IMG_EXTENSIONS = ['.jpg', '.jpeg', '.png', '.ppm', '.bmp', '.pgm', '.tif', '.tiff', 'webp']

class ImageFolder(DatasetFolder):
 """A generic data loader where the images are arranged in this way: ::

  root/dog/xxx.png
  root/dog/xxy.png
  root/dog/xxz.png

  root/cat/123.png
  root/cat/nsdf3.png
  root/cat/asd932_.png

 Args:
  root (string): Root directory path.
  transform (callable, optional): A function/transform that takes in an PIL image
   and returns a transformed version. E.g, ``transforms.RandomCrop``
  target_transform (callable, optional): A function/transform that takes in the
   target and transforms it.
  loader (callable, optional): A function to load an image given its path.

  Attributes:
  classes (list): List of the class names.
  class_to_idx (dict): Dict with items (class_name, class_index).
  imgs (list): List of (image path, class_index) tuples
 """
 def __init__(self, root, transform=None, target_transform=None,
     loader=default_loader):
  super(ImageFolder, self).__init__(root, loader, IMG_EXTENSIONS,
           transform=transform,
           target_transform=target_transform)
  self.imgs = self.samples

def has_file_allowed_extension(filename, extensions):
 """查看文件是否是支持的可扩展类型

 Args:
  filename (string): 文件路径
  extensions (iterable of strings): 可扩展类型列表，即能接受的图像文件类型

 Returns:
  bool: True if the filename ends with one of given extensions
 """
 filename_lower = filename.lower()
 return any(filename_lower.endswith(ext) for ext in extensions) # 返回True或False列表


def make_dataset(dir, class_to_idx, extensions):
 """
  返回形如[(图像路径, 该图像对应的类别索引值),(),...]
 """
 images = []
 dir = os.path.expanduser(dir)
 for target in sorted(class_to_idx.keys()):
  d = os.path.join(dir, target)
  if not os.path.isdir(d):
   continue

  for root, _, fnames in sorted(os.walk(d)): #层层遍历文件夹，返回当前文件夹路径，存在的所有文件夹名，存在的所有文件名
   for fname in sorted(fnames):
    if has_file_allowed_extension(fname, extensions):查看文件是否是支持的可扩展类型，是则继续
     path = os.path.join(root, fname)
     item = (path, class_to_idx[target])
     images.append(item)

 return images

class DatasetFolder(data.Dataset):
 """A generic data loader where the samples are arranged in this way: ::

  root/class_x/xxx.ext
  root/class_x/xxy.ext
  root/class_x/xxz.ext

  root/class_y/123.ext
  root/class_y/nsdf3.ext
  root/class_y/asd932_.ext

 Args:
  root (string): 根目录路径
  loader (callable): 根据给定的路径来加载样本的可调用函数
  extensions (list[string]): 可扩展类型列表，即能接受的图像文件类型.
  transform (callable, optional): 用于样本的transform函数，然后返回样本transform后的版本
   E.g, ``transforms.RandomCrop`` for images.
  target_transform (callable, optional): 用于样本标签的transform函数

  Attributes:
  classes (list): 类别名列表
  class_to_idx (dict): 项目(class_name, class_index)字典,如{'cat': 0, 'dog': 1}
  samples (list): (sample path, class_index) 元组列表，即(样本路径, 类别索引)
  targets (list): 在数据集中每张图片的类索引值，为列表
 """

 def __init__(self, root, loader, extensions, transform=None, target_transform=None):
  classes, class_to_idx = self._find_classes(root) # 得到类名和类索引，如['cat', 'dog']和{'cat': 0, 'dog': 1}
  # 返回形如[(图像路径, 该图像对应的类别索引值),(),...]，即对每个图像进行标记
  samples = make_dataset(root, class_to_idx, extensions) 
  if len(samples) == 0:
   raise(RuntimeError("Found 0 files in subfolders of: " + root + "\n"
        "Supported extensions are: " + ",".join(extensions)))

  self.root = root
  self.loader = loader
  self.extensions = extensions

  self.classes = classes
  self.class_to_idx = class_to_idx
  self.samples = samples
  self.targets = [s[1] for s in samples] #所有图像的类索引值组成的列表

  self.transform = transform
  self.target_transform = target_transform

 def _find_classes(self, dir):
  """
  在数据集中查找类文件夹。

  Args:
   dir (string): 根目录路径

  Returns:
   返回元组: (classes, class_to_idx)即(类名, 类索引)，其中classes即相应的目录名，如['cat', 'dog'];class_to_idx为形如{类名:类索引}的字典，如{'cat': 0, 'dog': 1}.

  Ensures:
   保证没有类名是另一个类目录的子目录
  """
  if sys.version_info >= (3, 5):
   # Faster and available in Python 3.5 and above
   classes = [d.name for d in os.scandir(dir) if d.is_dir()] #获得根目录dir的所有第一层子目录名
  else:
   classes = [d for d in os.listdir(dir) if os.path.isdir(os.path.join(dir, d))] #效果和上面的一样，只是版本不同方法不同
  classes.sort() #然后对类名进行排序
  class_to_idx = {classes[i]: i for i in range(len(classes))} #然后将类名和索引值一一对应的到相应字典，如{'cat': 0, 'dog': 1}
  return classes, class_to_idx #然后返回类名和类索引

 def __getitem__(self, index):
  """
  Args:
   index (int): Index

  Returns:
   tuple: (sample, target) where target is class_index of the target class.
  """
  path, target = self.samples[index]
  sample = self.loader(path) # 加载图片
  if self.transform is not None:
   sample = self.transform(sample)
  if self.target_transform is not None:
   target = self.target_transform(target)

  return sample, target

 def __len__(self):
  return len(self.samples)

 def __repr__(self):
  fmt_str = 'Dataset ' + self.__class__.__name__ + '\n'
  fmt_str += ' Number of datapoints: {}\n'.format(self.__len__())
  fmt_str += ' Root Location: {}\n'.format(self.root)
  tmp = ' Transforms (if any): '
  fmt_str += '{0}{1}\n'.format(tmp, self.transform.__repr__().replace('\n', '\n' + ' ' * len(tmp)))
  tmp = ' Target Transforms (if any): '
  fmt_str += '{0}{1}'.format(tmp, self.target_transform.__repr__().replace('\n', '\n' + ' ' * len(tmp)))
  return fmt_str

class CustomImageFolder(ImageFolder):
 """
  为了得到两张图(其中一张是随机选取的)的图像和索引值信息
 """
 def __init__(self, root, transform=None):
  super(CustomImageFolder, self).__init__(root, transform)
  self.indices = range(len(self)) #该文件夹中的长度

 def __getitem__(self, index1):
  index2 = random.choice(self.indices) #从[0,indices]中随机抽取一个数字,为了随机选取一张图

  path1 = self.imgs[index1][0] #此时的self.imgs等于self.samples，即内容为[(图像路径, 该图像对应的类别索引值),(),...]
  label1 = self.imgs[index1][1]
  path2 = self.imgs[index2][0]
  label2 = self.imgs[index2][1]

  img1 = self.loader(path1)
  img2 = self.loader(path2)
  if self.transform is not None:
   img1 = self.transform(img1)
   img2 = self.transform(img2)

  return img1, img2, label1, label2