What causes "Full trace: {} \n Output of the failed child process: {}" in apache/beam?

Inspect 'Output of the failed child process' in the exception for the real cause

apache/beam · error · RuntimeError

Full trace: \n Output of the failed child process

Error message

Full trace: {} \n Output of the failed child process: {}

What it means

RuntimeError raised from the subprocess wrapper `check_call` when the spawned child (again typically pip install) failed; it rewraps CalledProcessError with the full traceback and the child process's output for diagnosability.

Solutions

Inspect 'Output of the failed child process' in the exception for the real cause
Re-run the command locally with identical arguments/environment
Handle expected nonzero exits with processes.call instead of check_call, or catch RuntimeError
Fix flags, inputs, or credentials the child tool needs

Defensive patterns

Strategy: try-catch

Validate before calling

result = subprocess.run(['tool', '--version'], capture_output=True)
assert result.returncode == 0, result.stderr  # tool works before real invocation

Try / catch

try:
    processes.check_call('tool', '--do')
except RuntimeError as e:
    if 'Output of the failed child process' in str(e):
        logging.error('tool failed: %s', e)
        handle_child_failure()
    else:
        raise

Prevention

Smoke-test child tools with --version at startup
Use processes.call when nonzero exit codes are expected/acceptable
Log the embedded child output for diagnosis
Match CLI flags to the exact tool version deployed

When it happens

Trigger: Any processes.check_call(...) whose child process returns a nonzero exit code and whose argv[0][2] is not 'pip'.

Common situations: Failing shell utilities (grep with no matches returns 1, test scripts), wrong CLI flags, tools requiring auth or missing inputs inside pipeline workers.

Related errors

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/a92dfa60dfe52aa2. Report an issue: GitHub.

Appendix: source

Thrown at sdks/python/apache_beam/utils/processes.py:80

           \n Output of the failed child process: {} " \
          .format(traceback.format_exc(), error.output)) from error
    return out

  def check_call(*args, **kwargs):
    if force_shell:
      kwargs['shell'] = True
    try:
      out = subprocess.check_call(*args, **kwargs)
    except OSError as e:
      raise RuntimeError("Executable {} not found".format(args[0])) from e
    except subprocess.CalledProcessError as error:
      if isinstance(args, tuple) and (args[0][2] == "pip"):
        raise RuntimeError( \
          "Full traceback: {} \n Pip install failed for package: {} \
          \n Output from execution of subprocess: {}" \
          .format(traceback.format_exc(), args[0][6], error.output)) from error
      else:
        raise RuntimeError("Full trace: {} \
          \n Output of the failed child process: {}" \
          .format(traceback.format_exc(), error.output)) from error
    return out

  def check_output(*args, **kwargs):
    if force_shell:
      kwargs['shell'] = True
    try:
      out = subprocess.check_output(*args, **kwargs)
    except OSError as e:
      raise RuntimeError("Executable {} not found".format(args[0])) from e
    except subprocess.CalledProcessError as error:
      if isinstance(args, tuple) and (args[0][2] == "pip"):
        raise RuntimeError( \
          "Full traceback: {} \n Pip install failed for package: {} \
          \n Output from execution of subprocess: {}" \
          .format(traceback.format_exc(), args[0][6], error.output)) from error
      else:

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