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 by NullProvider._has() — a stub that always raises NotImplementedError because the base NullProvider has no knowledge of how to check file existence for an arbitrary PEP 302/451 loader. Only concrete subclasses (DefaultProvider, EggProvider, ZipProvider) override _has() with a real filesystem implementation. Hitting this means the provider in use is the un-extended base.

Solutions

Register your loader type with pkg_resources.register_loader_type(MyLoader, DefaultProvider) so resource ops use a concrete provider.
Ensure the module has a standard __file__-backed filesystem loader so EggProvider/DefaultProvider is selected automatically.
Avoid pkg_resources resource APIs for custom loaders; use importlib.resources instead.

Example fix

// before
# custom loader, NullProvider._has() raises
has = provider.has_resource('data.txt')
// after
import pkg_resources
pkg_resources.register_loader_type(MyLoader, pkg_resources.DefaultProvider)
has = provider.has_resource('data.txt')

Defensive patterns

Strategy: try-catch

Validate before calling

# Register loader before using resource APIs
import pkg_resources
pkg_resources.register_loader_type(type(module.__loader__), pkg_resources.DefaultProvider)

Try / catch

try:
    provider.has_resource(name)
except NotImplementedError:
    # fallback to importlib.resources or register loader

Prevention

Register custom loaders with register_loader_type().
Migrate to importlib.resources for loader-agnostic access.

When it happens

Trigger: A resource operation (has_resource/has_metadata) routed to a NullProvider instance because the module's loader type was not registered with register_loader_type() to map it to a concrete provider like DefaultProvider.

Common situations: Using a custom import hook/loader that is not registered; a namespace package or module loaded by an unusual loader (e.g. in a frozen app, embedded interpreter) that pkg_resources does not recognize.

Related errors

AI-assisted analysis of pypa/pip@f399c37189 (2026-08-08). Data as JSON: /api/errors/62bc28b7ca8f7e65. Report an issue: GitHub.

Appendix: 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."
            )

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