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这篇文章给大家分享的是有关Pytorch如何实现数据加载与数据预处理的内容。小编觉得挺实用的，因此分享给大家做个参考，一起跟随小编过来看看吧。
数据加载分为加载torchvision.datasets中的数据集以及加载自己使用的数据集两种情况。
torchvision.datasets中的数据集
torchvision.datasets中自带MNIST，Imagenet-12，CIFAR等数据集，所有的数据集都是torch.utils.data.Dataset的子类，都包含 _ _ len _ （获取数据集长度）和 _ getItem _ _ （获取数据集中每一项）两个子方法。
Dataset源码如上，可以看到其中包含了两个没有实现的子方法，之后所有的Dataet类都继承该类，并根据数据情况定制这两个子方法的具体实现。
因此当我们需要加载自己的数据集的时候也可以借鉴这种方法，只需要继承torch.utils.data.Dataset类并重写 init ,len,以及getitem这三个方法即可。这样组着的类可以直接作为参数传入到torch.util.data.DataLoader中去。
以CIFAR10为例 源码：
class torchvision.datasets.CIFAR10(root, train=True, transform=None, target_transform=None, download=False)
root (string) – Root directory of dataset where directory cifar-10-batches-py exists or will be saved to if download is set to True. train (bool, optional) – If True, creates dataset from training set, otherwise creates from test set. 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. download (bool, optional) – If true, downloads the dataset from the internet and puts it in root directory. If dataset is already downloaded, it is not downloaded again.
加载自己的数据集
对于torchvision.datasets中有两个不同的类，分别为DatasetFolder和ImageFolder,ImageFolder是继承自DatasetFolder。
下面我们通过源码来看一看folder文件中DatasetFolder和ImageFolder分别做了些什么
import torch.utils.data as data
from PIL import Image
import os
import os.path
def has_file_allowed_extension(filename, extensions): //检查输入是否是规定的扩展名
"""Checks if a file is an allowed extension.
Args:
filename (string): path to a file
Returns:
bool: True if the filename ends with a known image extension
"""
filename_lower = filename.lower()
return any(filename_lower.endswith(ext) for ext in extensions)
def find_classes(dir):
classes = [d for d in os.listdir(dir) if os.path.isdir(os.path.join(dir, d))] //获取root目录下所有的文件夹名称
classes.sort()
class_to_idx = {classes[i]: i for i in range(len(classes))} //生成类别名称与类别id的对应Dictionary
return classes, class_to_idx
def make_dataset(dir, class_to_idx, extensions):
images = []
dir = os.path.expanduser(dir)// 将~和~user转化为用户目录，对参数中出现~进行处理
for target in sorted(os.listdir(dir)):
d = os.path.join(dir, target)
if not os.path.isdir(d):
continue
for root, _, fnames in sorted(os.walk(d)): //os.work包含三个部分，root代表该目录路径 _代表该路径下的文件夹名称集合，fnames代表该路径下的文件名称集合
for fname in sorted(fnames):
if has_file_allowed_extension(fname, extensions):
path = os.path.join(root, fname)
item = (path, class_to_idx[target])
images.append(item) //生成（训练样本图像目录，训练样本所属类别）的元组
return images //返回上述元组的列表
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): Root directory path.
loader (callable): A function to load a sample given its path.
extensions (list[string]): A list of allowed extensions.
transform (callable, optional): A function/transform that takes in
a sample and returns a transformed version.
E.g, ``transforms.RandomCrop`` for images.
target_transform (callable, optional): A function/transform that takes
in the target and transforms it.
Attributes:
classes (list): List of the class names.
class_to_idx (dict): Dict with items (class_name, class_index).
samples (list): List of (sample path, class_index) tuples
"""
def __init__(self, root, loader, extensions, transform=None, target_transform=None):
classes, class_to_idx = find_classes(root)
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.transform = transform
self.target_transform = target_transform
def __getitem__(self, index):
"""
根据index获取sample 返回值为（sample，target）元组，同时如果该类输入参数中有transform和target_transform，torchvision.transforms类型的参数时，将获取的元组分别执行transform和target_transform中的数据转换方法。
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)
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): //定义输出对象格式 其中和__str__的区别是__repr__无论是print输出还是直接输出对象自身 都是以定义的格式进行输出，而__str__ 只有在print输出的时候会是以定义的格式进行输出
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
IMG_EXTENSIONS = ['.jpg', '.jpeg', '.png', '.ppm', '.bmp', '.pgm', '.tif']
def pil_loader(path):
# open path as file to avoid ResourceWarning (https://github.com/python-pillow/Pillow/issues/835)
with open(path, 'rb') as f:
img = Image.open(f)
return img.convert('RGB')
def accimage_loader(path):
import accimage
try:
return accimage.Image(path)
except IOError:
# Potentially a decoding problem, fall back to PIL.Image
return pil_loader(path)
def default_loader(path):
from torchvision import get_image_backend
if get_image_backend() == 'accimage':
return accimage_loader(path)
else:
return pil_loader(path)
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如果自己所要加载的数据组织形式如下
root/dog/xxx.png root/dog/xxy.png root/dog/xxz.png root/cat/123.png root/cat/nsdf3.png root/cat/asd932_.png
即不同类别的训练数据分别存储在不同的文件夹中，这些文件夹都在root（即形如 D:/animals 或者 /usr/animals ）路径下
class torchvision.datasets.ImageFolder(root, transform=None, target_transform=None, loader=<function default_loader>)
参数如下：
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 – A function to load an image given its path. 就是上述源码中 __getitem__(index) Parameters: index (int) – Index Returns: (sample, target) where target is class_index of the target class. Return type: tuple
可以通过torchvision.datasets.ImageFolder进行加载
img_data = torchvision.datasets.ImageFolder('D:/bnu/database/flower',
transform=transforms.Compose([
transforms.Scale(256),
transforms.CenterCrop(224),
transforms.ToTensor()])
)
print(len(img_data))
data_loader = torch.utils.data.DataLoader(img_data, batch_size=20,shuffle=True)
print(len(data_loader))对于所有的训练样本都在一个文件夹中 同时有一个对应的txt文件每一行分别是对应图像的路径以及其所属的类别，可以参照上述class写出对应的加载类
def default_loader(path):
return Image.open(path).convert('RGB')
class MyDataset(Dataset):
def __init__(self, txt, transform=None, target_transform=None, loader=default_loader):
fh = open(txt, 'r')
imgs = []
for line in fh:
line = line.strip('\n')
line = line.rstrip()
words = line.split()
imgs.append((words[0],int(words[1])))
self.imgs = imgs
self.transform = transform
self.target_transform = target_transform
self.loader = loader
def __getitem__(self, index):
fn, label = self.imgs[index]
img = self.loader(fn)
if self.transform is not None:
img = self.transform(img)
return img,label
def __len__(self):
return len(self.imgs)
train_data=MyDataset(txt='mnist_test.txt', transform=transforms.ToTensor())
data_loader = DataLoader(train_data, batch_size=100,shuffle=True)
print(len(data_loader))DataLoader解析
位于torch.util.data.DataLoader中 源代码
该接口的主要目的是将pytorch中已有的数据接口如torchvision.datasets.ImageFolder，或者自定义的数据读取接口转化按照
batch_size的大小封装为Tensor,即相当于在内置数据接口或者自定义数据接口的基础上增加一维，大小为batch_size的大小，
得到的数据在之后可以通过封装为Variable，作为模型的输出
_ _ init _ _中所需的参数如下
1. dataset torch.utils.data.Dataset类的子类，可以是torchvision.datasets.ImageFolder等内置类，也可是继承了torch.utils.data.Dataset的自定义类 2. batch_size 每一个batch中包含的样本个数，默认是1 3. shuffle 一般在训练集中采用，默认是false，设置为true则每一个epoch都会将训练样本打乱 4. sampler 训练样本选取策略，和shuffle是互斥的 如果 shuffle为true，该参数一定要为None 5. batch_sampler BatchSampler 一次产生一个 batch 的 indices，和sampler以及shuffle互斥，一般使用默认的即可 上述Sampler的源代码地址如下[源代码](https://github.com/pytorch/pytorch/blob/master/torch/utils/data/sampler.py) 6. num_workers 用于数据加载的线程数量 默认为0 即只有主线程用来加载数据 7. collate_fn 用来聚合数据生成mini_batch
使用的时候一般为如下使用方法：
train_data=torch.utils.data.DataLoader(...) for i, (input, target) in enumerate(train_data): ...
循环取DataLoader中的数据会触发类中_ _ iter __方法，查看源代码可知 其中调用的方法为 return _DataLoaderIter(self)，因此需要查看 DataLoaderIter 这一内部类
class DataLoaderIter(object):
"Iterates once over the DataLoader's dataset, as specified by the sampler"
def __init__(self, loader):
self.dataset = loader.dataset
self.collate_fn = loader.collate_fn
self.batch_sampler = loader.batch_sampler
self.num_workers = loader.num_workers
self.pin_memory = loader.pin_memory and torch.cuda.is_available()
self.timeout = loader.timeout
self.done_event = threading.Event()
self.sample_iter = iter(self.batch_sampler)
if self.num_workers > 0:
self.worker_init_fn = loader.worker_init_fn
self.index_queue = multiprocessing.SimpleQueue()
self.worker_result_queue = multiprocessing.SimpleQueue()
self.batches_outstanding = 0
self.worker_pids_set = False
self.shutdown = False
self.send_idx = 0
self.rcvd_idx = 0
self.reorder_dict = {}
base_seed = torch.LongTensor(1).random_()[0]
self.workers = [
multiprocessing.Process(
target=_worker_loop,
args=(self.dataset, self.index_queue, self.worker_result_queue, self.collate_fn,
base_seed + i, self.worker_init_fn, i))
for i in range(self.num_workers)]
if self.pin_memory or self.timeout > 0:
self.data_queue = queue.Queue()
self.worker_manager_thread = threading.Thread(
target=_worker_manager_loop,
args=(self.worker_result_queue, self.data_queue, self.done_event, self.pin_memory,
torch.cuda.current_device()))
self.worker_manager_thread.daemon = True
self.worker_manager_thread.start()
else:
self.data_queue = self.worker_result_queue
for w in self.workers:
w.daemon = True # ensure that the worker exits on process exit
w.start()
_update_worker_pids(id(self), tuple(w.pid for w in self.workers))
_set_SIGCHLD_handler()
self.worker_pids_set = True
# prime the prefetch loop
for _ in range(2 * self.num_workers):
self._put_indices()感谢各位的阅读！关于“Pytorch如何实现数据加载与数据预处理”这篇文章就分享到这里了，希望以上内容可以对大家有一定的帮助，让大家可以学到更多知识，如果觉得文章不错，可以把它分享出去让更多的人看到吧！
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