Consistent ArrayXD Python formatting + better NumPy/Pandas formatting
State: closed

Return a list of lists instead of a list of NumPy arrays when converting the variable-shaped `ArrayXD` to Python. Additionally, improve the NumPy conversion by returning a numeric NumPy array when the offsets are equal or a NumPy object array when they aren't, and allow converting the variable-shaped `ArrayXD` to Pandas.

(Reported in https://github.com/huggingface/datasets/issues/5719#issuecomment-1507579671)

Comments:
_The documentation is not available anymore as the PR was closed or merged._

<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) | 0.010459 / 0.011353 (-0.000894) | 0.007009 / 0.011008 (-0.003999) | 0.153885 / 0.038508 (0.115377) | 0.037308 / 0.023109 (0.014199) | 0.431931 / 0.275898 (0.156033) | 0.452940 / 0.323480 (0.129461) | 0.008572 / 0.007986 (0.000586) | 0.007479 / 0.004328 (0.003150) | 0.093835 / 0.004250 (0.089584) | 0.050172 / 0.037052 (0.013120) | 0.428855 / 0.258489 (0.170366) | 0.517814 / 0.293841 (0.223974) | 0.058558 / 0.128546 (-0.069988) | 0.019550 / 0.075646 (-0.056096) | 0.449837 / 0.419271 (0.030566) | 0.069710 / 0.043533 (0.026177) | 0.444163 / 0.255139 (0.189024) | 0.469003 / 0.283200 (0.185803) | 0.114665 / 0.141683 (-0.027018) | 1.822415 / 1.452155 (0.370261) | 1.956360 / 1.492716 (0.463644) |

### 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) | 0.237489 / 0.018006 (0.219483) | 0.556947 / 0.000490 (0.556457) | 0.006988 / 0.000200 (0.006789) | 0.000499 / 0.000054 (0.000444) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.037047 / 0.037411 (-0.000364) | 0.133973 / 0.014526 (0.119447) | 0.137072 / 0.176557 (-0.039485) | 0.201520 / 0.737135 (-0.535615) | 0.144177 / 0.296338 (-0.152161) |

### 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) | 0.694853 / 0.215209 (0.479644) | 6.805746 / 2.077655 (4.728091) | 2.717864 / 1.504120 (1.213744) | 2.360529 / 1.541195 (0.819335) | 2.384403 / 1.468490 (0.915913) | 1.337512 / 4.584777 (-3.247265) | 5.734090 / 3.745712 (1.988378) | 5.344909 / 5.269862 (0.075047) | 2.906218 / 4.565676 (-1.659458) | 0.160148 / 0.424275 (-0.264127) | 0.015159 / 0.007607 (0.007551) | 0.871356 / 0.226044 (0.645312) | 8.550965 / 2.268929 (6.282037) | 3.613522 / 55.444624 (-51.831103) | 2.868508 / 6.876477 (-4.007969) | 2.912263 / 2.142072 (0.770190) | 1.652548 / 4.805227 (-3.152680) | 0.274117 / 6.500664 (-6.226547) | 0.085911 / 0.075469 (0.010442) |

### 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) | 1.624798 / 1.841788 (-0.216989) | 18.413303 / 8.074308 (10.338995) | 21.742854 / 10.191392 (11.551462) | 0.255937 / 0.680424 (-0.424487) | 0.029492 / 0.534201 (-0.504709) | 0.541932 / 0.579283 (-0.037351) | 0.638594 / 0.434364 (0.204230) | 0.607427 / 0.540337 (0.067090) | 0.763046 / 1.386936 (-0.623890) |

</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) | 0.020543 / 0.011353 (0.009190) | 0.006079 / 0.011008 (-0.004929) | 0.100558 / 0.038508 (0.062050) | 0.039474 / 0.023109 (0.016365) | 0.468889 / 0.275898 (0.192991) | 0.477731 / 0.323480 (0.154251) | 0.006999 / 0.007986 (-0.000987) | 0.005845 / 0.004328 (0.001516) | 0.110022 / 0.004250 (0.105772) | 0.056885 / 0.037052 (0.019833) | 0.447296 / 0.258489 (0.188807) | 0.489007 / 0.293841 (0.195166) | 0.055086 / 0.128546 (-0.073460) | 0.020623 / 0.075646 (-0.055024) | 0.129599 / 0.419271 (-0.289672) | 0.064316 / 0.043533 (0.020784) | 0.446681 / 0.255139 (0.191542) | 0.488897 / 0.283200 (0.205698) | 0.119121 / 0.141683 (-0.022562) | 1.836248 / 1.452155 (0.384093) | 2.002456 / 1.492716 (0.509740) |

### 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) | 0.249344 / 0.018006 (0.231338) | 0.544320 / 0.000490 (0.543830) | 0.000459 / 0.000200 (0.000259) | 0.000088 / 0.000054 (0.000034) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.038771 / 0.037411 (0.001359) | 0.129527 / 0.014526 (0.115002) | 0.144681 / 0.176557 (-0.031876) | 0.208237 / 0.737135 (-0.528898) | 0.149502 / 0.296338 (-0.146836) |

### 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) | 0.668457 / 0.215209 (0.453248) | 6.729550 / 2.077655 (4.651895) | 2.741076 / 1.504120 (1.236956) | 2.394737 / 1.541195 (0.853542) | 2.415242 / 1.468490 (0.946752) | 1.322334 / 4.584777 (-3.262442) | 5.787454 / 3.745712 (2.041742) | 3.309847 / 5.269862 (-1.960015) | 2.199181 / 4.565676 (-2.366495) | 0.170740 / 0.424275 (-0.253535) | 0.015095 / 0.007607 (0.007487) | 0.864157 / 0.226044 (0.638112) | 8.701858 / 2.268929 (6.432929) | 3.617966 / 55.444624 (-51.826658) | 2.847144 / 6.876477 (-4.029332) | 3.011391 / 2.142072 (0.869319) | 1.595466 / 4.805227 (-3.209762) | 0.284010 / 6.500664 (-6.216654) | 0.091054 / 0.075469 (0.015585) |

### 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) | 1.702404 / 1.841788 (-0.139384) | 19.427130 / 8.074308 (11.352822) | 21.900446 / 10.191392 (11.709053) | 0.244088 / 0.680424 (-0.436336) | 0.027428 / 0.534201 (-0.506773) | 0.552226 / 0.579283 (-0.027057) | 0.653102 / 0.434364 (0.218738) | 0.635379 / 0.540337 (0.095042) | 0.771842 / 1.386936 (-0.615094) |

</details>
</details>

![](https://cml.dev/watermark.png#efde2a0b9ad937defc83e0ac3f14bbb90fb5f345 "CML watermark")


<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) | 0.006547 / 0.011353 (-0.004806) | 0.004569 / 0.011008 (-0.006439) | 0.097782 / 0.038508 (0.059274) | 0.028157 / 0.023109 (0.005048) | 0.319017 / 0.275898 (0.043119) | 0.340758 / 0.323480 (0.017278) | 0.005078 / 0.007986 (-0.002907) | 0.003343 / 0.004328 (-0.000985) | 0.074194 / 0.004250 (0.069944) | 0.037918 / 0.037052 (0.000866) | 0.310298 / 0.258489 (0.051809) | 0.349441 / 0.293841 (0.055600) | 0.030375 / 0.128546 (-0.098171) | 0.011527 / 0.075646 (-0.064119) | 0.320499 / 0.419271 (-0.098773) | 0.042639 / 0.043533 (-0.000894) | 0.312182 / 0.255139 (0.057043) | 0.329058 / 0.283200 (0.045858) | 0.085517 / 0.141683 (-0.056165) | 1.532603 / 1.452155 (0.080448) | 1.583996 / 1.492716 (0.091279) |

### 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) | 0.208286 / 0.018006 (0.190280) | 0.418696 / 0.000490 (0.418206) | 0.007051 / 0.000200 (0.006851) | 0.000409 / 0.000054 (0.000354) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.024055 / 0.037411 (-0.013356) | 0.098420 / 0.014526 (0.083894) | 0.104785 / 0.176557 (-0.071771) | 0.163618 / 0.737135 (-0.573517) | 0.110006 / 0.296338 (-0.186332) |

### 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) | 0.418756 / 0.215209 (0.203547) | 4.179557 / 2.077655 (2.101902) | 1.881708 / 1.504120 (0.377588) | 1.683393 / 1.541195 (0.142198) | 1.731909 / 1.468490 (0.263419) | 0.696674 / 4.584777 (-3.888103) | 3.384167 / 3.745712 (-0.361545) | 3.173479 / 5.269862 (-2.096382) | 1.620019 / 4.565676 (-2.945658) | 0.082850 / 0.424275 (-0.341426) | 0.012396 / 0.007607 (0.004789) | 0.519743 / 0.226044 (0.293699) | 5.208480 / 2.268929 (2.939552) | 2.312917 / 55.444624 (-53.131708) | 1.963486 / 6.876477 (-4.912991) | 2.084553 / 2.142072 (-0.057519) | 0.805486 / 4.805227 (-3.999742) | 0.153429 / 6.500664 (-6.347235) | 0.069451 / 0.075469 (-0.006018) |

### 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) | 1.197185 / 1.841788 (-0.644603) | 14.341005 / 8.074308 (6.266696) | 14.476162 / 10.191392 (4.284770) | 0.157372 / 0.680424 (-0.523052) | 0.016444 / 0.534201 (-0.517757) | 0.383721 / 0.579283 (-0.195562) | 0.380800 / 0.434364 (-0.053564) | 0.441137 / 0.540337 (-0.099200) | 0.524778 / 1.386936 (-0.862158) |

</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) | 0.006728 / 0.011353 (-0.004625) | 0.004536 / 0.011008 (-0.006472) | 0.076266 / 0.038508 (0.037757) | 0.028133 / 0.023109 (0.005024) | 0.351072 / 0.275898 (0.075174) | 0.375823 / 0.323480 (0.052344) | 0.005166 / 0.007986 (-0.002819) | 0.004717 / 0.004328 (0.000388) | 0.076130 / 0.004250 (0.071880) | 0.041354 / 0.037052 (0.004301) | 0.345904 / 0.258489 (0.087415) | 0.384119 / 0.293841 (0.090278) | 0.030759 / 0.128546 (-0.097787) | 0.011659 / 0.075646 (-0.063988) | 0.085269 / 0.419271 (-0.334002) | 0.042161 / 0.043533 (-0.001372) | 0.340806 / 0.255139 (0.085667) | 0.366832 / 0.283200 (0.083632) | 0.092187 / 0.141683 (-0.049495) | 1.520035 / 1.452155 (0.067880) | 1.603856 / 1.492716 (0.111140) |

### 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) | 0.237763 / 0.018006 (0.219757) | 0.413406 / 0.000490 (0.412916) | 0.000415 / 0.000200 (0.000215) | 0.000060 / 0.000054 (0.000005) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.026095 / 0.037411 (-0.011317) | 0.105775 / 0.014526 (0.091249) | 0.108452 / 0.176557 (-0.068105) | 0.160014 / 0.737135 (-0.577122) | 0.112385 / 0.296338 (-0.183953) |

### 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) | 0.437327 / 0.215209 (0.222118) | 4.374949 / 2.077655 (2.297294) | 2.090292 / 1.504120 (0.586172) | 1.885946 / 1.541195 (0.344752) | 1.946768 / 1.468490 (0.478278) | 0.704124 / 4.584777 (-3.880653) | 3.394994 / 3.745712 (-0.350718) | 1.905189 / 5.269862 (-3.364673) | 1.182300 / 4.565676 (-3.383376) | 0.082920 / 0.424275 (-0.341355) | 0.012781 / 0.007607 (0.005174) | 0.535467 / 0.226044 (0.309423) | 5.362799 / 2.268929 (3.093870) | 2.504825 / 55.444624 (-52.939799) | 2.180458 / 6.876477 (-4.696019) | 2.317750 / 2.142072 (0.175677) | 0.811182 / 4.805227 (-3.994045) | 0.151654 / 6.500664 (-6.349010) | 0.067925 / 0.075469 (-0.007544) |

### 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) | 1.290746 / 1.841788 (-0.551042) | 14.799309 / 8.074308 (6.725001) | 14.439722 / 10.191392 (4.248330) | 0.144358 / 0.680424 (-0.536066) | 0.016688 / 0.534201 (-0.517513) | 0.392907 / 0.579283 (-0.186376) | 0.383109 / 0.434364 (-0.051255) | 0.450069 / 0.540337 (-0.090269) | 0.532534 / 1.386936 (-0.854402) |

</details>
</details>

![](https://cml.dev/watermark.png#87c061032972509a2a1b4103763e62fb74912128 "CML watermark")


I turned it into a draft to fix the failing tests, but CI is now green, so there is no good reason for it :)