pypa/pip · error · NotImplementedError

Can't perform this operation for unregistered loader type

Error message

Can't perform this operation for unregistered loader type

What it means

Raised as NotImplementedError by NullProvider._has(path), the base implementation that checks whether a resource path exists. NullProvider is the catch-all provider registered for the generic `object` loader type (register_loader_type(object, NullProvider)); it has no filesystem semantics, so subclasses (DefaultProvider, EggProvider, ZipProvider) override _has with real implementations. If a module's loader type was never registered with a concrete provider, you get NullProvider and these stubs fire.

Source

Thrown at src/pip/_vendor/pkg_resources/__init__.py:1728

        namespace['__file__'] = script_filename
        if os.path.exists(script_filename):
            source = _read_utf8_with_fallback(script_filename)
            code = compile(source, script_filename, 'exec')
            exec(code, namespace, namespace)
        else:
            from linecache import cache

            cache[script_filename] = (
                len(script_text),
                0,
                script_text.split('\n'),
                script_filename,
            )
            script_code = compile(script_text, script_filename, 'exec')
            exec(script_code, namespace, namespace)

    def _has(self, path) -> bool:
        raise NotImplementedError(
            "Can't perform this operation for unregistered loader type"
        )

    def _isdir(self, path) -> bool:
        raise NotImplementedError(
            "Can't perform this operation for unregistered loader type"
        )

    def _listdir(self, path) -> list[str]:
        raise NotImplementedError(
            "Can't perform this operation for unregistered loader type"
        )

    def _fn(self, base: str | None, resource_name: str):
        if base is None:
            raise TypeError(
                "`base` parameter in `_fn` is `None`. Either override this method or check the parameter first."
            )

View on GitHub (pinned to d7d0d0a394)

Solutions

1. Register a concrete provider for your loader type with pkg_resources.register_loader_type(MyLoader, DefaultProvider) (or a custom provider subclass overriding _has/_isdir/_listdir/_get).
2. Use importlib.resources.files() / Traversable, which is the supported replacement and handles arbitrary loaders via importlib's own machinery.
3. Catch NotImplementedError and degrade gracefully if filesystem-style resource queries are unavailable for that loader.

Example fix

// before
pkg_resources.has_resource('mymod/data.txt')  # NotImplementedError on exotic loader

// after
from importlib.resources import files
exists = (files('mymod') / 'data.txt').is_file()

import pkg_resources
from pkg_resources import NullProvider, DefaultProvider

def can_has_resource(module):
    provider = pkg_resources.get_provider(module)
    # NullProvider._has is the unimplemented stub; concrete providers override it
    return type(provider)._has is not NullProvider._has

from pkg_resources import NullProvider

def has_concrete_provider(provider):
    return type(provider)._has is not NullProvider._has

try:
    exists = pkg_resources.has_resource(module, name)
except NotImplementedError:
    exists = None  # unsupported for this loader

When it happens

Trigger: Calling has_resource() (or any API delegating to _has, e.g. metadata_has_metadata checks) on a module whose import loader only maps to NullProvider, i.e. a loader type for which no richer provider was registered via register_loader_type().

Common situations: Custom PEP 302/451 import hooks, frozen/embedded modules, or runtime-generated modules whose loader type is not in the provider registry. The module is importable but pkg_resources cannot introspect its resource tree.

Related errors

• Can't perform this operation for loaders without 'get_data()
• `base` parameter in `_fn` is `None`. Either override this me
• Error decoding metadata for {self._wheel_name}: {e} in {name
• Wheel '{name}' located at {location} is invalid.
• Python 3.8 or later is required

AI-assisted analysis of pypa/pip@d7d0d0a394 (2026-08-04). Data as JSON: /data/errors/62bc28b7ca8f7e65.json. Report an issue: GitHub.