ContextProvider has no value¶

ContextValueNotSetError means a ContextProvider was resolved, directly or for a required parameter, on a container that holds no value for its type, and the provider has no default=.

Symptom¶

modern_di.exceptions.resolution.ContextValueNotSetError: Cannot resolve dependency chain:
  REQUEST  MyService (myapp.ctx:9)
  caused by: No context value is set for <class 'myapp.ctx.TenantId'> (scope REQUEST), needed for argument tenant. Pass context={...} to the container or call set_context(), or pass default= to the ContextProvider.
See: https://modern-di.modern-python.org/troubleshooting/context-not-set/

The top frame shows which provider failed, and the caused by line names the context type, the scope whose container was read, and the parameter it was needed for. A direct container.resolve(TenantId) raises the same error without the chain and without the needed for argument part. Inspect .context_type, .provider_scope, and .parameter_name (the parameter, or None for a direct resolve).

When a creator body calls container.resolve(TenantId) itself, the chain ends at that creator's provider and the message names no argument, because the value was not injected into a parameter.

Cause¶

1. The value is set on a container of a different scope¶

A ContextProvider reads only the container of its own scope, so a value set on a parent or a child container never reaches it. Set the value on the container whose scope matches the provider's scope, with context= when building it or set_context() afterwards; Context propagation shows both, and the rule in each direction.

If the value really is per request, declare the provider with scope=Scope.REQUEST. If it is one value for the whole app, declare it with scope=Scope.APP and set it on the APP container.

2. 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.

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

3. The value is optional¶

If a creator also runs where no value is set, make its parameter optional (tenant: TenantId | None = None): it then gets its default, or None. A parameter whose provider is another Factory that needs the value still raises. See Optional parameters.

To make the value optional for every consumer, direct resolves included, give the provider a default. See Optional context: default=:

tenant = providers.ContextProvider(TenantId, scope=Scope.REQUEST, default=None)