fix: Add error handling for empty Parquet files while indexing and corresponding tests

[bhimrazy]

What does this PR do ?

This PR adds error handling to the ParquetDir iterator to raise a clear exception when no Parquet files are found in the specified directory. Additionally, it includes tests to verify this behavior.

This change prevents unnecessary execution of the indexing process and avoids generating an empty index.json file when no data is available, and provides a clear error message to help the user identify and resolve the issue.

Resolves #600

[Copilot]

Pull Request Overview

This PR adds explicit error handling for empty Parquet directories and corresponding tests to ensure a clear failure when no files are present.

• Raise a RuntimeError in ParquetDir.__iter__ when self.files is empty.
• Add test_no_parquet_files to verify that indexing fails with the new error for remote stores.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/litdata/utilities/parquet.py	Check self.files and raise RuntimeError with guidance when no .parquet files exist
tests/streaming/test_parquet.py	Add parameterized test_no_parquet_files to assert the new error in various backends

Comments suppressed due to low confidence (1)

tests/streaming/test_parquet.py:208

• Consider adding a test case for a local directory with no .parquet files (e.g., using tmp_path / an empty temp folder) to ensure the error is raised for filesystem paths as well as remote URLs.

@pytest.mark.parametrize(

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79%. Comparing base (2402f45) to head (b38962c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #601   +/-   ##
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  Coverage    79%    79%           
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  Files        41     41           
  Lines      6136   6138    +2     
===================================
+ Hits       4836   4838    +2     
  Misses     1300   1300           

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[tchaton]

Nice !