Title: Better cast error when generating dataset
State: closed

I want to improve the error message for datasets like https://huggingface.co/datasets/m-a-p/COIG-CQIA

Cc @albertvillanova @severo is this new error ok ? Or should I use a dedicated error class ?

New:

```python
Traceback (most recent call last):
  File "/Users/quentinlhoest/hf/datasets/src/datasets/builder.py", line 1920, in _prepare_split_single
    writer.write_table(table)
  File "/Users/quentinlhoest/hf/datasets/src/datasets/arrow_writer.py", line 574, in write_table
    pa_table = table_cast(pa_table, self._schema)
  File "/Users/quentinlhoest/hf/datasets/src/datasets/table.py", line 2322, in table_cast
    return cast_table_to_schema(table, schema)
  File "/Users/quentinlhoest/hf/datasets/src/datasets/table.py", line 2276, in cast_table_to_schema
    raise CastError(
datasets.table.CastError: Couldn't cast
instruction: string
other: string
index: string
domain: list<item: string>
  child 0, item: string
output: string
task_type: struct<major: list<item: string>, minor: list<item: string>>
  child 0, major: list<item: string>
      child 0, item: string
  child 1, minor: list<item: string>
      child 0, item: string
task_name_in_eng: string
input: string
to
{'answer_from': Value(dtype='string', id=None), 'instruction': Value(dtype='string', id=None), 'human_verified': Value(dtype='bool', id=None), 'domain': Sequence(feature=Value(dtype='string', id=None), length=-1, id=None), 'output': Value(dtype='string', id=None), 'task_type': {'major': Sequence(feature=Value(dtype='string', id=None), length=-1, id=None), 'minor': Sequence(feature=Value(dtype='string', id=None), length=-1, id=None)}, 'copyright': Value(dtype='string', id=None), 'input': Value(dtype='string', id=None)}
because column names don't match

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/quentinlhoest/hf/datasets/playground/ttest.py", line 74, in <module>
    load_dataset("m-a-p/COIG-CQIA")
  File "/Users/quentinlhoest/hf/datasets/src/datasets/load.py", line 2529, in load_dataset
    builder_instance.download_and_prepare(
  File "/Users/quentinlhoest/hf/datasets/src/datasets/builder.py", line 936, in download_and_prepare
    self._download_and_prepare(
  File "/Users/quentinlhoest/hf/datasets/src/datasets/builder.py", line 1031, in _download_and_prepare
    self._prepare_split(split_generator, **prepare_split_kwargs)
  File "/Users/quentinlhoest/hf/datasets/src/datasets/builder.py", line 1791, in _prepare_split
    for job_id, done, content in self._prepare_split_single(
  File "/Users/quentinlhoest/hf/datasets/src/datasets/builder.py", line 1922, in _prepare_split_single
    raise DatasetGenerationCastError.from_cast_error(
datasets.exceptions.DatasetGenerationCastError: An error occurred while generating the dataset

All the data files must have the same columns, but at some point there are 3 new columns (other, index, task_name_in_eng) and 3 missing columns (answer_from, copyright, human_verified).

This happened while the json dataset builder was generating data using

hf://datasets/m-a-p/COIG-CQIA/coig_pc/coig_pc_core_sample.json (at revision b7b7ecf290f6515036c7c04bd8537228ac2eb474)

Please either edit the data files to have matching columns, or separate them into different configurations (see docs at https://hf.co/docs/hub/datasets-manual-configuration#multiple-configurations)
```

Previously:

```python
Traceback (most recent call last):
  File "/Users/quentinlhoest/hf/datasets/src/datasets/builder.py", line 1931, in _prepare_split_single
    writer.write_table(table)
  File "/Users/quentinlhoest/hf/datasets/src/datasets/arrow_writer.py", line 574, in write_table
    pa_table = table_cast(pa_table, self._schema)
  File "/Users/quentinlhoest/hf/datasets/src/datasets/table.py", line 2295, in table_cast
    return cast_table_to_schema(table, schema)
  File "/Users/quentinlhoest/hf/datasets/src/datasets/table.py", line 2253, in cast_table_to_schema
    raise ValueError(f"Couldn't cast\n{table.schema}\nto\n{features}\nbecause column names don't match")
ValueError: Couldn't cast
task_type: struct<major: list<item: string>, minor: list<item: string>>
  child 0, major: list<item: string>
      child 0, item: string
  child 1, minor: list<item: string>
      child 0, item: string
other: string
instruction: string
task_name_in_eng: string
domain: list<item: string>
  child 0, item: string
index: string
output: string
input: string
to
{'human_verified': Value(dtype='bool', id=None), 'task_type': {'major': Sequence(feature=Value(dtype='string', id=None), length=-1, id=None), 'minor': Sequence(feature=Value(dtype='string', id=None), length=-1, id=None)}, 'answer_from': Value(dtype='string', id=None), 'copyright': Value(dtype='string', id=None), 'instruction': Value(dtype='string', id=None), 'domain': Sequence(feature=Value(dtype='string', id=None), length=-1, id=None), 'output': Value(dtype='string', id=None), 'input': Value(dtype='string', id=None)}
because column names don't match

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/quentinlhoest/hf/datasets/playground/ttest.py", line 74, in <module>
    load_dataset("m-a-p/COIG-CQIA")
  File "/Users/quentinlhoest/hf/datasets/src/datasets/load.py", line 2529, in load_dataset
    builder_instance.download_and_prepare(
  File "/Users/quentinlhoest/hf/datasets/src/datasets/builder.py", line 949, in download_and_prepare
    self._download_and_prepare(
  File "/Users/quentinlhoest/hf/datasets/src/datasets/builder.py", line 1044, in _download_and_prepare
    self._prepare_split(split_generator, **prepare_split_kwargs)
  File "/Users/quentinlhoest/hf/datasets/src/datasets/builder.py", line 1804, in _prepare_split
    for job_id, done, content in self._prepare_split_single(
  File "/Users/quentinlhoest/hf/datasets/src/datasets/builder.py", line 1949, in _prepare_split_single
    raise DatasetGenerationError("An error occurred while generating the dataset") from e
datasets.builder.DatasetGenerationError: An error occurred while generating the dataset
```

Comments: The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_6509). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.I created `DatatasetGenerationCastError` in `exceptions.py` that inherits from `DatasetGenerationError` (for backward compatibility) that inherits from `DatasetsError`.

I also added a help message at the end of the error:

```
Please either edit the data files to have matching columns, or separate them into different configurations (see docs at https://hf.co/docs/hub/datasets-manual-configuration#multiple-configurations)
```<details>
<summary>Show benchmarks</summary>

PyArrow==8.0.0

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.004991 / 0.011353 (-0.006361) | 0.003362 / 0.011008 (-0.007646) | 0.062093 / 0.038508 (0.023585) | 0.051533 / 0.023109 (0.028424) | 0.247508 / 0.275898 (-0.028390) | 0.275593 / 0.323480 (-0.047886) | 0.003828 / 0.007986 (-0.004158) | 0.002573 / 0.004328 (-0.001755) | 0.047727 / 0.004250 (0.043477) | 0.037029 / 0.037052 (-0.000023) | 0.250359 / 0.258489 (-0.008130) | 0.282640 / 0.293841 (-0.011201) | 0.027853 / 0.128546 (-0.100693) | 0.010247 / 0.075646 (-0.065400) | 0.206826 / 0.419271 (-0.212445) | 0.035837 / 0.043533 (-0.007695) | 0.251795 / 0.255139 (-0.003344) | 0.275654 / 0.283200 (-0.007545) | 0.017722 / 0.141683 (-0.123960) | 1.120287 / 1.452155 (-0.331868) | 1.203087 / 1.492716 (-0.289630) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.092320 / 0.018006 (0.074314) | 0.300079 / 0.000490 (0.299589) | 0.000211 / 0.000200 (0.000011) | 0.000044 / 0.000054 (-0.000011) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.018193 / 0.037411 (-0.019218) | 0.061310 / 0.014526 (0.046784) | 0.072433 / 0.176557 (-0.104124) | 0.119092 / 0.737135 (-0.618043) | 0.074044 / 0.296338 (-0.222294) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.297184 / 0.215209 (0.081975) | 2.805197 / 2.077655 (0.727543) | 1.521326 / 1.504120 (0.017206) | 1.374321 / 1.541195 (-0.166874) | 1.388767 / 1.468490 (-0.079723) | 0.571865 / 4.584777 (-4.012912) | 2.385213 / 3.745712 (-1.360499) | 2.726840 / 5.269862 (-2.543021) | 1.725352 / 4.565676 (-2.840325) | 0.063012 / 0.424275 (-0.361263) | 0.004911 / 0.007607 (-0.002697) | 0.336430 / 0.226044 (0.110385) | 3.390616 / 2.268929 (1.121688) | 1.846398 / 55.444624 (-53.598227) | 1.576797 / 6.876477 (-5.299680) | 1.579445 / 2.142072 (-0.562627) | 0.652515 / 4.805227 (-4.152712) | 0.118393 / 6.500664 (-6.382271) | 0.042155 / 0.075469 (-0.033314) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.942269 / 1.841788 (-0.899518) | 11.318258 / 8.074308 (3.243950) | 10.299948 / 10.191392 (0.108556) | 0.136088 / 0.680424 (-0.544336) | 0.013682 / 0.534201 (-0.520519) | 0.287549 / 0.579283 (-0.291734) | 0.258346 / 0.434364 (-0.176018) | 0.337146 / 0.540337 (-0.203191) | 0.443922 / 1.386936 (-0.943014) |

</details>
PyArrow==latest

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.005302 / 0.011353 (-0.006051) | 0.003234 / 0.011008 (-0.007774) | 0.049159 / 0.038508 (0.010651) | 0.050459 / 0.023109 (0.027350) | 0.273718 / 0.275898 (-0.002180) | 0.296997 / 0.323480 (-0.026483) | 0.003948 / 0.007986 (-0.004038) | 0.002590 / 0.004328 (-0.001739) | 0.048129 / 0.004250 (0.043879) | 0.039369 / 0.037052 (0.002317) | 0.276469 / 0.258489 (0.017980) | 0.306359 / 0.293841 (0.012519) | 0.028864 / 0.128546 (-0.099682) | 0.010253 / 0.075646 (-0.065394) | 0.058264 / 0.419271 (-0.361008) | 0.032451 / 0.043533 (-0.011082) | 0.277336 / 0.255139 (0.022197) | 0.296137 / 0.283200 (0.012937) | 0.018094 / 0.141683 (-0.123589) | 1.119539 / 1.452155 (-0.332615) | 1.163116 / 1.492716 (-0.329600) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.092578 / 0.018006 (0.074572) | 0.300756 / 0.000490 (0.300267) | 0.000222 / 0.000200 (0.000022) | 0.000044 / 0.000054 (-0.000010) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.022333 / 0.037411 (-0.015078) | 0.076632 / 0.014526 (0.062107) | 0.087829 / 0.176557 (-0.088727) | 0.127686 / 0.737135 (-0.609449) | 0.091314 / 0.296338 (-0.205024) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.297499 / 0.215209 (0.082290) | 2.889775 / 2.077655 (0.812120) | 1.598976 / 1.504120 (0.094856) | 1.478805 / 1.541195 (-0.062389) | 1.481818 / 1.468490 (0.013328) | 0.557972 / 4.584777 (-4.026804) | 2.453248 / 3.745712 (-1.292464) | 2.771823 / 5.269862 (-2.498039) | 1.721527 / 4.565676 (-2.844150) | 0.062786 / 0.424275 (-0.361489) | 0.005298 / 0.007607 (-0.002309) | 0.346660 / 0.226044 (0.120615) | 3.412262 / 2.268929 (1.143334) | 1.940240 / 55.444624 (-53.504384) | 1.654015 / 6.876477 (-5.222461) | 1.652039 / 2.142072 (-0.490034) | 0.636870 / 4.805227 (-4.168357) | 0.116213 / 6.500664 (-6.384451) | 0.040937 / 0.075469 (-0.034532) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 1.001605 / 1.841788 (-0.840183) | 11.986592 / 8.074308 (3.912284) | 10.231288 / 10.191392 (0.039896) | 0.130242 / 0.680424 (-0.550182) | 0.015764 / 0.534201 (-0.518437) | 0.289257 / 0.579283 (-0.290026) | 0.275996 / 0.434364 (-0.158368) | 0.323089 / 0.540337 (-0.217248) | 0.556383 / 1.386936 (-0.830553) |

</details>
</details>

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