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Fix ArrowWriter closes stream at exit #1971

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merged 17 commits into from
Mar 10, 2021
Merged

Fix ArrowWriter closes stream at exit #1971

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49d14fb
Test ArrowWriter closes stream at exit
albertvillanova Mar 2, 2021
d3c0e0b
Implement ArrowWriter as context manager and close stream at exit
albertvillanova Mar 2, 2021
19cd459
Specify exception class
albertvillanova Mar 2, 2021
b28967f
Use ArrowWriter as context manager
albertvillanova Mar 2, 2021
8c230ee
Fix ArrowWriter.close
albertvillanova Mar 2, 2021
4d50e95
Merge remote-tracking branch 'upstream/master' into fix-arrow-writer
albertvillanova Mar 2, 2021
2a7a861
Fix ArrowWriter.close
albertvillanova Mar 2, 2021
5c7af22
Test stream already closed at ArrowWriter context exit
albertvillanova Mar 2, 2021
ba368f5
Revert "Use ArrowWriter as context manager"
albertvillanova Mar 2, 2021
d2e123d
Use ArrowWriter as context manager in tests
albertvillanova Mar 2, 2021
84ad7f3
Generalize Exception type in ArrowWriter.close
albertvillanova Mar 2, 2021
c521ffe
Use ArrowWriter as context manager in benchmarks
albertvillanova Mar 2, 2021
7d241ce
Use ArrowWriter as context manager in arrow_writer and builder
albertvillanova Mar 2, 2021
e00d72a
Use ArrowWriter as context manager in arrow_dataset
albertvillanova Mar 2, 2021
7f287fb
Use ArrowWriter as context manager in metric
albertvillanova Mar 2, 2021
10d9258
Revert "Use ArrowWriter as context manager in metric"
albertvillanova Mar 9, 2021
6f1a701
Merge remote-tracking branch 'upstream/master' into fix-arrow-writer
albertvillanova Mar 10, 2021
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3 changes: 2 additions & 1 deletion src/datasets/metric.py
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Original file line number Diff line number Diff line change
Expand Up @@ -336,7 +336,8 @@ def _finalize(self):
from all the nodes if main node or all_process is True.
"""
if self.writer is not None:
self.writer.finalize()
with self.writer as writer:
writer.finalize()
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The usage of the with statement here can make the user think that the writer opens and creates a file here.
This is not the case since finalize just writes down the latest examples and close the file.

It would be prettier to use .close() instead of a with statement here, to remove this ambiguity.

The with statement should be used to define the start and end point of the usage of the writer (file opened until closed).
However in this case the writer is an attribute of the Metric object and so there's no code block with a defined start/end.
In this case I think it's better to use .close() instead. You can even do try/except to make sure that close() is called even if finalize fails.

It would makes things clearer IMO.

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@albertvillanova albertvillanova Mar 9, 2021 •
edited
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@lhoestq, I think the main point is in your sentence:

You can even do try/except to make sure that close() is called even if finalize fails.

Exactly! This is why I decided to open this PR: an exception was raised, and the resource was not closed. If an exception is raised within .finalize(), the file will no be closed. So that the solution would be: try... except... finally: close().

And that is exactly the use case for with: use a context manager instead of try... except... finally: close().
See: https://docs.python.org/3/reference/compound_stmts.html#the-with-statement

This allows common try…except…finally usage patterns to be encapsulated for convenient reuse.

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In this case maybe you can you add in the writer's docstring that the with can also be used only when calling finalize ?
As a user I'd expect the with statement of a Writer class to be necessarily used from start of writing till closing, but here you can write and then use the with statement to close. Let me know if that makes sense.

That's definitely not a big deal but I'd rather have things said explicitly :)

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Yes @lhoestq, you are right. One should use the context manager from start of writing until closing. I left some usages of writer without context manager, because the refactoring was not immediate, for a follow-up PR. Maybe I can remove the with in this case, to finish this PR. And then open another PR for the remaining non-trivial cases?

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Sounds good !

self.writer = None
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Hi @lhoestq , do you think this is the cause of the test error? I am trying to understand why... It happens with DistributedDatasetTest.test_load_dataset_distributed. Maybe you know why...

if self.filelock is not None:
self.filelock.release()
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