* Check that the tests **fail** before the PR. 🚨

* Then check that the tests **pass** after the PR. ✅

* Many PRs don't have tests, you can **remind** them to add tests, or you can even **suggest** some tests yourself. That's one of the things that consume most time and you can help a lot with that.

* Then also comment what you tried, that way I'll know that you checked it. 🤓

## Create a Pull Request

## OAuth2

OAuth2 is a specification that defines several ways to handle authentication and authorization.

It is quite an extensive specification and covers several complex use cases.

It includes ways to authenticate using a "third party".

That's what all the systems with "login with Facebook, Google, Twitter, GitHub" use underneath.

### OAuth 1

title: ''
labels: community, triage
assignees: ''

---

**Is your feature request related to a problem? Please describe.**
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

**Describe the solution you'd like**
A clear and concise description of what you want to happen.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

-->

#### What type of PR is this?

<!--
Add one of the following kinds:
/kind bug
/kind cleanup
/kind documentation
/kind feature

Optionally add one or more of the following kinds if applicable:
/kind api-change
/kind deprecation
/kind failing-test
/kind flake
/kind regression
-->

#### What this PR does / why we need it:

**Describe the bug**
A clear and concise description of what the bug is.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

**Expected behavior**
A clear and concise description of what you expected to happen.

**Screenshots**
If applicable, add screenshots to help explain your problem.

    attributes:
      label: Example
      description: >
        Please provide an example usage of the feature that would be different with the improvement.
      render: java
    validations:
      required: true

  - type: textarea
    attributes:
      label: Current Behavior
      description: What does the feature currently do?
    validations:
      required: true

  - type: textarea
    attributes:

  - name: Feature Request
    about: To suggest an idea or ask about a feature, please start with a question saying what you would like to achieve. There might be a way to do it already.
    url: https://github.com/fastapi/fastapi/discussions/categories/questions
  - name: Show and tell
    about: Show what you built with FastAPI or to be used with FastAPI.
    url: https://github.com/fastapi/fastapi/discussions/categories/show-and-tell

)

// SHA256Mismatch - when content sha256 does not match with what was sent from client.
type SHA256Mismatch struct {
	ExpectedSHA256   string
	CalculatedSHA256 string
}

func (e SHA256Mismatch) Error() string {
	return "Bad sha256: Expected " + e.ExpectedSHA256 + " does not match calculated " + e.CalculatedSHA256
}

// BadDigest - Content-MD5 you specified did not match what we received.
type BadDigest struct {
	ExpectedMD5   string

In this case, you could want to document how that external API *should* look like. What *path operation* it should have, what body it should expect, what response it should return, etc.

## An app with callbacks

Let's see all this with an example.

Imagine you develop an app that allows creating invoices.

These invoices will have an `id`, `title` (optional), `customer`, and `total`.

    }

    /**
     * The logger name for a component with a non-null hint is <code>role.hint</code>.
     * <b>Warning</b>: this does not conform to logger name as class name convention.
     * (and what about <code>null</code> and <code>default</code> hint equivalence?)
     */
    public Logger getLoggerForComponent(String role, String hint) {
        return (null == hint
                ? getLoggerForComponent(role)