pytorch数据集对象如何知道在for循环中使用时是否已到达终点？

Question

我正在编写一个定制的pytorch数据集。在__init__中，dataset对象加载包含特定数据的文件。但在我的程序中，我只希望访问部分数据（如果有帮助的话，实现训练/有效切割）。起初我认为这种行为是通过重写__len__来控制的，但结果证明修改__len__没有帮助。一个简单的例子如下：

from torch.utils.data import Dataset, DataLoader
import torch

class NewDS(Dataset):
    def __init__(self):
        self.data = torch.randn(10,2) # suppose there are 10 items in the data file
    
    def __len__(self):
        return len(self.data)-5 # But I only want to access the first 5 items
        
    def __getitem__(self, index):
        return self.data[index]

ds = NewDS()
for i, x in enumerate(ds):
    print(i)

输出为0到9，而期望的行为为0到4

在这样的for循环中使用此dataset对象时，该对象如何知道枚举已结束？任何其他达到类似效果的方法都是受欢迎的

Answer 1

您正在使用Dataset类创建自定义数据加载器，同时使用for循环枚举它。这不是它的工作原理。对于枚举，必须将Dataset传递给DataLoader类。你的代码会像这样工作的很好

from torch.utils.data import Dataset, DataLoader
import torch

class NewDS(Dataset):
    def __init__(self):
        self.data = torch.randn(10,2) # suppose there are 10 items in the data file
    
    def __len__(self):
        return len(self.data)-5 # But I only want to access the first 5 items
        
    def __getitem__(self, index):
        return self.data[index]

ds = NewDS()
for i, x in range(len(ds)): #if you do dont want to use DataLoader, then dont use enumerate
    print(i, ds[i])
#output 
tensor([-0.2351,  1.3037])
tensor([ 0.4032, -0.2739])
tensor([-0.5687, -0.7300])
tensor([0.5418, 0.8572])
tensor([ 1.9973, -0.2939])

dl = DataLoader(ds, batch_size=1) # pass the ds object to DataLoader 

for i, x in enumerate(dl): # now you can use enumarate
    print(i, x)
#output
tensor([-0.2351,  1.3037])
tensor([ 0.4032, -0.2739])
tensor([-0.5687, -0.7300])
tensor([0.5418, 0.8572])
tensor([ 1.9973, -0.2939])

更多详情可在本官方网站pytorch tutorial上阅读

Answer 2

您可以使用^{}获取数据的子集

top_five = torch.utils.data.Subset(ds, indices=range(5))  # Get first five items
for i, x in enumerate(top_five):
    print(i)
0
1
2
3
4

---

循环中的^{}将返回项，直到它获得^{}异常

len(ds)         # Returned modified length
5

# `enumerate` will call `next` method on iterable each time in loop.
#  and When no more data available a StopIteration exception is raised instead.
iter_ds = iter(ds)
print(next(iter_ds))
print(next(iter_ds))
print(next(iter_ds))
print(next(iter_ds))
print(next(iter_ds))
print(next(iter_ds))
print(next(iter_ds))
print(next(iter_ds))
print(next(iter_ds))
print(next(iter_ds))

print(next(iter_ds))  #11th time StopIteration exception raised as no item left to iterate in iterable

输出：

tensor([-1.5952, -0.0826])
tensor([-2.2254,  0.2461])
tensor([-0.8268,  0.1956])
tensor([ 0.3157, -0.3403])
tensor([0.8971, 1.1255])
tensor([0.3922, 1.3184])
tensor([-0.4311, -0.8898])
tensor([ 0.1128, -0.5708])
tensor([-0.5403, -0.9036])
tensor([0.6550, 1.6777])

                                     -
StopIteration                             Traceback (most recent call last)
<ipython-input-99-7a9910e027c3> in <module>
     10 print(next(iter_ds))
     11 
 -> 12 print(next(iter_ds))  #11th time StopIteration exception raised as no item left to iterate

StopIteration: