Fix JSON builder when missing keys in first row

[albertvillanova]

Until now, the JSON builder only considered the keys present in the first element of the list:

• Either explicitly: by passing index 0 in dataset[0].keys()
• Or implicitly: pa.Table.from_pylist(dataset), where "schema (default None): If not passed, will be inferred from the first row of the mapping values"

This PR fixes the bug by considering the union of the keys present in all the rows.

Fix #5726.

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[lhoestq]

Sounds good ! :)

[mariosasko]

Thanks!

[mariosasko]

I think it's worth noting that Pandas uses Cython to perform this conversion: https://github.com/pandas-dev/pandas/blob/478d340667831908b5b4bf09a2787a11a14560c9/pandas/core/internals/construction.py#L775

I benchmarked the SST datasets converted to a list of dictionaries, and the Pandas version is 26x faster than ours (53.4 ms vs. 1.41s).

[albertvillanova]

Thanks for the investigation.

[github-actions]

Show benchmarks

PyArrow==8.0.0

Show updated benchmarks!

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)	0.009262 / 0.011353 (-0.002091)	0.006157 / 0.011008 (-0.004851)	0.125960 / 0.038508 (0.087451)	0.036213 / 0.023109 (0.013104)	0.399331 / 0.275898 (0.123433)	0.453597 / 0.323480 (0.130117)	0.006990 / 0.007986 (-0.000995)	0.007320 / 0.004328 (0.002991)	0.100321 / 0.004250 (0.096070)	0.048870 / 0.037052 (0.011818)	0.396284 / 0.258489 (0.137795)	0.475619 / 0.293841 (0.181778)	0.052329 / 0.128546 (-0.076217)	0.019564 / 0.075646 (-0.056083)	0.430942 / 0.419271 (0.011670)	0.063224 / 0.043533 (0.019692)	0.391717 / 0.255139 (0.136578)	0.448342 / 0.283200 (0.165142)	0.114055 / 0.141683 (-0.027628)	1.793204 / 1.452155 (0.341049)	1.895151 / 1.492716 (0.402435)

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)	0.283699 / 0.018006 (0.265693)	0.597194 / 0.000490 (0.596704)	0.007143 / 0.000200 (0.006944)	0.000602 / 0.000054 (0.000548)

Benchmark: benchmark_indices_mapping.json

metric	select	shard	shuffle	sort	train_test_split
new / old (diff)	0.034761 / 0.037411 (-0.002651)	0.124555 / 0.014526 (0.110030)	0.149126 / 0.176557 (-0.027430)	0.220335 / 0.737135 (-0.516801)	0.153109 / 0.296338 (-0.143229)

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)	0.620210 / 0.215209 (0.405001)	6.229937 / 2.077655 (4.152282)	2.615203 / 1.504120 (1.111083)	2.239337 / 1.541195 (0.698143)	2.262138 / 1.468490 (0.793648)	1.196498 / 4.584777 (-3.388279)	5.609932 / 3.745712 (1.864220)	3.031347 / 5.269862 (-2.238515)	2.025530 / 4.565676 (-2.540146)	0.139828 / 0.424275 (-0.284447)	0.015476 / 0.007607 (0.007869)	0.768964 / 0.226044 (0.542920)	7.728677 / 2.268929 (5.459748)	3.336407 / 55.444624 (-52.108217)	2.700055 / 6.876477 (-4.176422)	2.765223 / 2.142072 (0.623151)	1.409073 / 4.805227 (-3.396155)	0.246849 / 6.500664 (-6.253815)	0.081231 / 0.075469 (0.005762)

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)	1.593836 / 1.841788 (-0.247952)	18.020525 / 8.074308 (9.946216)	21.766822 / 10.191392 (11.575430)	0.258615 / 0.680424 (-0.421809)	0.026895 / 0.534201 (-0.507306)	0.529823 / 0.579283 (-0.049460)	0.623470 / 0.434364 (0.189106)	0.628171 / 0.540337 (0.087833)	0.745249 / 1.386936 (-0.641687)

PyArrow==latest

Show updated benchmarks!

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)	0.008624 / 0.011353 (-0.002729)	0.006317 / 0.011008 (-0.004691)	0.097315 / 0.038508 (0.058807)	0.035217 / 0.023109 (0.012108)	0.440197 / 0.275898 (0.164299)	0.473863 / 0.323480 (0.150383)	0.006722 / 0.007986 (-0.001264)	0.006444 / 0.004328 (0.002116)	0.102056 / 0.004250 (0.097806)	0.047142 / 0.037052 (0.010089)	0.452476 / 0.258489 (0.193986)	0.487619 / 0.293841 (0.193778)	0.052456 / 0.128546 (-0.076090)	0.018735 / 0.075646 (-0.056911)	0.114656 / 0.419271 (-0.304616)	0.062577 / 0.043533 (0.019044)	0.444471 / 0.255139 (0.189332)	0.494264 / 0.283200 (0.211065)	0.117112 / 0.141683 (-0.024571)	1.848965 / 1.452155 (0.396810)	1.984008 / 1.492716 (0.491292)

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)	0.290494 / 0.018006 (0.272488)	0.588415 / 0.000490 (0.587925)	0.000459 / 0.000200 (0.000259)	0.000080 / 0.000054 (0.000026)

Benchmark: benchmark_indices_mapping.json

metric	select	shard	shuffle	sort	train_test_split
new / old (diff)	0.032873 / 0.037411 (-0.004538)	0.131139 / 0.014526 (0.116614)	0.140268 / 0.176557 (-0.036289)	0.204561 / 0.737135 (-0.532574)	0.147443 / 0.296338 (-0.148895)

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)	0.636899 / 0.215209 (0.421690)	6.236139 / 2.077655 (4.158484)	2.801468 / 1.504120 (1.297348)	2.398808 / 1.541195 (0.857613)	2.493150 / 1.468490 (1.024659)	1.228845 / 4.584777 (-3.355932)	5.675874 / 3.745712 (1.930162)	3.084939 / 5.269862 (-2.184922)	2.061310 / 4.565676 (-2.504367)	0.142285 / 0.424275 (-0.281990)	0.014972 / 0.007607 (0.007365)	0.786599 / 0.226044 (0.560555)	7.876036 / 2.268929 (5.607107)	3.476136 / 55.444624 (-51.968489)	2.847922 / 6.876477 (-4.028555)	3.040326 / 2.142072 (0.898253)	1.448538 / 4.805227 (-3.356690)	0.257230 / 6.500664 (-6.243434)	0.085137 / 0.075469 (0.009668)

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)	1.668173 / 1.841788 (-0.173615)	18.668520 / 8.074308 (10.594212)	20.535542 / 10.191392 (10.344150)	0.244580 / 0.680424 (-0.435844)	0.026364 / 0.534201 (-0.507837)	0.531753 / 0.579283 (-0.047530)	0.616578 / 0.434364 (0.182214)	0.618906 / 0.540337 (0.078569)	0.738785 / 1.386936 (-0.648151)