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ChildContainerRegistrationError

Symptom

Container.add_providers() raises it when called on a child container:

modern_di.exceptions.registration.ChildContainerRegistrationError: Providers can only be registered on a root container: the providers registry is shared tree-wide, so registering on a child container (scope REQUEST) would mutate every container in the tree. Pass groups= to the root Container or call add_providers on it instead.
See: https://modern-di.modern-python.org/troubleshooting/child-container-registration-error/

.container_scope holds the child container's scope.

Cause

Every container in a tree shares one providers registry, so a registration on a child would change what every other container in the tree resolves. add_providers() accepts calls only on a root.

Fix

Register on the root container, either with groups= when you build it or with add_providers() afterwards:

# Broken
request_container.add_providers(late_provider)

# Works
app_container.add_providers(late_provider)

Any root works, whatever its scope. Register at startup, because add_providers() is not coordinated with resolves running at the same time on other threads.

See also