The problem is once FastAPI sees item: Item in your route definition, it will try to initialize an Item type from the request body, and you can't declare your model's fields to be optional sometimes depending on some conditional, such as depending on which route it is used.
I have 3 solutions:
Solution #1: Separate Models
I would say that having separate models for the POST and PATCH payloads seems to be the more logical and readable approach. It might lead to duplicated code, yes, but I think clearly defining which route has an all-required or an all-optional model balances out the maintainability cost.
The FastAPI docs has a section for partially updating models with PUT or PATCH that uses Optional fields, and there's a note at the end that says something similar:
Notice that the input model is still validated.
So, if you want to receive partial updates that can omit all the attributes, you need to have a model with all the attributes marked as optional (with default values or None).
So...
class NewItem(BaseModel):
name: str
description: str
price: float
tax: float
class UpdateItem(BaseModel):
name: Optional[str] = None
description: Optional[str] = None
price: Optional[float] = None
tax: Optional[float] = None
@app.post('/items', response_model=NewItem)
async def post_item(item: NewItem):
return item
@app.patch('/items/{item_id}',
response_model=UpdateItem,
response_model_exclude_none=True)
async def update_item(item_id: str, item: UpdateItem):
return item
Solution #2: Declare as All-Required, but Manually Validate for PATCH
You can define your model to have all-required fields, then define your payload as a regular Body parameter on the PATCH route, and then initialize the actual Item object "manually" depending on what's available in the payload.
from fastapi import Body
from typing import Dict
class Item(BaseModel):
name: str
description: str
price: float
tax: float
@app.post('/items', response_model=Item)
async def post_item(item: Item):
return item
@app.patch('/items/{item_id}', response_model=Item)
async def update_item(item_id: str, payload: Dict = Body(...)):
item = Item(
name=payload.get('name', ''),
description=payload.get('description', ''),
price=payload.get('price', 0.0),
tax=payload.get('tax', 0.0),
)
return item
Here, the Item object is initialized with whatever is in the payload, or some default if there isn't one. You'll have to manually validate if none of the expected fields are passed, ex.:
from fastapi import HTTPException
@app.patch('/items/{item_id}', response_model=Item)
async def update_item(item_id: str, payload: Dict = Body(...)):
# Get intersection of keys/fields
# Must have at least 1 common
if not (set(payload.keys()) & set(Item.__fields__)):
raise HTTPException(status_code=400, detail='No common fields')
...
$ cat test2.json
{
"asda": "1923"
}
$ curl -i -H'Content-Type: application/json' --data @test2.json --request PATCH localhost:8000/items/1
HTTP/1.1 400 Bad Request
content-type: application/json
{"detail":"No common fields"}
The behavior for the POST route is as expected: all the fields must be passed.
Solution #3: Declare as All-Optional But Manually Validate for POST
Pydantic's BaseModel's dict method has exclude_defaults and exclude_none options for:
exclude_defaults: whether fields which are equal to their default values (whether set or otherwise) should be excluded from the returned dictionary; default False
exclude_none: whether fields which are equal to None should be excluded from the returned dictionary; default False
This means, for both POST and PATCH routes, you can use the same Item model, but now with all Optional[T] = None fields. The same item: Item parameter can also be used.
class Item(BaseModel):
name: Optional[str] = None
description: Optional[str] = None
price: Optional[float] = None
tax: Optional[float] = None
On the POST route, if not all the fields were set, then exclude_defaults and exclude_none will return an incomplete dict, so you can raise an error. Else, you can use the item as your new Item.
@app.post('/items', response_model=Item)
async def post_item(item: Item):
new_item_values = item.dict(exclude_defaults=True, exclude_none=True)
# Check if exactly same set of keys/fields
if set(new_item_values.keys()) != set(Item.__fields__):
raise HTTPException(status_code=400, detail='Missing some fields..')
# Use `item` or `new_item_values`
return item
$ cat test_empty.json
{
}
$ curl -i -H'Content-Type: application/json' --data @test_empty.json --request POST localhost:8000/items
HTTP/1.1 400 Bad Request
content-type: application/json
{"detail":"Missing some fields.."}
$ cat test_incomplete.json
{
"name": "test-name",
"tax": 0.44
}
$ curl -i -H'Content-Type: application/json' --data @test_incomplete.json --request POST localhost:8000/items
HTTP/1.1 400 Bad Request
content-type: application/json
{"detail":"Missing some fields.."}
$ cat test_ok.json
{
"name": "test-name",
"description": "test-description",
"price": 123.456,
"tax": 0.44
}
$ curl -i -H'Content-Type: application/json' --data @test_ok.json --request POST localhost:8000/items
HTTP/1.1 200 OK
content-type: application/json
{"name":"test-name","description":"test-description","price":123.456,"tax":0.44}
On the PATCH route, if at least 1 value is not default/None, then that will be your update data. Use the same validation from Solution 2 to fail if none of the expected fields were passed in.
@app.patch('/items/{item_id}', response_model=Item)
async def update_item(item_id: str, item: Item):
update_item_values = item.dict(exclude_defaults=True, exclude_none=True)
# Get intersection of keys/fields
# Must have at least 1 common
if not (set(update_item_values.keys()) & set(Item.__fields__)):
raise HTTPException(status_code=400, detail='No common fields')
update_item = Item(**update_item_values)
return update_item
$ cat test2.json
{
"asda": "1923"
}
$ curl -i -s -H'Content-Type: application/json' --data @test2.json --request PATCH localhost:8000/items/1
HTTP/1.1 400 Bad Request
content-type: application/json
{"detail":"No common fields"}
$ cat test2.json
{
"description": "test-description"
}
$ curl -i -s -H'Content-Type: application/json' --data @test2.json --request PATCH localhost:8000/items/1
HTTP/1.1 200 OK
content-type: application/json
{"name":null,"description":"test-description","price":null,"tax":null}
