PyTorch：数据读取2 - Dataloader

蔚落 2022-12-05 15:08 239阅读 0赞

[\-柚子皮-][-_-]

# nlp中的dataloader的使用 #

`torch.utils.data.DataLoader`中的参数：

*  dataset ([*Dataset*][Dataset]) – dataset from which to load the data.
 *  batch\_size ([*int*][int], *optional*) – how many samples per batch to load (default: 1).
 *  shuffle ([*bool*][bool], *optional*) – set to `True` to have the data reshuffled at every epoch (default: False).
 *  sampler ([*Sampler*][Sampler], *optional*) – defines the strategy to draw samples from the dataset. If specified, `shuffle` must be False.
 *  batch\_sampler ([*Sampler*][Sampler], *optional*) – like sampler, but returns a batch of indices at a time. Mutually exclusive with batch\_size, shuffle, sampler, and drop\_last.
 *  num\_workers ([*int*][int], *optional*) – how many subprocesses to use for data loading. 0 means that the data will be loaded in the main process. (default: 0)
 *  collate\_fn (*callable*\*, \**optional*) – merges a list of samples to form a mini-batch.
 *  pin\_memory ([*bool*][bool], *optional*) – If `True`, the data loader will copy tensors into CUDA pinned memory before returning them.
 *  drop\_last ([*bool*][bool], *optional*) – set to `True` to drop the last incomplete batch, if the dataset size is not divisible by the batch size. If `False` and the size of dataset is not divisible by the batch size, then the last batch will be smaller. (default: False)
 *  timeout (*numeric*, *optional*) – if positive, the timeout value for collecting a batch from workers. Should always be non-negative. (default: 0)
 *  worker\_init\_fn (*callable*, *optional*) – If not None, this will be called on each worker subprocess with the worker id (an int in `[0, num_workers - 1]`) as input, after seeding and before data loading. (default: None)

### 返回值 ###

返回值是一个实现了\_\_iter\_\_的对象，可以使用for循环进行迭代，或者转换成迭代器取第一条batch数据查看。

for循环进行迭代时返回的每条数据就是(batch\_size,\*)大小的。

### 常用操作 ###

**使用示例**

self.data\_loader = torch.utils.data.DataLoader(  
            dataset=self.dataset, collate\_fn=self.collate\_fn,  
            batch\_size=batch\_size, shuffle=if\_shuffle, num\_workers=args.num\_workers)

**batch数目**

batch\_num = len(train\_dataloader)

**获取dataset中的第一条数据**

train\_data\_loader.dataset.\_\_getitem\_\_(0)

获取dataloader中batch中的第一条数据

def get\_one\_data(item\_dict, i):  
    return \{k: v\[i\] for k, v in item\_dict.items()\}

print(get\_one\_data(next(iter(train\_data\_loader)), 1))

或者

for index, item\_dict in enumerate(train\_data\_loader):  
        print(get\_one\_data(item\_dict, 1))  
        exit()

# 自定义dataloader #

Dataloader的处理逻辑是先通过Dataset类里面的 `__getitem__` 函数获取单个数据，然后组合成batch，再使用collate\_fn所指定的函数对这个batch做一些操作（比如每个batch中实际lengths，padding，cuda之类的）。

## 自定义collate\_fn ##

因为`dataloader`是有`batch_size`参数的，我们可以通过自定义`collate_fn=myfunction`来设计数据收集的方式，意思是已经通过上面的`Dataset`类中的`__getitem__`函数采样了`batch_size`数据，以一个包的形式传递给`collate_fn`所指定的函数。

示例1：通过`collate_fn`进行解包

def myfunction(data):
        A,B,path,hop=zip(*data)
        print('A:',A," B:",B," path:",path," hop:",hop)
        raise Exception('utils collate_fun 147')
        return A,B,path,hop

for index,item in enumerate(dataloaders['train'])
        A,B,path.hop=item

Note: 需要在外面对dataloaders进行for调用，后再断点或者exit()，否则不会真正执行collate\_fn，这样就不会print了。  
`示例2：nlp`任务中，经常在`collate_fn`指定的函数里面做padding，将同一个batch中不一样长的句子padding成一样长。

def myfunction(data):
        src, tgt, original_src, original_tgt = zip(*data)
    
        src_len = [len(s) for s in src]
        src_pad = torch.zeros(len(src), max(src_len)).long()
        for i, s in enumerate(src):
            end = src_len[i]
            src_pad[i, :end] = torch.LongTensor(s[end-1::-1])
    
        tgt_len = [len(s) for s in tgt]
        tgt_pad = torch.zeros(len(tgt), max(tgt_len)).long()
        for i, s in enumerate(tgt):
            end = tgt_len[i]
            tgt_pad[i, :end] = torch.LongTensor(s)[:end]
    
        return src_pad, tgt_pad, \
               torch.LongTensor(src_len), torch.LongTensor(tgt_len), \
               original_src, original_tgt

### 一些问题 ###

\[[为什么pytorch DataLoader在numpy数组和列表上的行为有所不同？][pytorch DataLoader_numpy]\]

**1 import问题**

使用torch.utils.data.DataLoader时，pycharm中无法直接点击进入代码。

**2 num\_workers设置过大问题**

num\_workers如果设置过大，资源不够，会出错：Process finished with exit code 139 (interrupted by signal 11: SIGSEGV)  
\[[运行tensorflow程序bug：Process finished with exit code 139 (interrupted by signal 11: SIGSEGV)][tensorflow_bug_Process finished with exit code 139 _interrupted by signal 11_ SIGSEGV]\]

\[[python模块导入及属性：import][python_import]\]

\[[https://github.com/pytorch/pytorch/issues/41794][https_github.com_pytorch_pytorch_issues_41794]\]

from: [\-柚子皮-][-_-]

ref: \[[https://www.jianshu.com/p/8ea7fba72673][https_www.jianshu.com_p_8ea7fba72673]\]

[-_-]: https://me.csdn.net/pipisorry
[Dataset]: https://pytorch.org/docs/stable/data.html?highlight=dataloader#torch.utils.data.Dataset
[int]: https://docs.python.org/3/library/functions.html#int
[bool]: https://docs.python.org/3/library/functions.html#bool
[Sampler]: https://pytorch.org/docs/stable/data.html?highlight=dataloader#torch.utils.data.Sampler
[pytorch DataLoader_numpy]: https://www.thinbug.com/q/52818145
[tensorflow_bug_Process finished with exit code 139 _interrupted by signal 11_ SIGSEGV]: https://blog.csdn.net/firesolider/article/details/103290563?utm_medium=distribute.pc_relevant_t0.none-task-blog-searchFromBaidu-1.control&depth_1-utm_source=distribute.pc_relevant_t0.none-task-blog-searchFromBaidu-1.control
[python_import]: https://youzipi.blog.csdn.net/article/details/43313197
[https_github.com_pytorch_pytorch_issues_41794]: https://github.com/pytorch/pytorch/issues/41794
[https_www.jianshu.com_p_8ea7fba72673]: https://www.jianshu.com/p/8ea7fba72673