Extra Models¶
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Continuing with the previous example, it will be common to have more than one related model.
This is especially the case for user models, because:
- The input model needs to be able to have a password.
- The output model should not have a password.
- The database model would probably need to have a hashed password.
Danger
Never store user's plaintext passwords. Always store a "secure hash" that you can then verify.
If you don't know, you will learn what a "password hash" is in the security chapters.
Multiple models¶
Here's a general idea of how the models could look like with their password fields and the places where they are used:
from fastapi import FastAPI
from pydantic import BaseModel, EmailStr
app = FastAPI()
class UserIn(BaseModel):
username: str
password: str
email: EmailStr
full_name: str | None = None
class UserOut(BaseModel):
username: str
email: EmailStr
full_name: str | None = None
class UserInDB(BaseModel):
username: str
hashed_password: str
email: EmailStr
full_name: str | None = None
def fake_password_hasher(raw_password: str):
return "supersecret" + raw_password
def fake_save_user(user_in: UserIn):
hashed_password = fake_password_hasher(user_in.password)
user_in_db = UserInDB(**user_in.dict(), hashed_password=hashed_password)
print("User saved! ..not really")
return user_in_db
@app.post("/user/", response_model=UserOut)
async def create_user(user_in: UserIn):
user_saved = fake_save_user(user_in)
return user_saved
from typing import Union
from fastapi import FastAPI
from pydantic import BaseModel, EmailStr
app = FastAPI()
class UserIn(BaseModel):
username: str
password: str
email: EmailStr
full_name: Union[str, None] = None
class UserOut(BaseModel):
username: str
email: EmailStr
full_name: Union[str, None] = None
class UserInDB(BaseModel):
username: str
hashed_password: str
email: EmailStr
full_name: Union[str, None] = None
def fake_password_hasher(raw_password: str):
return "supersecret" + raw_password
def fake_save_user(user_in: UserIn):
hashed_password = fake_password_hasher(user_in.password)
user_in_db = UserInDB(**user_in.dict(), hashed_password=hashed_password)
print("User saved! ..not really")
return user_in_db
@app.post("/user/", response_model=UserOut)
async def create_user(user_in: UserIn):
user_saved = fake_save_user(user_in)
return user_saved
About **user_in.dict()¶
Pydantic's .dict()¶
user_in is a Pydantic model of class UserIn.
Pydantic models have a .dict() method that returns a dict with the model's data.
So, if we create a Pydantic object user_in like:
user_in = UserIn(username="john", password="secret", email="john.doe@example.com")
and then we call:
user_dict = user_in.dict()
we now have a dict with the data in the variable user_dict (it's a dict instead of a Pydantic model object).
And if we call:
print(user_dict)
we would get a Python dict with:
{
'username': 'john',
'password': 'secret',
'email': 'john.doe@example.com',
'full_name': None,
}
Unwrapping a dict¶
If we take a dict like user_dict and pass it to a function (or class) with **user_dict, Python will "unwrap" it. It will pass the keys and values of the user_dict directly as key-value arguments.
So, continuing with the user_dict from above, writing:
UserInDB(**user_dict)
Would result in something equivalent to:
UserInDB(
username="john",
password="secret",
email="john.doe@example.com",
full_name=None,
)
Or more exactly, using user_dict directly, with whatever contents it might have in the future:
UserInDB(
username = user_dict["username"],
password = user_dict["password"],
email = user_dict["email"],
full_name = user_dict["full_name"],
)
A Pydantic model from the contents of another¶
As in the example above we got user_dict from user_in.dict(), this code:
user_dict = user_in.dict()
UserInDB(**user_dict)
would be equivalent to:
UserInDB(**user_in.dict())
...because user_in.dict() is a dict, and then we make Python "unwrap" it by passing it to UserInDB prepended with **.
So, we get a Pydantic model from the data in another Pydantic model.
Unwrapping a dict and extra keywords¶
And then adding the extra keyword argument hashed_password=hashed_password, like in:
UserInDB(**user_in.dict(), hashed_password=hashed_password)
...ends up being like:
UserInDB(
username = user_dict["username"],
password = user_dict["password"],
email = user_dict["email"],
full_name = user_dict["full_name"],
hashed_password = hashed_password,
)
Warning
The supporting additional functions are just to demo a possible flow of the data, but they of course are not providing any real security.
Reduce duplication¶
Reducing code duplication is one of the core ideas in FastAPI.
As code duplication increments the chances of bugs, security issues, code desynchronization issues (when you update in one place but not in the others), etc.
And these models are all sharing a lot of the data and duplicating attribute names and types.
We could do better.
We can declare a UserBase model that serves as a base for our other models. And then we can make subclasses of that model that inherit its attributes (type declarations, validation, etc).
All the data conversion, validation, documentation, etc. will still work as normally.
That way, we can declare just the differences between the models (with plaintext password, with hashed_password and without password):
from fastapi import FastAPI
from pydantic import BaseModel, EmailStr
app = FastAPI()
class UserBase(BaseModel):
username: str
email: EmailStr
full_name: str | None = None
class UserIn(UserBase):
password: str
class UserOut(UserBase):
pass
class UserInDB(UserBase):
hashed_password: str
def fake_password_hasher(raw_password: str):
return "supersecret" + raw_password
def fake_save_user(user_in: UserIn):
hashed_password = fake_password_hasher(user_in.password)
user_in_db = UserInDB(**user_in.dict(), hashed_password=hashed_password)
print("User saved! ..not really")
return user_in_db
@app.post("/user/", response_model=UserOut)
async def create_user(user_in: UserIn):
user_saved = fake_save_user(user_in)
return user_saved
from typing import Union
from fastapi import FastAPI
from pydantic import BaseModel, EmailStr
app = FastAPI()
class UserBase(BaseModel):
username: str
email: EmailStr
full_name: Union[str, None] = None
class UserIn(UserBase):
password: str
class UserOut(UserBase):
pass
class UserInDB(UserBase):
hashed_password: str
def fake_password_hasher(raw_password: str):
return "supersecret" + raw_password
def fake_save_user(user_in: UserIn):
hashed_password = fake_password_hasher(user_in.password)
user_in_db = UserInDB(**user_in.dict(), hashed_password=hashed_password)
print("User saved! ..not really")
return user_in_db
@app.post("/user/", response_model=UserOut)
async def create_user(user_in: UserIn):
user_saved = fake_save_user(user_in)
return user_saved
Union or anyOf¶
You can declare a response to be the Union of two types, that means, that the response would be any of the two.
It will be defined in OpenAPI with anyOf.
To do that, use the standard Python type hint typing.Union:
Note
When defining a Union, include the most specific type first, followed by the less specific type. In the example below, the more specific PlaneItem comes before CarItem in Union[PlaneItem, CarItem].
from typing import Union
from fastapi import FastAPI
from pydantic import BaseModel
app = FastAPI()
class BaseItem(BaseModel):
description: str
type: str
class CarItem(BaseItem):
type: str = "car"
class PlaneItem(BaseItem):
type: str = "plane"
size: int
items = {
"item1": {"description": "All my friends drive a low rider", "type": "car"},
"item2": {
"description": "Music is my aeroplane, it's my aeroplane",
"type": "plane",
"size": 5,
},
}
@app.get("/items/{item_id}", response_model=Union[PlaneItem, CarItem])
async def read_item(item_id: str):
return items[item_id]
from typing import Union
from fastapi import FastAPI
from pydantic import BaseModel
app = FastAPI()
class BaseItem(BaseModel):
description: str
type: str
class CarItem(BaseItem):
type: str = "car"
class PlaneItem(BaseItem):
type: str = "plane"
size: int
items = {
"item1": {"description": "All my friends drive a low rider", "type": "car"},
"item2": {
"description": "Music is my aeroplane, it's my aeroplane",
"type": "plane",
"size": 5,
},
}
@app.get("/items/{item_id}", response_model=Union[PlaneItem, CarItem])
async def read_item(item_id: str):
return items[item_id]
Union in Python 3.10¶
In this example we pass Union[PlaneItem, CarItem] as the value of the argument response_model.
Because we are passing it as a value to an argument instead of putting it in a type annotation, we have to use Union even in Python 3.10.
If it was in a type annotation we could have used the vertical bar, as:
some_variable: PlaneItem | CarItem
But if we put that in response_model=PlaneItem | CarItem we would get an error, because Python would try to perform an invalid operation between PlaneItem and CarItem instead of interpreting that as a type annotation.
List of models¶
The same way, you can declare responses of lists of objects.
For that, use the standard Python typing.List (or just list in Python 3.9 and above):
from fastapi import FastAPI
from pydantic import BaseModel
app = FastAPI()
class Item(BaseModel):
name: str
description: str
items = [
{"name": "Foo", "description": "There comes my hero"},
{"name": "Red", "description": "It's my aeroplane"},
]
@app.get("/items/", response_model=list[Item])
async def read_items():
return items
from typing import List
from fastapi import FastAPI
from pydantic import BaseModel
app = FastAPI()
class Item(BaseModel):
name: str
description: str
items = [
{"name": "Foo", "description": "There comes my hero"},
{"name": "Red", "description": "It's my aeroplane"},
]
@app.get("/items/", response_model=List[Item])
async def read_items():
return items
Response with arbitrary dict¶
You can also declare a response using a plain arbitrary dict, declaring just the type of the keys and values, without using a Pydantic model.
This is useful if you don't know the valid field/attribute names (that would be needed for a Pydantic model) beforehand.
In this case, you can use typing.Dict (or just dict in Python 3.9 and above):
from fastapi import FastAPI
app = FastAPI()
@app.get("/keyword-weights/", response_model=dict[str, float])
async def read_keyword_weights():
return {"foo": 2.3, "bar": 3.4}
from typing import Dict
from fastapi import FastAPI
app = FastAPI()
@app.get("/keyword-weights/", response_model=Dict[str, float])
async def read_keyword_weights():
return {"foo": 2.3, "bar": 3.4}
Recap¶
Use multiple Pydantic models and inherit freely for each case.
You don't need to have a single data model per entity if that entity must be able to have different "states". As the case with the user "entity" with a state including password, password_hash and no password.