* 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.**
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**Describe the solution you'd like**
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A clear and concise description of any alternative solutions or features you've considered.

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#### What type of PR is this?

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

**Describe the bug**
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        a full-time Guava team member. [Feedback](https://stackoverflow.com/a/4543114) from our
        users indicates that they really appreciate Guava's high power-to-weight ratio. It's
        important to us to keep Guava as easy to use and understand as we can. That means boiling
        features down to compact but powerful abstractions, and controlling feature bloat carefully.

  - 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

You could create an API with a *path operation* that could trigger a request to an *external API* created by someone else (probably the same developer that would be *using* your API).

The process that happens when your API app calls the *external API* is named a "callback". Because the software that the external developer wrote sends a request to your API and then your API *calls back*, sending a request to an *external API* (that was probably created by the same developer).

import org.slf4j.LoggerFactory;

/**
 * Use an SLF4J {@link org.slf4j.ILoggerFactory} as a backing for a Plexus
 * {@link org.codehaus.plexus.logging.LoggerManager},
 * ignoring Plexus logger API parts that are not classical and probably not really used.
 *
 * @since 3.1
 */
public class Slf4jLoggerManager implements LoggerManager {

    private ILoggerFactory loggerFactory;

    public Slf4jLoggerManager() {