Hi. I am training a CNN model using torch text and getting a strange error when trying to perform evaluation on my dev set.
I have my data loaded into two CSV’s and have their location stored in pathD and test_path.
def twitter_data(text_field, label_field, **kargs):
    train_data = data.TabularDataset(
    path=pathD, format='csv',
    fields=[('twitter_text', text_field),
            ('label', label_field)])
    dev_data = data.TabularDataset(
    path=test_path, format='csv',
    fields=[('twitter_text', text_field),
            ('label', label_field)])

    
    text_field.build_vocab(train_data, dev_data, vectors=GloVe(name='twitter.27B', dim=200))
    label_field.build_vocab(train_data, dev_data)
    train_iter, dev_iter = data.Iterator.splits(
                                (train_data, dev_data), 
                                batch_sizes=(args.batch_size, len(dev_data)),
                                **kargs)
    return train_iter, dev_iter

text_field = data.Field(lower=True)
label_field = data.Field(sequential=False)
train_iter, dev_iter = twitter_data(text_field, label_field, device=-1, repeat=False)

This code works fine and returns an iter for both the train and dev set. When I train, I run the following code and all is dandy.

optimizer = torch.optim.Adam(model.parameters(), lr=args.lr, weight_decay = args.weight_decay)
scheduler = StepLR(optimizer, step_size = 5, gamma = .25)
model.train()
for epoch in range(1, args.epochs+1):
    scheduler.step()
    for batch in train_iter:
        ....

Though when I try to call my eval() function, I get an error on the last line here.

def eval(data_iter, model, args):
model.eval()
corrects, avg_loss = 0, 0
for batch in data_iter:    # error occurs here

The stack trace appears as follows.
for batch in data_iter:
File “/usr/local/lib/python3.5/dist-packages/torchtext/data/iterator.py”, line 164, in iter
self.init_epoch()
File “/usr/local/lib/python3.5/dist-packages/torchtext/data/iterator.py”, line 140, in init_epoch
self.create_batches()
File “/usr/local/lib/python3.5/dist-packages/torchtext/data/iterator.py”, line 151, in create_batches
self.batches = batch(self.data(), self.batch_size, self.batch_size_fn)
File “/usr/local/lib/python3.5/dist-packages/torchtext/data/iterator.py”, line 125, in data
xs = sorted(self.dataset, key=self.sort_key)
TypeError: unorderable types: Example() < Example()

The confusing thing is the error persists even when I cheat and use the same csv file for train and dev splits, so there isn’t an issue with the dev set file.