jax-ml/jax · error · RuntimeError

Cannot determine the ``__name__`` of the caller.

Error message

Cannot determine the ``__name__`` of the caller.

What it means

jax._src.lazy_loader.attach builds a PEP 562 lazy module by inspecting the caller frame's globals for __name__. If sys._getframe(1).f_globals has no __name__ key (e.g. exec'd namespace or unusual import machinery), it raises this RuntimeError because it cannot know which module to patch with lazily imported attributes.

Source

Thrown at jax/_src/lazy_loader.py:40

def attach(package_name: str, submodules: Sequence[str]) -> tuple[
    Callable[[str], Any],
    Callable[[], list[str]],
    list[str],
]:
  """Lazily loads submodules of a package.

  Returns:
    A tuple of ``__getattr__``, ``__dir__`` function and ``__all__`` --
    a list of available global names, which can be used to replace the
    corresponding definitions in the package.

  Raises:
    RuntimeError: If the ``__name__`` of the caller cannot be determined.
  """
  owner_name = sys._getframe(1).f_globals.get("__name__")
  if owner_name is None:
    raise RuntimeError("Cannot determine the ``__name__`` of the caller.")

  __all__ = list(submodules)

  def __getattr__(name: str) -> Any:
    if name in submodules:
      value = importlib.import_module(f"{package_name}.{name}")
      # Update module-level globals to avoid calling ``__getattr__`` again
      # for this ``name``.
      assert owner_name is not None  # pyrefly#40
      setattr(sys.modules[owner_name], name, value)
      return value
    raise AttributeError(f"module '{package_name}' has no attribute '{name}'")

  def __dir__() -> list[str]:
    return __all__

  return __getattr__, __dir__, __all__

View on GitHub (pinned to 1e1c6a8fc0)

Solutions

1. Call attach at module top level in a real .py module so the caller frame has __name__
2. Pass a namespace that includes __name__ if you must exec the code
3. Avoid wrapping the attach call inside exec/eval or lambda frames
4. Update JAX — this is internal API misuse, not a user configuration issue

import sys
frame_globals = sys._getframe(1).f_globals
assert '__name__' in frame_globals, 'caller must be a real module'

try:
    lazy_loader.attach(...)
except RuntimeError:
    # execute inside a real module file instead
    ...

When it happens

Trigger: Calling lazy_loader.attach from code executed via exec() with a bare dict globals, from a REPL-like namespace without __name__, or from an embedding context where the frame globals lack __name__.

Common situations: Almost never seen by end users; occurs in exotic embedding, notebook kernels, or custom import hooks that strip __name__ from globals, or when attach is invoked inside a function called through C extensions where frame introspection is unusual.

Related errors

• Accumulator aval mismatch: expected {aval}, got {acc.aval}
• {name} cannot accept args with unreduced_kind={mat.unreduced
• module '{package_name}' has no attribute '{name}'
• Unsupported method: {method}
• Unknown action: {action}

AI-assisted analysis of jax-ml/jax@1e1c6a8fc0 (2026-08-27). Data as JSON: /api/errors/64fc032550ceaa56. Report an issue: GitHub.