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ContextProvider has no value

A ContextProvider(context_type=SomeType) resolves by looking up SomeType in the container's context registry. If no value was registered, the outcome depends on how the provider is consumed: resolving it directly returns None, while injecting it into a Factory parameter that has no value raises ArgumentResolutionError — unless that parameter has a default (the default is used; None is not injected) or is nullable X | None (then None is injected).

Understanding the error

Cannot resolve dependency chain:
  REQUEST  MyService
  caused by: Argument tenant of type <class 'TenantId'> cannot be resolved. Trying to build dependency <class 'MyService'>.

The error is an ArgumentResolutionError rendered as a chain: the top frame shows which provider failed, and the caused by line names the specific parameter that could not be wired. The parameter cannot be resolved because the ContextProvider for TenantId has no value in this container's context registry — nothing was set for that type on this container.

Common causes

1. set_context was called on the wrong container (scope mismatch)

Context never propagates between containers. A ContextProvider reads the context registry of the container at the provider's own scope. For a REQUEST-scoped provider, only the request container's registry is ever consulted — setting the value on the parent has no effect, regardless of build order.

# ❌ Broken: TenantId provider has scope=Scope.REQUEST, so it reads the REQUEST
# container's registry. Setting it on the APP parent does nothing.
app_container.set_context(TenantId, TenantId("acme"))     # ignored for REQUEST-scoped providers
request_container = app_container.build_child_container(scope=Scope.REQUEST)

Fix: set the value on the container whose scope matches the provider's scope:

# Option A: pass directly to the child when building it
request_container = app_container.build_child_container(
    scope=Scope.REQUEST,
    context={TenantId: TenantId("acme")},
)

# Option B: set on the request container after building it
request_container = app_container.build_child_container(scope=Scope.REQUEST)
request_container.set_context(TenantId, TenantId("acme"))

2. The ContextProvider's scope doesn't match where you set the context

ContextProvider(scope=Scope.APP, context_type=TenantId) looks up the value on the APP container. If you set_context on the REQUEST child container, the APP-scope provider doesn't see it.

Fix: match the scope. If the value is per-request, declare ContextProvider(scope=Scope.REQUEST, ...) and set_context on the request container (or pass via build_child_container(context=...)).

3. Framework integration didn't inject the expected request

Framework integrations (modern-di-fastapi, modern-di-litestar) register the per-request Request/WebSocket automatically. If your code expects, say, fastapi.Request but you're outside the framework's request lifecycle (a background task, a CLI command), no Request is in context and the lookup fails.

Fix: only depend on framework-injected context inside the framework's request handling. For background tasks, build the REQUEST child container yourself and pass the necessary context.

See also

  • Context Provider — the full ContextProvider and set_context API.
  • Scopes — per-container context registries, why context never propagates between containers.
  • Async resources via lifespan — the canonical "construct in lifespan, inject as context" pattern.