Running through a dataloader in Pytorch using Google Colab - python-3.x

Question

I am trying to use Pytorch to run classification on a dataset of images of cats and dogs. In my code I am so far downloading the data and going into the folder train which has two folders in it called "cats" and "dogs." I am then trying to load this data into a dataloader and iterate through batches, but it is giving me some error I don't understand in the iteration step.
Since it is Google Colabs I have code in there for downloading data and installing libraries. Any other advice on my code so far would be appreciated as well.
!pip install torch
!pip install torchvision
from __future__ import print_function, division
import os
import torch
import pandas as pd
import numpy as np
# For showing and formatting images
import matplotlib.pyplot as plt
import matplotlib.image as mpimg
# For importing datasets into pytorch
import torchvision.datasets as dataset
# Used for dataloaders
import torch.utils.data as data
# For pretrained resnet34 model
import torchvision.models as models
# For optimisation function
import torch.nn as nn
import torch.optim as optim
!wget http://files.fast.ai/data/dogscats.zip
!unzip dogscats.zip
batch_size = 256
train_raw = dataset.ImageFolder(PATH+"train", transform=transforms.ToTensor())
train_loader = data.DataLoader(train_raw, batch_size=batch_size, shuffle=True)
for batch_idx, (data, target) in enumerate(train_loader):
print("Data: ", batch_idx)
The error comes up on the last lines and is below:
RuntimeErrorTraceback (most recent call last)
<ipython-input-66-c32dd0c1b880> in <module>()
----> 1 for batch_idx, (data, target) in enumerate(train_loader):
2 print("Data: ", batch_idx)
3
/usr/local/lib/python2.7/dist-packages/torch/utils/data/dataloader.pyc in __next__(self)
257 if self.num_workers == 0: # same-process loading
258 indices = next(self.sample_iter) # may raise StopIteration
--> 259 batch = self.collate_fn([self.dataset[i] for i in indices])
260 if self.pin_memory:
261 batch = pin_memory_batch(batch)
/usr/local/lib/python2.7/dist-packages/torch/utils/data/dataloader.pyc in default_collate(batch)
133 elif isinstance(batch[0], collections.Sequence):
134 transposed = zip(*batch)
--> 135 return [default_collate(samples) for samples in transposed]
136
137 raise TypeError((error_msg.format(type(batch[0]))))
/usr/local/lib/python2.7/dist-packages/torch/utils/data/dataloader.pyc in default_collate(batch)
110 storage = batch[0].storage()._new_shared(numel)
111 out = batch[0].new(storage)
--> 112 return torch.stack(batch, 0, out=out)
113 elif elem_type.__module__ == 'numpy' and elem_type.__name__ != 'str_' \
114 and elem_type.__name__ != 'string_':
/usr/local/lib/python2.7/dist-packages/torch/functional.pyc in stack(sequence, dim, out)
62 inputs = [t.unsqueeze(dim) for t in sequence]
63 if out is None:
---> 64 return torch.cat(inputs, dim)
65 else:
66 return torch.cat(inputs, dim, out=out)
RuntimeError: invalid argument 0: Sizes of tensors must match except in dimension 0. Got 400 and 487 in dimension 2 at /pytorch/torch/lib/TH/generic/THTensorMath.c:2897
Thanks

Answer 1

I think the main problem was images being of different size . I may have understood ImageFolder in other way but, i think you don't need labels for images if the directory structure is as specified in pytorch and pytorch will figure out the labels for you.
I would also add more things to your transform that automatically resizes every images from the folder such as:
normalize = transforms.Normalize(
mean=[0.485, 0.456, 0.406],
std=[0.229, 0.224, 0.225]
)
transform = transforms.Compose(
[transforms.ToTensor(),transforms.Resize((224,224)),
normalize])
Also you can use other tricks to make your DataLoader much faster such as adding batch_size and number of cpu workers such as:
testloader = DataLoader(testset, batch_size=16,
shuffle=False, num_workers=4)
I think this will make you pipeline much faster.

Answer 2

I see two problems in your code first you are importing import torch.utils.data as data and again replacing that in the data loader. Please keep the imported module and your variable name in separate namespace. I think this error could be because of different sizes of data returned by dataloder(images) and labels. As you can see there is an error in concatenation because the first dimension ie. the label size and number of images in folder do not match. Hope this helps.

Answer 3

I think I was wrong in my comment to Manoj Acharya, the problem was in the batch_size being put into the dataloader. I read the below source and it seems you can't batch images together with different sizes:
https://medium.com/#yvanscher/pytorch-tip-yielding-image-sizes-6a776eb4115b
So in my code after changing the data variable Manoj points out I changed the batch_size to 1 and the program stopped failing. I want to put it in batches though so I added a further transform CenterCrop() to resize all images to the same size. Below is my new code:
!pip install torch
!pip install torchvision
from __future__ import print_function, division
import os
import torch
import pandas as pd
import numpy as np
# For showing and formatting images
import matplotlib.pyplot as plt
import matplotlib.image as mpimg
# For importing datasets into pytorch
import torchvision.datasets as dataset
# Used for dataloaders
from torch.utils.data import DataLoader
# For pretrained resnet34 model
import torchvision.models as models
# For optimisation function
import torch.nn as nn
import torch.optim as optim
# For turning data into tensors
import torchvision.transforms as transforms
!wget http://files.fast.ai/data/dogscats.zip
!unzip dogscats.zip
batch_size = 256
sz = 224
train_raw = dataset.ImageFolder(PATH+"train", transform=transforms.Compose([transforms.CenterCrop(sz),transforms.ToTensor()]))
train_loader = DataLoader(train_raw,batch_size=batch_size, shuffle=True)
for batch_idx, (data, target) in enumerate(train_loader):
print("Data: ", batch_idx)
Thanks