## Decision

The *Build Tool Team* has decided to use Architectural Decision Records (aka ADR) to track decisions we want to follow.

The main logic with ADRs is to describe (architectural) decisions made:

* To provide best practices and solutions we (as the *build tool* team) want to promote.
* To avoid asking the same thing multiple times during code review.

# Development - Contributing

First, you might want to see the basic ways to [help FastAPI and get help](help-fastapi.md){.internal-link target=_blank}.

## Developing

If you already cloned the <a href="https://github.com/fastapi/fastapi" class="external-link" target="_blank">fastapi repository</a> and you want to deep dive in the code, here are some guidelines to set up your environment.

### Virtual environment

			t.Errorf("Test %d: filtered headers do not match expected headers - got: %v , want: %v", i, test.Header, test.ExpectedHeader)
		}
		RemoveSensitiveEntries(metadata)
		if !areKeysEqual(test.ExpectedHeader, metadata) {
			t.Errorf("Test %d: filtered headers do not match expected headers - got: %v , want: %v", i, test.Header, test.ExpectedHeader)
		}
	}

                    type.defaultBaselines,
                    display = ParameterDisplay.PROMPT,
                    allowEmpty = true,
                    description = "The baselines you want to run performance tests against. Empty means default baseline.",
                )
                param("env.PERFORMANCE_STAGE", stage.stageName.toString())

You can import it directly from `fastapi`:

```python
from fastapi import Request
```

/// tip

When you want to define dependencies that should be compatible with both HTTP and WebSockets, you can define a parameter that takes an `HTTPConnection` instead of a `Request` or a `WebSocket`.

///

And you could have a single dependency that requires several other dependencies with `yield`, etc.

You can have any combinations of dependencies that you want.

**FastAPI** will make sure everything is run in the correct order.

/// note | Technical Details

func bitrotVerify(r io.Reader, wantSize, partSize int64, algo BitrotAlgorithm, want []byte, shardSize int64) error {
	if algo != HighwayHash256S {
		h := algo.New()
		if n, err := io.Copy(h, r); err != nil || n != wantSize {
			// Premature failure in reading the object, file is corrupt.
			return errFileCorrupt
		}
		if !bytes.Equal(h.Sum(nil), want) {
			return errFileCorrupt
		}
		return nil
	}

* <a href="https://jinja.palletsprojects.com" target="_blank"><code>jinja2</code></a> - Required if you want to use the default template configuration.

Let's say you have a custom exception `UnicornException` that you (or a library you use) might `raise`.

And you want to handle this exception globally with FastAPI.

You could add a custom exception handler with `@app.exception_handler()`:

{* ../../docs_src/handling_errors/tutorial003_py39.py hl[5:7,13:18,24] *}

 * This is only necessary for default components using @Typed that want to support overriding.
 *
 * As a non-default component this now gets a negative priority relative to other implementations
 * of the same interface. Since we want to allow overriding this doesn't matter in this case.
 * (if it did we could add @Priority of 0 to match the priority given to default components.)
 *