Fix IndexError in apache_beam.utils.processes when pip subprocess fails with short command

[shaheeramjad]

Problem

When a pip subprocess fails and the command is passed as a short list (e.g. ['python', '-m', 'pip', 'install', 'pkg']), the exception handler in sdks/python/apache_beam/utils/processes.py raised IndexError instead of the intended RuntimeError with traceback and pip output.

The pip-specific branch used a hardcoded index 6 for the "package name" when formatting the error message (.format(..., args[0][6], ...)). For pip install <pkg> the command list has only 5 elements (indices 0–4), so args[0][6] caused IndexError and users saw a crash instead of a clear error message.

Solution

• Added _pip_package_from_args(args)
  Returns a safe string for the "package" field: uses args[0][6] only when args is a tuple, args[0] is a list/tuple, and len(args[0]) > 6; otherwise returns "see output below".

• Guarded the pip branch
  Replaced (args[0][2] == "pip") with len(args[0]) > 2 and args[0][2] == "pip" so we never index args[0][2] on a short list.

• Updated call, check_call, and check_output
  All three now use _pip_package_from_args(args) instead of args[0][6] when building the pip error message.

Files changed

• sdks/python/apache_beam/utils/processes.py

  • New helper _pip_package_from_args(args) (with docstring).
  • Pip branch in call, check_call, and check_output: length check + use of helper for package string.

• sdks/python/apache_beam/utils/processes_test.py

  • TestErrorHandlingCheckCall.test_check_call_pip_short_command_no_index_error
  • TestErrorHandlingCheckOutput.test_check_output_pip_short_command_no_index_error
  • TestErrorHandlingCall.test_call_pip_short_command_no_index_error
    Each test uses a short pip command (['python', '-m', 'pip', 'install', 'nonexistent-package-xyz']), triggers CalledProcessError, and asserts that a RuntimeError is raised (not IndexError) and that the message contains the subprocess output and "see output below".

Testing

• All 12 tests in apache_beam.utils.processes_test pass, including the 3 new tests and the existing ones (e.g. long pip command with package at index 6).
• Run: python -m unittest apache_beam.utils.processes_test -v

Backward compatibility

• Behavior is unchanged for commands with 7+ elements (e.g. existing pip download --dest /var <pkg> ... still shows the package name at index 6).
• Only the previously broken case (short command) is fixed; no API or contract changes.

Related

• Fixes: Fixes #37515
• Component: Python SDK

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[gemini-code-assist]

Summary of Changes

Hello @shaheeramjad, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly improves the error handling within Apache Beam's Python SDK by addressing a critical IndexError that could occur during pip subprocess failures. The changes ensure that users receive a meaningful RuntimeError with relevant output, rather than an application crash, when pip commands with short argument lists fail. This enhances the stability and diagnostic capabilities of the system without altering existing API contracts.

Highlights

• Bug Fix: IndexError in Pip Subprocess Error Handling: Resolved an IndexError that occurred in apache_beam.utils.processes when a pip subprocess failed with a short command list (e.g., ['python', '-m', 'pip', 'install', 'pkg']). Previously, this would crash the application instead of raising a RuntimeError with a descriptive message.
• New Helper Function: _pip_package_from_args: Introduced a new internal helper function, _pip_package_from_args, to safely extract the package name from the command arguments. This function now returns 'see output below' if the command list is too short to prevent IndexError.
• Enhanced Robustness in Pip Command Checks: Added explicit length checks (len(args[0]) > 2) before attempting to access args[0][2] to determine if the command is a 'pip' command, further preventing IndexError with malformed or short command lists.
• Updated Subprocess Call Functions: Modified call, check_call, and check_output functions to utilize the new _pip_package_from_args helper when constructing error messages for failed pip subprocesses.
• New Test Cases: Added three new test cases (test_check_call_pip_short_command_no_index_error, test_check_output_pip_short_command_no_index_error, test_call_pip_short_command_no_index_error) to apache_beam.utils.processes_test.py to specifically validate the fix for short pip commands.

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Changelog

• CHANGES.md
  • Added an entry documenting the fix for IndexError in apache_beam.utils.processes when pip subprocess fails with short command.
• sdks/python/apache_beam/utils/processes.py
  • Introduced _pip_package_from_args(args) to safely extract package names for pip error messages.
  • Modified call, check_call, and check_output to include length checks for args[0] and use _pip_package_from_args when formatting pip-related RuntimeError messages.
• sdks/python/apache_beam/utils/processes_test.py
  • Added test_check_call_pip_short_command_no_index_error to verify check_call handles short pip commands without IndexError.
  • Added test_check_output_pip_short_command_no_index_error to verify check_output handles short pip commands without IndexError.
  • Added test_call_pip_short_command_no_index_error to verify call handles short pip commands without IndexError.

Activity

• The pull request was authored by shaheeramjad.
• The changes address and fix issue #37515.
• The CHANGES.md file has been updated to reflect this bugfix.
• New tests have been added to cover the specific scenario of short pip commands, and all 12 existing tests in apache_beam.utils.processes_test continue to pass.
• The changes maintain backward compatibility, only fixing previously broken behavior without altering existing functionality for longer commands.

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[shaheeramjad]

Hi @damccorm Can you review it
Regards !!

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[shaheeramjad]

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