How does FastAPI resolve and cache dependencies for one request?

Explains dependency graphs, sub-dependencies, request-local caching, failures, and deliberate cache bypass.

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FastAPI builds a dependency graph from the path operation and every declared Depends. It resolves sub-dependencies before the callable that needs them and passes each result into its consumer. By default, the result of the same dependency is reused within one request, so two branches can share one authenticated user or session instead of repeating the work. use_cache=False asks FastAPI to call that dependency again, and an exception from a dependency can end request processing before the handler runs.

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Overview
FastAPI dependencies are callables in a per-operation graph, not magic global objects.
FastAPI builds a dependency graph from the path operation and every declared Depends. It resolves sub-dependencies before the callable that needs them and passes each result into its consumer. By default, the result of the same dependency is reused within one request, so two branches can share one authenticated user or session instead of repeating the work. use_cache=False asks FastAPI to call that dependency again, and an exception from a dependency can end request processing before the handler runs.
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Request caching is keyed by the dependency invocation FastAPI has planned, not by an application-wide singleton. That makes it useful for request context and resource acquisition, but it can surprise code that expects a fresh timestamp, nonce, or read on every injection. I use sub-dependencies to compose boundary concerns such as authentication and tenant lookup, while keeping business orchestration in explicit services. The graph remains inspectable in the endpoint signature rather than becoming a hidden lookup mechanism.

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Describing dependency caching as a process-wide singleton.
Using use_cache=False without considering repeated I/O.
Putting irreversible business side effects in dependency resolution.
Treating Depends as an arbitrary object lookup from anywhere.

How does FastAPI resolve and cache dependencies for one request?

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