[Solved] What is wrong with this FastAPI TrustedHostMiddleware usage? ```python app.... What is wrong with this FastAPI TrustedHostMiddleware usage? | FastAPI

FastAPI - Middleware and Hooks

What is wrong with this FastAPI TrustedHostMiddleware usage? ```python app.add_middleware(TrustedHostMiddleware, allowed_hosts=['example.com']) app.add_middleware(TrustedHostMiddleware, allowed_hosts=['localhost']) ```

AAdding TrustedHostMiddleware twice causes multiple independent checks to apply

Ballowed_hosts must be a single string, not list

CMiddleware cannot be added after route definitions

Dallowed_hosts parameter name is incorrect

Step-by-Step Solution

Solution:

Step 1: Understand middleware stacking in FastAPI
Adding the same middleware twice stacks separate instances, each with its own allowed_hosts list.
Step 2: Analyze the effect on trusted hosts
The request must pass all checks (host in both lists). Here, no overlap, so effectively blocks all; lists do not merge.
Final Answer:
Adding TrustedHostMiddleware twice causes multiple independent checks to apply -> Option A
Quick Check:
Multiple TrustedHostMiddleware calls stack independent checks = C [OK]

Quick Trick: Add TrustedHostMiddleware once with all hosts in list [OK]

Common Mistakes:

MISTAKES

Adding middleware multiple times expecting merge
Wrong parameter type
Adding middleware after routes

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What is wrong with this FastAPI TrustedHostMiddleware usage?

Step 1: Understand middleware stacking in FastAPI

Step 2: Analyze the effect on trusted hosts

Final Answer:

Quick Check:

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