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Header Parameters

You can define Header parameters the same way you define Query, Path and Cookie parameters.

Import Header

First import Header:

from fastapi import FastAPI, Header

app = FastAPI()


@app.get("/items/")
async def read_items(*, user_agent: str = Header(None)):
    return {"User-Agent": user_agent}

Declare Header parameters

Then declare the header parameters using the same structure as with Path, Query and Cookie.

The first value is the default value, you can pass all the extra validation or annotation parameters:

from fastapi import FastAPI, Header

app = FastAPI()


@app.get("/items/")
async def read_items(*, user_agent: str = Header(None)):
    return {"User-Agent": user_agent}

Technical Details

Header is a "sister" class of Path, Query and Cookie. It also inherits from the same common Param class.

But remember that when you import Query, Path, Header, and others from fastapi, those are actually functions that return classes of the same name.

Info

To declare headers, you need to use Header, because otherwise the parameters would be interpreted as query parameters.

Automatic conversion

Header has a little extra functionality on top of what Path, Query and Cookie provide.

Most of the standard headers are separated by a "hyphen" character, also known as the "minus symbol" (-).

But a variable like user-agent is invalid in Python.

So, by default, Header will convert the parameter names characters from underscore (_) to hyphen (-) to extract and document the headers.

Also, HTTP headers are case-insensitive, so, you can declare them with standard Python style (also known as "snake_case").

So, you can use user_agent as you normally would in Python code, instead of needing to capitalize the first letters as User_Agent or something similar.

If for some reason you need to disable automatic conversion of underscores to hyphens, set the parameter convert_underscores of Header to False:

from fastapi import FastAPI, Header

app = FastAPI()


@app.get("/items/")
async def read_items(*, strange_header: str = Header(None, convert_underscores=False)):
    return {"strange_header": strange_header}

Warning

Before setting convert_underscores to False, bear in mind that some HTTP proxies and servers disallow the usage of headers with underscores.

Duplicate headers

It is possible to receive duplicate headers. That means, the same header with multiple values.

You can define those cases using a list in the type declaration.

You will receive all the values from the duplicate header as a Python list.

For example, to declare a header of X-Token that can appear more than once, you can write:

from typing import List

from fastapi import FastAPI, Header

app = FastAPI()


@app.get("/items/")
async def read_items(x_token: List[str] = Header(None)):
    return {"X-Token values": x_token}

If you communicate with that path operation sending two HTTP headers like:

X-Token: foo
X-Token: bar

The response would be like:

{
    "X-Token values": [
        "bar",
        "foo"
    ]
}

Recap

Declare headers with Header, using the same common pattern as Query, Path and Cookie.

And don't worry about underscores in your variables, FastAPI will take care of converting them.