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What will happen if you forget to use File(...) when declaring multiple file uploads in FastAPI?

medium📝 component behavior Q5 of 15
FastAPI - File Handling
What will happen if you forget to use File(...) when declaring multiple file uploads in FastAPI?
async def upload(files: List[UploadFile]):
AFastAPI will raise a validation error at runtime
BFiles will upload normally without issues
CThe endpoint will accept only one file
DThe server will crash immediately
Step-by-Step Solution
Solution:

  1. Step 1: Understand the role of File(...) in FastAPI

    File(...) marks the parameter as a file upload and required. Without it, FastAPI cannot parse files correctly.

  2. Step 2: Predict behavior without File(...)

    Missing File(...) causes FastAPI to raise a validation error because it expects a different data type.

  3. Final Answer:

    FastAPI will raise a validation error at runtime -> Option A

  4. Quick Check:

    Missing File(...) causes validation error [OK]
Quick Trick: Always use File(...) to mark file parameters required [OK]
Common Mistakes:
MISTAKES
  • Assuming files upload without File(...)
  • Using UploadFile without File(...)
  • Expecting silent failure instead of error

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