cocoindex-io/cocoindex · error · TypeError

use_mount() requires a ComponentSubpath when the function ha

Error message

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

What it means

use_mount() runs a function as a child processing component and returns its value directly. The component subpath is normally derived from the function's __name__; when the callable has no usable __name__ (e.g. a lambda, functools.partial, or other anonymous callable), CocoIndex cannot derive a stable path and refuses to guess, since stable component paths are how runs are matched across updates.

Source

Thrown at python/cocoindex/_internal/api.py:270

    Example:
        target = await coco.use_mount(declare_table_target, table_name)

        # With explicit subpath:
        target = await coco.use_mount(
            coco.component_subpath("setup"), declare_table_target, table_name
        )
    """
    check_cancellation()
    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(
                "use_mount() requires a ComponentSubpath when the function has no "
                "__name__. Provide an explicit subpath as the first argument."
            )
        subpath = ComponentSubpath(Symbol(name))

    check_not_in_process_live("coco.use_mount")

    if is_live_component_class(processor_fn):
        raise TypeError(
            "LiveComponent classes cannot be used with use_mount(). "
            "Use mount() instead."
        )

    parent_ctx = get_context_from_ctx()
    child_path = build_child_path(parent_ctx, subpath)
    # One explicit value travels through core: capture once, pass to both
    # the processor (ContextVar restore in the wrapper) and the engine call.
    deadline_context = _deadline_for_engine()

View on GitHub (pinned to e84aa99b32)

Solutions

Pass an explicit ComponentSubpath as the first argument: coco.use_mount(coco.component_subpath('my-step'), fn, *args)
Use a named function (def) instead of a lambda/partial so the subpath can be auto-derived
Set fn.__name__ on the callable if it is safe and the name is meaningful

Example fix

// before
result = await coco.use_mount(lambda ctx: build(ctx), ctx)
// after
result = await coco.use_mount(coco.component_subpath("build"), build, ctx)

Defensive patterns

Strategy: validation

Validate before calling

def has_name(fn):
    return getattr(fn, "__name__", None) not in (None, "<lambda>")

if not has_name(fn):
    fn = coco.component_subpath("my-step")  # pass subpath explicitly

Type guard

def is_named_fn(fn) -> bool:
    return callable(fn) and getattr(fn, "__name__", "<lambda>") != "<lambda>"

Prevention

Avoid lambdas/partials as mounted functions; prefer named @coco.fn functions
Add functools.wraps to all custom decorators
Pass an explicit component_subpath whenever the callable may be anonymous

When it happens

Trigger: Calling coco.use_mount(lambda ...: ..., ...) or coco.use_mount(functools.partial(fn, x)) without passing an explicit ComponentSubpath as the first positional argument.

Common situations: Passing lambdas or partially-applied functions inline for quick one-off mounts; wrapping functions with decorators that strip __name__; dynamically generated callables.

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

AI-assisted analysis of cocoindex-io/cocoindex@e84aa99b32 (2026-09-08). Data as JSON: /api/errors/8ab682f9d4e57cb0. Report an issue: GitHub.