How do you distinguish missing fields from explicit nulls in a FastAPI PATCH endpoint?

Models partial updates without confusing omitted fields, explicit nulls, defaults, or unauthorized changes.

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TL;DR

I use a dedicated update model whose fields represent only values the operation may change. Pydantic's model_fields_set records which fields the client explicitly supplied, and model_dump(exclude_unset=True) can produce that subset. An omitted field means leave the stored value unchanged; a supplied null means clear it only when the contract allows null. A defaulted value is different again, because it may appear on the model even though the client did not send it.

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PATCH semantics, Pydantic v2 field-state tracking, nullability, and safe update mapping.

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A partial update needs to preserve the difference between 'leave it alone' and 'clear it.'
I use a dedicated update model whose fields represent only values the operation may change. Pydantic's model_fields_set records which fields the client explicitly supplied, and model_dump(exclude_unset=True) can produce that subset. An omitted field means leave the stored value unchanged; a supplied null means clear it only when the contract allows null. A defaulted value is different again, because it may appear on the model even though the client did not send it.
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Details

Using every optional field as if omission and null were identical.
Calling model_dump() without preserving which fields were supplied.
Blindly applying the patch dictionary to an ORM object.
Leaving nested merge behavior undefined.

How do you distinguish missing fields from explicit nulls in a FastAPI PATCH endpoint?

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