What is the "could not find answer" warning in squad.py
Hello, 

I am trying to run run_squad.py for BERT (italian-cased) with an italian version of squad.

During the creation of features from dataset, I got some answer skipped like in the following: 
<img width="478" alt="Screenshot 2019-12-30 at 23 30 19" src="https://user-images.githubusercontent.com/26765504/71603304-81081e80-2b5c-11ea-8333-73608e3141a7.png">

Can you tell why is this happening and if this influences the overall accuracy of the training?

This means that the script that converts the examples to features can't find the answers it should be finding. Rather than trying to predict those, it ignores them.

This means that these examples won't be used for training, reducing the total number of examples that will be used. If it is a small portion of the total number of examples, it shouldn't impact the resulting accuracy much. If it is a significant portion of the examples then it would be a good idea to look into it to see if there's a quick fix.<|||||>Hi @LysandreJik, thanks for the clarification. I noticed that for some of my data it happens that the the "text" field in "answers" field may differ from the one present in the "context" just because of some upper/lower letters mismatch. Do you think this could be avoided by using an uncased model?<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
<|||||>@antocapp I had your same logs with a cased model. Now I'm using an uncased model, putting the flag `--do_lower_case` in [run_squad.py](https://github.com/huggingface/transformers/blob/master/examples/question-answering/run_squad.py) I expected not to have those warnings, instead they appeared anyway. 

I took a look in the doc and I saw that in [run_squad.py](https://github.com/huggingface/transformers/blob/master/examples/question-answering/run_squad.py), examples are passed to the function `squad_convert_examples_to_features` [in this line](https://github.com/huggingface/transformers/blob/master/examples/question-answering/run_squad.py#L448).

```python
features, dataset = squad_convert_examples_to_features(
            examples=examples,
            tokenizer=tokenizer,
            max_seq_length=args.max_seq_length,
            doc_stride=args.doc_stride,
            max_query_length=args.max_query_length,
            is_training=not evaluate,
            return_dataset="pt",
            threads=args.threads,
        )
```

The tokenizer is created passing the argument `--do_lower_case` so it should tokenize putting the lower case to every token. Anyway the warning you see comes within [squad_convert_example_to_features](https://github.com/huggingface/transformers/blob/930153e7d2d658267b7630a047a4bfc85b86042d/src/transformers/data/processors/squad.py#L91) declaration.

```python
def squad_convert_example_to_features(
    example, max_seq_length, doc_stride, max_query_length, padding_strategy, is_training
):
    features = []
    if is_training and not example.is_impossible:
        # Get start and end position
        start_position = example.start_position
        end_position = example.end_position

        # If the answer cannot be found in the text, then skip this example.
        actual_text = " ".join(example.doc_tokens[start_position : (end_position + 1)])
        cleaned_answer_text = " ".join(whitespace_tokenize(example.answer_text))
        if actual_text.find(cleaned_answer_text) == -1:
            logger.warning("Could not find answer: '%s' vs. '%s'", actual_text, cleaned_answer_text)
            return []

    tok_to_orig_index = []
    orig_to_tok_index = []
    all_doc_tokens = []
    for (i, token) in enumerate(example.doc_tokens):
        orig_to_tok_index.append(len(all_doc_tokens))
        sub_tokens = tokenizer.tokenize(token)
        for sub_token in sub_tokens:
            tok_to_orig_index.append(i)
            all_doc_tokens.append(sub_token)
# code continues...
```

As you can see `actual_text` and `cleaned_answer_text` use `example.doc_tokens` and `example.answer_text` which already contain upper_case! `cleaned_answer_text` is searched within `actual_text` considering upper-case letters different from lower_case letters, so an example like _'Mantenere i precetti' vs 'mantenere i precetti'_ (like you told in the issue) would be discarded. Indeed the `tokenizer` hasn't tokenized yet in those lines so, even if the features could be created with lower_case, that check makes that example to be discardes, even if it could be considered!

So what I made, is putting a `lower()` on every field of example before passing it to that function, changin [run_squad.py](https://github.com/huggingface/transformers/blob/master/examples/question-answering/run_squad.py) in this way:
```python
# other code...
else:
            processor = SquadV2Processor() if args.version_2_with_negative else SquadV1Processor()
            if evaluate:
                examples = processor.get_dev_examples(args.data_dir, filename=args.predict_file)
            else:
                examples = processor.get_train_examples(args.data_dir, filename=args.train_file)
        
        if args.do_lower_case:
            logger.info("Putting lower case to examples...")
            for example in examples:
                example.doc_tokens = [token.lower() for token in example.doc_tokens]
                example.question_text = example.question_text.lower()
                example.context_text = example.context_text.lower()
                if example.answer_text is not None: # for dev set
                    example.answer_text = example.answer_text.lower()
            
        features, dataset = squad_convert_examples_to_features(
            examples=examples,
            tokenizer=tokenizer,
            max_seq_length=args.max_seq_length,
            doc_stride=args.doc_stride,
            max_query_length=args.max_query_length,
            is_training=not evaluate,
            return_dataset="pt",
            threads=args.threads,
        )
```

I don't know if this can improve the results, but it avoids some discarded examples for sure 😊
@LysandreJik, is this a bug 🐛 ? Or maybe was there another trivial method to fix this?<|||||>Hi @paulthemagno, lowering every example was the same thing I did to solve the warning, although I lowered the dataset instead of editing the run_squad.py; but it is indeed the same thing.

I uploaded just toady a model on the HF model hub (https://huggingface.co/antoniocappiello/bert-base-italian-uncased-squad-it).

This was trained based on `dbmdz/bert-base-italian-uncased`; I tried also a training with Musixmatch Umberto but the F1 and EM were slightly lower (like 1 point percentage lower). 

But maybe running several experiments with different hyperparameters could lead to better results. 