apache/beam · error · ValueError

We were unable to execute the subprocess

Error message

We were unable to execute the subprocess "%s" to automatically build prism.
Alternatively provide an alternate binary with the --prism_location flag.
Captured output:
 %s

What it means

In _install_from_source, Prism is auto-built by invoking a subprocess ('go install' via go run). If the subprocess completes with a nonzero return code (and it did get executed), the library raises ValueError showing the subprocess args and its captured stdout/stderr so the user can supply a binary manually.

Solutions

  1. Inspect the 'Captured output' in the message for the underlying go command failure
  2. Fix network/proxy access to the Go module proxy (set GOPROXY/HTTPS_PROXY as needed)
  3. Run the same 'go install ...' command manually to reproduce and fix the error
  4. Provide a prebuilt prism binary via --prism_location to bypass automatic builds

Example fix

// before
--runner=PrismRunner  # auto-build fails behind proxy
// after
export HTTPS_PROXY=http://proxy:8080
# or
--prism_location=/usr/local/bin/prism
Defensive patterns

Strategy: try-catch

Validate before calling

import subprocess
r = subprocess.run(['go', 'install', 'github.com/apache/beam/sdks/v2/go/cmd/prism@latest'], capture_output=True)
if r.returncode != 0:
    print(r.stdout, r.stderr)  # fix go env/network before running Beam

Type guard

def can_build_prism() -> bool:
    import shutil, subprocess
    if shutil.which('go') is None:
        return False
    return subprocess.run(['go', 'env', 'GOMODCACHE'], capture_output=True).returncode == 0

Try / catch

try:
    server = PrismJobServer(options)
except ValueError as e:
    if 'unable to execute the subprocess' in str(e):
        logger.error('Prism auto-build failed; see captured output:\n%s', e)
        raise SystemExit(3)

Prevention

When it happens

Trigger: Automatic prism build subprocess (process.args, typically 'go install github.com/apache/beam/sdks/v2/go/cmd/prism@...') returns nonzero after being launched; process.stdout is decoded and embedded in the error.

Common situations: Network failures fetching the Go module from proxy.golang.org; Go version mismatch with the module; module version/tag that doesn't exist; proxies blocking downloads in corporate environments.

Related errors

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/b668fae1500d4147. Report an issue: GitHub.

Appendix: source

Thrown at sdks/python/apache_beam/runners/portability/prism_runner.py:448

    # Go is installed and claims we're not in a Go module that has access to
    # the Prism package.

    # Fallback to using the @latest version of prism, which works everywhere.
    _LOGGER.info(
        'Installing prism from "%s@latest" into "%s".',
        PRISMPKG,
        PrismJobServer.BIN_CACHE)
    process = subprocess.run(["go", "install", PRISMPKG + "@latest"],
                             stdout=subprocess.PIPE,
                             stderr=subprocess.STDOUT,
                             env=envdict,
                             check=False)

    if process.returncode == 0:
      return '%s/prism' % (PrismJobServer.BIN_CACHE)

    output = process.stdout.decode("utf-8")
    raise ValueError(
        'We were unable to execute the subprocess "%s" to automatically '
        'build prism.\nAlternatively provide an alternate binary with the '
        '--prism_location flag.'
        '\nCaptured output:\n %s' % (process.args, output))

  def _resolve_source_path(self) -> str:
    """Resolves and returns the source for the Prism binary.

    The resolution follows this order:

    1. A user-provided location (local path, GCS, or URL).
    2. A pre-built binary from GitHub for a release version.
    3. Build from local Go source for a development version.
    """
    if self._path:
      return self._resolve_from_location_override(self._path, self._version)

    if '.dev' not in self._version:

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