tiangolo.fastapi/docs/tutorial/header-params.md

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

Import Header

First import Header:

{!./tutorial/src/header_params/tutorial001.py!}

Declare Header parameteres

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 parameteres:

{!./tutorial/src/header_params/tutorial001.py!}

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

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

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:

{!./tutorial/src/header_params/tutorial002.py!}

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

Recap

Declare headeres 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.