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Application Basics
FastAPI app uvicorn development server path operation path parameter query parameter request body Pydantic model response model status code response headers
Validation
field constraints optional fields nested models list validation custom validator model validator query constraints path constraints header parameters cookie parameters
Dependencies
Depends basics dependency with parameters class dependency sub-dependencies dependency caching yield dependency database session dependency authentication dependency router dependencies global dependencies
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Security
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Async & Integration
async endpoint sync endpoint behavior background tasks WebSocket endpoint lifespan context file upload form data external API with httpx async database query task cancellation
Testing
TestClient dependency override async test request validation test auth test WebSocket test mock external API startup lifespan test response schema test error handler test
Production
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FastAPI / Validation

header parameters

Learn header parameters in FastAPI with a dedicated explanation, a topic-specific example, edge guidance, and a mastery check.

12 min read • Beginner
fastapi-019.py
python 
from fastapi import Header
@app.get("/trace")
def trace(x_request_id:str|None=Header(default=None)): return {"request_id":x_request_id}
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