Chain filtering is leaking
State: closed | Author: DrMatters (NONE)
Labels: name: bug

## Describe the bug
As there's no support for lists within dataset fields, I convert my lists to json-string format. However, the bug described is occurring even when the data format is 'string'.
These samples show that filtering behavior diverges from what's expected when chaining filterings.
On sample 2 the second filtering leads to "leaking" of data that should've been filtered on the first filtering into the results.

## Steps to reproduce the bug
Sample 1:
```python
import datasets
import json

items = [[1, 2], [3], [4]]
jsoned_items = map(json.dumps, [[1, 2], [3], [4]])
ds = datasets.Dataset.from_dict({'a': jsoned_items})
print(list(ds))
# > Prints: [{'a': '[1, 2]'}, {'a': '[3]'}, {'a': '[4]'}] as expected

filtered = ds

# get all lists that are shorter than 2
filtered = filtered.filter(lambda x: len(json.loads(x['a'])) < 2, load_from_cache_file=False)
print(list(filtered))
# > Prints: [{'a': '[3]'}, {'a': '[4]'}] as expected

# get all lists, which have a value bigger than 3 on its zero index
filtered = filtered.filter(lambda x: json.loads(x['a'])[0] > 3, load_from_cache_file=False)
print(list(filtered))
# > Should be: [{'a': [4]}]
# > Prints: [{'a': [3]}]
```
Sample 2:
```python
import datasets
import json

items = [[1, 2], [3], [4]]
jsoned_items = map(json.dumps, [[1, 2], [3], [4]])
ds = datasets.Dataset.from_dict({'a': jsoned_items})
print(list(ds))
# > Prints: [{'a': '[1, 2]'}, {'a': '[3]'}, {'a': '[4]'}]

filtered = ds

# get all lists, which have a value bigger than 3 on its zero index
filtered = filtered.filter(lambda x: json.loads(x['a'])[0] > 3, load_from_cache_file=False)
print(list(filtered))
# > Prints: [{'a': '[4]'}] as expected

# get all lists that are shorter than 2
filtered = filtered.filter(lambda x: len(json.loads(x['a'])) < 2, load_from_cache_file=False)
print(list(filtered))
# > Prints: [{'a': '[1, 2]'}]
# > Should be: [{'a': '[4]'}] (remain intact)
```

## Expected results
Expected and actual results are attached to the code snippets.

## Actual results
Expected and actual results are attached to the code snippets.

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.12.1
- Platform: Windows-10-10.0.19042-SP0
- Python version: 3.9.7
- PyArrow version: 5.0.0


Comments: ### Update:
I wrote a bit cleaner code snippet (without transforming to json) that can expose leaking.
```python
import datasets
import json

items = ['ab', 'c', 'df']

ds = datasets.Dataset.from_dict({'col': items})
print(list(ds))
# > Prints: [{'col': 'ab'}, {'col': 'c'}, {'col': 'df'}]

filtered = ds

# get all items that are starting with a character with ascii code bigger than 'a'
filtered = filtered.filter(lambda x: x['col'][0] > 'a', load_from_cache_file=False)
print(list(filtered))
# > Prints: [{'col': 'c'}, {'col': 'df'}] as expected

# get all items that are shorter than 2
filtered = filtered.filter(lambda x: len(x['col']) < 2, load_from_cache_file=False)
print(list(filtered))
# > Prints: [{'col': 'ab'}] -> this is a leaked item from the first filter
# > Should be: [{'col': 'c'}]
```

Thanks for reporting. I'm looking into it

I just pushed a fix ! We'll do a new release soon.
In the meantime feel free to install `datasets` from source to play with it

Thanks, I'm already using it from your branch!