# Dependencies with yield { #dependencies-with-yield }

FastAPI supports dependencies that do some <abbr title='sometimes also called "exit code", "cleanup code", "teardown code", "closing code", "context manager exit code", etc.'>extra steps after finishing</abbr>.

To do this, use `yield` instead of `return`, and write the extra steps (code) after.

/// tip

Make sure to use `yield` one single time per dependency.

///

/// note | Technical Details

                  exit 1
              fi

     */
    public static void main(final String[] args) throws Exception {
        if (args.length < 1) {
            logger.error("usage: TestLocking [-t <numThreads>] [-i <numIter>] [-d <delay>] url");
            System.exit(1);
        }

        final TestLocking t = new TestLocking();
        t.ltime = System.currentTimeMillis();

        for (int ai = 0; ai < args.length; ai++) {
            if (args[ai].equals("-t")) {

          coordinatorWakeUpAt = now + minDelayNanos
          try {
            backend.coordinatorWait(this@TaskRunner, minDelayNanos)
          } catch (_: InterruptedException) {
            // Will cause all tasks to exit unless more are scheduled!
            cancelAll()
          } finally {
            coordinatorWaiting = false
          }
        }
      }
    }
  }

* It can do something to that **response** or run any needed code.
* Then it returns the **response**.

/// note | Technical Details

If you have dependencies with `yield`, the exit code will run *after* the middleware.

If there were any background tasks (covered in the [Background Tasks](background-tasks.md){.internal-link target=_blank} section, you will see it later), they will run *after* all the middleware.

        return connection.socket()
      }
    }

    return null
  }

  private fun timeoutExit(cause: IOException?): IOException? {
    if (timeoutEarlyExit) return cause
    if (!timeout.exit()) return cause

    val e = InterruptedIOException("timeout")
    if (cause != null) e.initCause(cause)
    return e
  }

  /**

                boolean hasShared = shareList.stream().anyMatch(s -> s.toLowerCase().contains("shared"));

                assertTrue(hasPublic || hasShared, "Should contain expected shares");
                return; // Success, exit retry loop

            } catch (SmbAuthException e) {
                // Some configurations may not allow share listing without specific permissions

	pr, pw := xioutil.WaitPipe()
	go func() {
		pw.CloseWithError(er.getObjectWithFileInfo(ctx, bucket, object, off, length, pw, fi, metaArr, onlineDisks))
	}()

	// Cleanup function to cause the go routine above to exit, in
	// case of incomplete read.
	pipeCloser := func() {
		pr.CloseWithError(nil)
	}

	if !unlockOnDefer {
		return fn(pr, h, pipeCloser, nsUnlocker)
	}

	return fn(pr, h, pipeCloser)
}

FastAPI 0.106.0, raising exceptions after `yield` was not possible, the exit code in dependencies with `yield` was executed *after* the response was sent, so [Exception Handlers](https://fastapi.tiangolo.com/tutorial/handling-errors/#install-custom-exception-handlers) would have already run.

This was designed this way mainly to allow using the same objects "yielded" by dependencies inside of background tasks, because the exit code would be executed after the background tasks were finished....