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

Read the embedded 'Output of the failed child process' to find the child's actual error

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 `call` when a child process (typically a pip invocation it special-cases) exits non-zero; the message embeds the Python traceback plus the failed child's captured output so the root cause is visible from the parent.

Solutions

Read the embedded 'Output of the failed child process' to find the child's actual error
Run the same command manually to reproduce and fix its arguments
Check the child's exit code and required inputs/environment
Add error handling around call() so nonzero exits are handled rather than crashing the pipeline

Defensive patterns

Strategy: try-catch

Validate before calling

# dry-run the command locally first
result = subprocess.run(cmd_args, capture_output=True)
assert result.returncode == 0, result.stderr

Try / catch

try:
    out = processes.call('my-tool', '--flag', 'value')
except RuntimeError as e:
    if 'Output of the failed child process' in str(e):
        logging.error('child failed: %s', e)  # includes captured child output
        return fallback_result()
    raise

Prevention

Validate arguments and inputs of child commands before invoking
Capture and read the embedded child output when debugging
Use processes.call (not raising on nonzero) when nonzero exits are expected
Keep command flags compatible with the tool version installed in the runtime image

When it happens

Trigger: Any processes.call(...) where the child process completes with a nonzero exit code and the command is not detected as pip (args[0][2] != 'pip').

Common situations: Shell command typos, wrong arguments to CLI tools, tools failing due to missing inputs, commands whose output was captured (stderr/stdout redirection) causing CalledProcessError to carry output.

Related errors

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/dcf91a2bb5a4668b. Report an issue: GitHub.

Appendix: source

Thrown at sdks/python/apache_beam/utils/processes.py:61

  Popen = subprocess.Popen

else:

  def call(*args, **kwargs):
    if force_shell:
      kwargs['shell'] = True
    try:
      out = subprocess.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_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:

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