Validations in FastAPI

There are 3 objects with which we can execute validations, consult these are, Path and Fields. The 3 have the same methods and in fact, inherit a common "param" class.

String validation

For validations we import the Query object:

from fastapi import FastAPI, Query

app = FastAPI()

@app.get("/items/")
async def read_items(
    q: str
    | None = Query(
        default=None,
        alias="item-query",
        title="Query string",
        description="Query string for the items to search in the database that have a good match",
        min_length=3,
        max_length=50,
        regex="^fixedquery$",
        deprecated=True,
    )
):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})
    return results

Number validations

from fastapi import FastAPI, Path, Query

app = FastAPI()

@app.get("/items/{item_id}")
async def read_items(
    *,
    item_id: int = Path(title="The ID of the item to get", ge=0, le=1000),
    q: str,
    size: float = Query(gt=0, lt=10.5)
):
    results = {"item_id": item_id}
    if q:
        results.update({"q": q})
    return results

gt: greater than, ge: greater than or equal, lt: less than, le: less than or equal

Get a list

from typing import List

from fastapi import FastAPI, Query

app = FastAPI()

@app.get("/items/")
async def read_items(q: List[str] = Query(default=["foo", "bar"])):
    query_items = {"q": q}
    return query_items

Required field

Para que un campo sea requerido simplemente debe ser definido en la función de la ruta y no recibir un valor por defecto, a continuación, se muestran dos formas alternativas para hacer que un parámetro sea requerido.

Ellipsis

from fastapi import FastAPI, Query

app = FastAPI()

@app.get("/items/")
async def read_items(q: str | None = Query(default=..., min_length=3)):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})
    return results

Require

from fastapi import FastAPI, Query
from pydantic import Required

app = FastAPI()

@app.get("/items/")
async def read_items(q: str = Query(default=Required, min_length=3)):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})
    return results

Obtain parameters by name and not by order

In the following example, 3 parameters are obtained, but when a parameter passes as first parameter, then each parameter will be associated with its key, indistinctly of the order in which they are declared and obtained.

from fastapi import FastAPI, Path
from pydantic import BaseModel

app = FastAPI()

class Item(BaseModel):
    name: str
    description: str | None = None
    price: float
    tax: float | None = None

@app.put("/items/{item_id}")
async def update_item(
    *,
    item_id: int = Path(title="The ID of the item to get", ge=0, le=1000),
    q: str | None = None,
    item: Item | None = None,
):
    results = {"item_id": item_id}
    if q:
        results.update({"q": q})
    if item:
        results.update({"item": item})
    return results

Receive a JSON as parameter

Receive a JSON

from fastapi import FastAPI, Path from pydantic import BaseModel app = FastAPI() class Item(BaseModel): name: str description: str | None = None price: float tax: float | None = None @app.put("/items/{item_id}") async def update_item( *, item_id: int = Path(title="The ID of the item to get", ge=0, le=1000), q: str | None = None, item: Item | None = None, ): results = {"item_id": item_id} if q: results.update({"q": q}) if item: results.update({"item": item}) return results

In this case a JSON is received in this way:

{
    "name": "Foo",
    "description": "The pretender",
    "price": 42.0,
    "tax": 3.2
}

Receive a JSON that corresponds to various classes

Upon receiving parameters that correspond to the Basemodel classes, Fastapi expects a specific JSON structure, where you can even obtain more values of the JSON body:

from fastapi import Body, FastAPI
from pydantic import BaseModel

app = FastAPI()

class Item(BaseModel):
    name: str
    description: str | None = None
    price: float
    tax: float | None = None

class User(BaseModel):
    username: str
    full_name: str | None = None

@app.put("/items/{item_id}")
async def update_item(item_id: int, item: Item, user: User, importance: int = Body()):
    results = {"item_id": item_id, "item": item, "user": user, "importance": importance}
    return results

In this case the JSON must have the following structure:

{
    "item": {
        "name": "Foo",
        "description": "The pretender",
        "price": 42.0,
        "tax": 3.2
    },
    "user": {
        "username": "dave",
        "full_name": "Dave Grohl"
    },
    "importance": 5
}

Receive JSON content within a key

from fastapi import Body, FastAPI
from pydantic import BaseModel

app = FastAPI()

class Item(BaseModel):
    name: str
    description: str | None = None
    price: float
    tax: float | None = None

@app.put("/items/{item_id}")
async def update_item(item_id: int, item: Item = Body(embed=True)):
    results = {"item_id": item_id, "item": item}
    return results

JSON's structure would be as follows:

{
    "item": {
        "name": "Foo",
        "description": "The pretender",
        "price": 42.0,
        "tax": 3.2
    }
}

in place of:

{
    "name": "Foo",
    "description": "The pretender",
    "price": 42.0,
    "tax": 3.2
}

A class containing another class

from fastapi import Body, FastAPI
from pydantic import BaseModel, Field

app = FastAPI()

class Item(BaseModel):
    name: str
    description: str | None = Field(
        default=None, title="The description of the item", max_length=300
    )
    price: float = Field(gt=0, description="The price must be greater than zero")
    tax: float | None = None

@app.put("/items/{item_id}")
async def update_item(item_id: int, item: Item = Body(embed=True)):
    results = {"item_id": item_id, "item": item}
    return results

The expected json is in this way

{
    "name": "Foo",
    "description": "The pretender",
    "price": 42.0,
    "tax": 3.2,
    "tags": ["rock", "metal", "bar"],
    "image": {
        "url": "http://example.com/baz.jpg",
        "name": "The Foo live"
    }
}

Subclasses lists

from fastapi import FastAPI
from pydantic import BaseModel, HttpUrl

app = FastAPI()

class Image(BaseModel):
    url: HttpUrl
    name: str

class Item(BaseModel):
    name: str
    description: str | None = None
    price: float
    tax: float | None = None
    tags: set[str] = set()
    images: list[Image] | None = None

@app.put("/items/{item_id}")
async def update_item(item_id: int, item: Item):
    results = {"item_id": item_id, "item": item}
    return results

JSON:

{
    "name": "Foo",
    "description": "The pretender",
    "price": 42.0,
    "tax": 3.2,
    "tags": [
        "rock",
        "metal",
        "bar"
    ],
    "images": [
        {
            "url": "http://example.com/baz.jpg",
            "name": "The Foo live"
        },
        {
            "url": "http://example.com/dave.jpg",
            "name": "The Baz"
        }
    ]
}

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