLabels = []
  arrImageClasses = []
  for nImageIdx in range(len(arrAllImageNames)):
    if(i == 0):
      arrImages = []
      arrImageNames = []
      arrImageClasses = []
      
    i += 1

    strClass = arrAllImageClasses[nImageIdx]
    strImageName = arrAllImageNames[nImageIdx]

    #angle = random.randint(0, 90)
    img = loadImage(join(working_path, "images/", strClass, strImageName)) #, angle)
    arrImg = img_to_array(img)

    #arrImg = datagen.random_transform(arrImg) #/ 255.
    #arrImg = add_noise(arrImg)

    arrImg = np.array(arrImg, dtype="float32")  

    arrImages.append(arrImg)
    arrImageNames.append(strImageName)
    arrImageClasses.append(strClass)

    if i == BATCH_SIZE:
      i = 0
      yield np.array(arrImages), arrImageNames, arrImageClasses
  
  raise StopIteration()	

As usual, load image using this generator:

def ShowImgSimple(img, label):
  print(label)

  fig = plt.figure()

  fig.add_subplot(1, 1, 1)
  plt.imshow(img, cmap='gray')

  plt.show()
  plt.close()

And display it:

Using the generator above, we can load all test images and run prediction on them:

def getAllTestImages():
  global embedding_model

  arrAllImageNames = []
  arrAllImageClasses = []

  for cClass in arrLabeledData:
    for nIdx in range(int(len(cClass['image_names']) * (TRAINING_IMAGES_PERCENT + VALIDATION_IMAGES_PERCENT)), len(cClass['image_names'])): 
      arrAllImageNames.append(cClass['image_names'][nIdx])
      arrAllImageClasses.append(cClass['class'])

  test_preds  = []
  test_file_names = []
  test_class_names = []

  for imgs, fnames, classes in data_generator_simple(arrAllImageNames, arrAllImageClasses):
    predicts = embedding_model.predict(imgs)
    predicts = predicts.tolist()
    test_preds += predicts
    test_file_names += fnames
    test_class_names += classes
  test_preds = np.array(test_preds)

  return test_preds, test_file_names, test_class_names	

By the way, we can get accuracies for all our predictions:

def getAccuracy(test_preds, test_file_names, test_class_names):

  nTotalSuccess = 0

  for i, arrPredictedProbabilities in enumerate(test_preds):
    nPredictedClassIdx = arrPredictedProbabilities.argmax()
    gt_class = test_class_names[i]
    predicted_class = arrClasses[nPredictedClassIdx]
    if(predicted_class == gt_class):
      nTotalSuccess += 1
    else:
      print("GT: ", gt_class, "; Pred: ", predicted_class, "; Probabilitires: ", arrPredictedProbabilities[0], ", ", arrPredictedProbabilities[1])
      img = loadImage(join(working_path, "images/", gt_class, test_file_names[i]))#, 0)
      plt.imshow(img)
      plt.show()

  nSuccess = nTotalSuccess / (i+1)

  return nSuccess	

Finally, here is the function that STARTS the training. It has somewhat confusing name "test":

def test(EPOCHS, nL2, optimizer, learning_rate, bCumulativeLearning):
  global embedding_model
  
  embedding_model = trainNetwork(EPOCHS, nL2, optimizer, bCumulativeLearning)
  print("loading best model")
  embedding_model = loadModel(embedding_model, True)

  test_preds, test_file_names, test_class_names = getAllTestImages()

  # print("test_preds[0], test_file_names[0], test_class_names[0]: ", test_preds[0], test_file_names[0], test_class_names[0])

  nSuccess = getAccuracy(test_preds, test_file_names, test_class_names)

  print(">>> Accuracy on test set:", nSuccess, "<<<")	

We can now call this function and therefore start training:

opt = tf.keras.optimizers.Adam(0.0002) ##Adamax(lr=0.0001, beta_1=0.9, beta_2=0.999, epsilon=None, decay=0.0)
nL2 = 0.4

if(bDoTraining):
  EPOCHS = 50
  learning_rate=0.001

  np.random.seed(7)
  test(EPOCHS, nL2, opt, learning_rate, bCumulativeLearning=False)

  embedding_model = loadModel(embedding_model, True)
  embedding_model.save(best_weights_filepath)    # A full model is saved

After training is complete, we can run predictions on all test data:

if(bDoTraining):
  nClassIdx = np.random.randint(len(arrLabeledData))
  cls = arrLabeledData[nClassIdx]

  nMinIdx, nMaxIdx = getClassMinMax(cls, False)
  nImageIdx = random.randint(nMinIdx, nMaxIdx)

  for i, nImageIdx in enumerate(range(nMinIdx, nMaxIdx)):
    print(i+1, "of", nMaxIdx - nMinIdx)
    img = loadImage(join(working_path, "images/", arrLabeledData[nClassIdx]['class'], arrLabeledData[nClassIdx]['image_names'][nImageIdx]))#, 0)

    arrImg = img_to_array(img)
    arrImg = np.array(arrImg, dtype="float32")  

    # ---

    test_preds = embedding_model.predict(arrImg.reshape(1, IMAGE_SIZE, IMAGE_SIZE, 3))

    nIdx = test_preds.argmax()
    if(nClassIdx != nIdx):
      print("GT: ", arrLabeledData[nClassIdx]['class'], "; Pred: ", arrClasses[nIdx])
      plt.imshow(img)
      plt.show()	

Ok, our model is trained and tested on a test data set. Now we can actually USE it: we can load a HUGE set of images and classify them (see comments in code). Note that this code is written to work with image file names convention from previous step:

# Same as above in "test" section, but this time we process images from output folder
# The "/content/drive/My Drive/01_Output/" is the output of the previous step, remember, we goi pairs of images, and 
# now need to figure which ones are avers and which ones are revers?

images_source_path = "/content/drive/My Drive/01_Output/"

# We will save images by new names (with "head" or "tail" suffix) in this folder
images_dest_path = working_path + "images_processed/"

arrSourceImageNames = [f for f in listdir(images_source_path) if isfile(join(images_source_path, f))]

# Create model and load its weights (ones we got during training)
embedding_model = createModel(nL2, opt)
embedding_model = loadModel(embedding_model, True)

# Dictionary will store image names and counter: see below for details
dictNames = {}

nTotal = len(arrSourceImageNames)
for i, file_name in enumerate(arrSourceImageNames):

  image_path = join(images_source_path, file_name)
  img = loadImage(image_path)

  arrImg = img_to_array(img)
  arrImg = np.array(arrImg, dtype="float32")  

  # ---

  # For image, predict its class
  test_preds = embedding_model.predict(arrImg.reshape(1, IMAGE_SIZE, IMAGE_SIZE, 3))

  nIdx = test_preds.argmax()
  #print(i+1, "of", nTotal, ": ", arrClasses[nIdx])
  #plt.imshow(img)
  #plt.show()

  # Split image name
  word_list = file_name.split(".")  # ['0_000_00', 'png']
  image_name = word_list[0]
  image_ext = word_list[1]

  plt.imsave(images_dest_path + image_name + "_" + arrClasses[nIdx] + ".png", img)

  # Now we need to move source file to trash, but make it zero size first so it doesn't take space there
  open(image_path, 'w').close() #overwrite and make the file blank instead
  os.remove(image_path)

  if(i%100 == 0):
    print(i, " of ", nTotal)

  # File names look like 123496110_07_03.
  # Here 123496110 is the file root name, 07 is number of a coin in that image (some images contain >1 coins), and 03 is a number of images of that coin 
  #    (Say, we have 169860023_000.jpg, 169860023_001.jpg, 169860023_002.jpg, one coin per image, tail-tail-head. Then at step 1 we will get
  #    169860023_00_00, 169860023_00_01, and 169860023_00_02) 
  # We append _head or _tail: 169860023_00_00_tail(.png)
  # In dictNames we keep pairs 169860023_00 + flag. Flag == 0 if no heads, no tails, 1 if heads / no tails, 2 if tails / no heads and 3 if has both
  # When scanning is complete, we delete files that have flag != 3    

  arrImageNameParts = image_name.split("_")  # ['169860023', '000', '00']
  # We do not need "000" here, as it is just number of an image in a group of images for that coin. We need name (169860023) of course, plus 
  # number of a coin (00)
  coin_name = arrImageNameParts[0] + "_" + arrImageNameParts[2]

  if(arrClasses[nIdx] == "head"):
      if coin_name in dictNames:
        dictNames[coin_name] = dictNames[coin_name] | 1
      else:
        dictNames[coin_name] = 1
  else:
      if coin_name in dictNames:
        dictNames[coin_name] = dictNames[coin_name] | 2
      else:
        dictNames[coin_name] = 2

#print(dictNames)

# Now we need to delete all files for which dictNames[coin_name] != 3
print("Deleting files that do not have both head and tail")

nDeleted = 0
for i, file_name in enumerate(arrSourceImageNames):
  image_path = join(images_dest_path, file_name)
  
  word_list = file_name.split(".")  # ['0_000_00', 'png']
  
  image_name = word_list[0]
  image_ext = word_list[1]  

  arrImageNameParts = image_name.split("_")  # ['169860023', '000', '00']
  # We do not need "000" here, as it is just number of an image in a group of images for that coin. We need name (169860023) of course, plus 
  # number of a coin (00)
  coin_name = arrImageNameParts[0] + "_" + arrImageNameParts[2]  

  if (coin_name not in dictNames) or (dictNames[coin_name] != 3):
    open(image_path, 'w').close() #overwrite and make the file blank instead
    os.remove(image_path)

  if(i%100 == 0):
    print(i, " of ", nTotal)

print("Deleted", nDeleted)	

As the result, we have file names with "_head" or "_tail" suffix, and coins that have no pair are removed.