# 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

 *
 * <ul>
 *   <li>Implementations can be generally assumed to prioritize memory efficiency, then speed of
 *       access, and lastly speed of creation.
 *   <li>The {@code copyOf} methods will sometimes recognize that the actual copy operation is
 *       unnecessary; for example, {@code copyOf(copyOf(anArrayList))} should copy the data only

 *
 * <ul>
 *   <li>Implementations can be generally assumed to prioritize memory efficiency, then speed of
 *       access, and lastly speed of creation.
 *   <li>The {@code copyOf} methods will sometimes recognize that the actual copy operation is
 *       unnecessary; for example, {@code copyOf(copyOf(anArrayList))} should copy the data only

     *
     * In theory, we can quickly determine the size of any Collection. However, thanks to
     * regrettable implementations like our own Sets.filter, Collection.size() is sometimes a
     * linear-time operation, and it can even have side effects. Thus, we limit the special case to
     * List, which is _even more likely_ to have size() implemented to be fast and side-effect-free.
     *

   * provided that the function is capable of accepting null input. The transformed multimap might
   * contain null values, if the function sometimes gives a null result.
   *
   * <p>The returned multimap is not thread-safe or serializable, even if the underlying multimap
   * is. The {@code equals} and {@code hashCode} methods of the returned multimap are meaningless,

   * provided that the function is capable of accepting null input. The transformed multimap might
   * contain null values, if the function sometimes gives a null result.
   *
   * <p>The returned multimap is not thread-safe or serializable, even if the underlying multimap
   * is. The {@code equals} and {@code hashCode} methods of the returned multimap are meaningless,

### Concurrency and Burgers { #concurrency-and-burgers }

This idea of **asynchronous** code described above is also sometimes called **"concurrency"**. It is different from **"parallelism"**.

**Concurrency** and **parallelism** both relate to "different things happening more or less at the same time".

import java.util.List;
import java.util.Set;
import org.jspecify.annotations.Nullable;

/**
 * A collection that supports order-independent equality, like {@link Set}, but may have duplicate
 * elements. A multiset is also sometimes called a <i>bag</i>.
 *
 * <p>Elements of a multiset that are equal to one another are referred to as <i>occurrences</i> of
 * the same single element. The total number of occurrences of an element in a multiset is called

    return hextetsToIPv6String(hextets) + scopeWithDelimiter((Inet6Address) ip);
  }

  private static String scopeWithDelimiter(Inet6Address ip) {
    // getHostAddress on android sometimes maps the scope ID to an invalid interface name; if the
    // mapped interface isn't present, fallback to use the scope ID (which has no validation against
    // present interfaces)

                if (LogStream.level >= 6) {
                    Hexdump.hexdump(log, BUF, 4, n);
                }
            }
            /* For some reason this can sometimes get broken up into another
             * "NBSS Continuation Message" frame according to WireShark
             */
            out.write(BUF, 0, 4 + n);
        }
    }

    /**