How does FastAPI resolve and cache dependencies for one request?

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

1Say itAnswer out loud first

2CompareCompare with Core answer

3RefineRefine with Stronger and Advanced

TL;DR

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.

Full answer below

Practice focus

Whether the candidate understands dependency execution and can avoid using it as an invisible global service locator.

Answer first

Answer path

Core answer first, then checkpoints toward a stronger answer.

Start
Core answer
Start here. This is the direct spoken answer to practice first.
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.
Checkpoint
Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
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.

Review signals

Use these to catch weak phrasing, shallow reasoning, and risky claims.

Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

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?

Practice focus

Answer path

Overview

Review signals

Follow-up prompts

Continue the section

Tech stacks

Skill areas

Suggest an improvement

Found this answer useful?