Let's continue from the previous example. We wanted to **annotate the function with one type**, but we wanted to be able to return from the function something that actually includes **more data**.

We want FastAPI to keep **filtering** the data using the response model. So that even though the function returns more data, the response will only include the fields declared in the response model.

* You can then further modify the `response` before returning it.

{* ../../docs_src/middleware/tutorial001_py39.py hl[8:9,11,14] *}

/// tip

Keep in mind that custom proprietary headers can be added <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers" class="external-link" target="_blank">using the `X-` prefix</a>.

Just use any kind of model, any kind of class, any kind of database that you need for your application. **FastAPI** has you covered with the dependency injection system.

## Code size { #code-size }

This example might seem verbose. Keep in mind that we are mixing security, data models, utility functions and *path operations* in the same file.

But here's the key point.

The security and dependency injection stuff is written once.

            noMoreParts = true
            return null
          }

          2, 3 -> {
            // " " or "\t" Ignore whitespace and keep looking.
            whitespace = true
            continue@afterBoundaryLoop
          }

          -1 -> throw ProtocolException("unexpected characters after boundary")
        }
      }

In many cases it's not very obvious how environment variables would be useful and applicable right away. But they keep showing up in many different scenarios when you are developing, so it's good to know about them.

        /**
         * Logic duplicated from [org.gradle.configuration.DefaultImportsReader].
         * Please keep this code in sync.
         */
        internal
        fun generateImportPackages(): List<String> {
            /*

		}
		if len(remainCaches) > metacacheMaxEntries {
			// Sort oldest last...
			sort.Slice(remainCaches, func(i, j int) bool {
				return remainCaches[i].lastHandout.Before(remainCaches[j].lastHandout)
			})
			// Keep first metacacheMaxEntries...
			for _, cache := range remainCaches[metacacheMaxEntries:] {
				if time.Since(cache.lastHandout) > metacacheMaxClientWait {
					remove[cache.id] = struct{}{}
				}
			}
		}
	}

   * </pre>
   *
   * @throws IllegalArgumentException if there is a loop in the causal chain
   */
  public static Throwable getRootCause(Throwable throwable) {
    // Keep a second pointer that slowly walks the causal chain. If the fast pointer ever catches
    // the slower pointer, then there's a loop.
    Throwable slowPointer = throwable;
    boolean advanceSlowPointer = false;

      }
      return false;
    }

    /**
     * Returns a string in the format specified by {@link MoreObjects#toStringHelper(Object)}.
     *
     * <p>After calling this method, you can keep adding more properties to later call toString()
     * again and get a more complete representation of the same object; but properties cannot be
     * removed, so this only allows limited reuse of the helper instance. The helper allows

With fast fallback, OkHttp attempts to connect to multiple web servers concurrently. It keeps whichever route connects first and cancels all of the others. Its rules are:

 * Prefer to alternate IP addresses from different address families, (IPv6 / IPv4), starting with IPv6.
 * Don't start a new attempt until 250 ms after the most recent attempt was started.
 * Keep whichever TCP connection succeeds first and cancel all the others.