cocoindex-io/cocoindex · error · TypeError

mount() requires a ComponentSubpath when the function has no

Error message

mount() requires a ComponentSubpath when the function has no __name__. Provide an explicit subpath as the first argument.

What it means

mount() runs a function as a child processing component. It derives the component subpath from the function's __name__, which must be stable across runs for change detection and cleanup. Anonymous callables (lambdas, partials) have no __name__, so mount() requires an explicit ComponentSubpath.

Source

Thrown at python/cocoindex/_internal/api.py:390

    Example:
        await coco.mount(process_file, file, target)

        # With explicit subpath:
        await coco.mount(coco.component_subpath("process", filename), process_file, file, target)
    """
    check_cancellation()
    check_not_in_process_live("coco.mount")

    if pos_args and isinstance(pos_args[0], ComponentSubpath):
        subpath = pos_args[0]
        processor_fn = pos_args[1]
        args = pos_args[2:]
    else:
        processor_fn = pos_args[0]
        args = pos_args[1:]
        name = _default_subpath_name(processor_fn)
        if name is None:
            raise TypeError(
                "mount() requires a ComponentSubpath when the function has no "
                "__name__. Provide an explicit subpath as the first argument."
            )
        subpath = ComponentSubpath(Symbol(name))

    parent_ctx = get_context_from_ctx()
    child_path = build_child_path(parent_ctx, subpath)

    if is_live_component_class(processor_fn):
        instance = processor_fn(*args, **kwargs)
        return await _mount_live_component(parent_ctx, child_path, instance)

    processor = create_core_component_processor(
        processor_fn, parent_ctx._env, child_path, args, kwargs
    )
    resolved = parent_ctx.resolve_exception_handler(
        stable_path=child_path.to_string(),
        processor_name=getattr(processor_fn, "__qualname__", None),

View on GitHub (pinned to e84aa99b32)

Solutions

1. Pass an explicit subpath first: coco.mount(coco.component_subpath('step'), fn, *args)
2. Define a named function (def) instead of the anonymous callable
3. Add @functools.wraps to custom decorators so wrapped functions keep __name__

Example fix

// before
await coco.mount(lambda f, t: t.declare_file(f.name, await f.read_text()), f, target)
// after
await coco.mount(coco.component_subpath("copy-file"), copy_file, f, target)

subpath = coco.component_subpath("step") if getattr(fn, "__name__", "<lambda>") == "<lambda>" else None
await coco.mount(subpath, fn, *args) if subpath else await coco.mount(fn, *args)

def mountable(fn) -> bool:
    return callable(fn) and bool(getattr(fn, "__name__", ""))

try:
    await coco.mount(fn, *args)
except TypeError as e:
    if "requires a ComponentSubpath" in str(e):
        await coco.mount(coco.component_subpath("step"), fn, *args)

When it happens

Trigger: Calling coco.mount(lambda ...: ..., ...) or coco.mount(functools.partial(fn), ...) with no explicit ComponentSubpath as the first argument.

Common situations: Inline lambdas in pipelines; decorated functions losing __name__ (missing functools.wraps); factory-produced closures.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• use_mount() requires a ComponentSubpath when the function ha
• mount_each() requires a ComponentSubpath when the function h
• LiveComponent classes cannot be used with use_mount(). Use m
• Cannot use sync 'with coco.runtime()' from within an async e
• coco.use_state() cannot be called inside a `with coco.compon

AI-assisted analysis of cocoindex-io/cocoindex@e84aa99b32 (2026-09-08). Data as JSON: /api/errors/ce7c28de3d9cc233. Report an issue: GitHub.