## 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

Follow the instructions to create and activate a [virtual environment](virtual-environments.md){.internal-link target=_blank} for the internal code of `fastapi`.

import static org.junit.jupiter.api.Assertions.assertThrows;
import static org.junit.jupiter.api.Assertions.assertTrue;

// It would be cool if there was a hook that I could use to set up a test environment.
// I want to set up a local/remote repositories for testing but I don't want to have
// to change them when I change the layout of the repositories. So I want to generate

You can limit the lines used from the docstring of a *path operation function* for OpenAPI.

Adding an `\f` (an escaped "form feed" character) causes **FastAPI** to truncate the output used for OpenAPI at this point.

It won't show up in the documentation, but other tools (such as Sphinx) will be able to use the rest.

{* ../../docs_src/path_operation_advanced_configuration/tutorial004_py310.py hl[17:27] *}

## Additional Responses { #additional-responses }

import static java.util.Objects.requireNonNull;

/**
 * System {@link Logger}. Uses provided {@link PrintStream}s or {@link System#err} ones as fallback.
 * This logger is used in case of "early failures" (when no logging may be set up yet).
 */
public class SystemLogger implements Logger {
    private final PrintWriter out;
    private final Level threshold;

    public SystemLogger(@Nullable OutputStream out) {
        this(out, null);
    }

  private final TearDownStack stack = new TearDownStack();

  @Override
  protected void setUp() throws Exception {
    super.setUp();

    handler = new TestLogHandler();

    // You could also apply it higher up the Logger hierarchy than this
    ExampleClassUnderTest.logger.addHandler(handler);

    ExampleClassUnderTest.logger.setUseParentHandlers(false); // optional

    stack.addTearDown(
        new TearDown() {

    runs-on: ubuntu-latest
    steps:
      - name: Dump GitHub context
        env:
          GITHUB_CONTEXT: ${{ toJson(github) }}
        run: echo "$GITHUB_CONTEXT"
      - uses: actions/checkout@v6
      - name: Set up Python
        uses: actions/setup-python@v6
        with:
          python-version: "3.11"
      - name: Setup uv
        uses: astral-sh/setup-uv@v7
        with:
          version: "0.4.15"

import org.apache.maven.api.annotations.Nonnull;
import org.apache.maven.api.annotations.Nullable;

/**
 * Interface used to locate the root directory for a given project.
 *
 * The root locator is usually looked up from the plexus container.
 * One notable exception is the computation of the early {@code session.rootDirectory}
 * property which happens very early.  The implementation used in this case

@GwtIncompatible
public final class UncaughtExceptionHandlers {
  private UncaughtExceptionHandlers() {}

  /**
   * Returns an exception handler that exits the system. This is particularly useful for the main
   * thread, which may start up other, non-daemon threads, but fail to fully initialize the
   * application successfully.
   *
   * <p>Example usage:
   *
   * <pre>
   * public static void main(String[] args) {

   * - method reference: Inside Google, CacheBuilder is used from the implementation of a custom
   *   ClassLoader that is sometimes used as a system classloader. That's a problem because
   *   method-reference linking tries to look up the system classloader, and it fails because there
   *   isn't one yet.
   *
   * - lambda: Outside Google, we got a report of a similar problem in
   *   https://github.com/google/guava/issues/6565
   */

	evict := c.size > 0 && c.evictList.Length() > c.size
	// Verify size not exceeded
	if evict {
		c.removeOldest()
	}
	return evict
}

// Get looks up a key's value from the cache.
func (c *LRU[K, V]) Get(key K) (value V, ok bool) {
	c.mu.Lock()
	defer c.mu.Unlock()
	var ent *Entry[K, V]
	if ent, ok = c.items[key]; ok {
		// Expired item check