<img src="/img/tutorial/dependencies/image02.png">

## Shortcut

But you see that we are having some code repetition here, writing `CommonQueryParams` twice:

=== "Python 3.8+"

    ```Python
    commons: Annotated[CommonQueryParams, Depends(CommonQueryParams)]
    ```

=== "Python 3.8+ non-Annotated"

 *  * [onHeaders] (unless canceled)
 *  * [onData] (optional sequence of data frames)
 *
 * As a stream ID is scoped to a single HTTP/2 connection, implementations which target multiple
 * connections should expect repetition of stream IDs.
 *
 * Return true to request cancellation of a pushed stream.  Note that this does not guarantee
 * future frames won't arrive on the stream ID.
 */
interface PushObserver {
  /**

It has an integrated dependency injection system, inspired by Angular 2. It requires pre-registering the "injectables" (like all the other dependency injection systems I know), so, it adds to the verbosity and code repetition.

As the parameters are described with TypeScript types (similar to Python type hints), editor support is quite good.

* Have shared logic (the same code logic again and again).
* Share database connections.
* Enforce security, authentication, role requirements, etc.
* And many other things...

All these, while minimizing code repetition.

## First Steps

Let's see a very simple example. It will be so simple that it is not very useful, for now.

But this way we can focus on how the **Dependency Injection** system works.

Group       = "(" Expression ")" .
Option      = "[" Expression "]" .
Repetition  = "{" Expression "}" .
</pre>

<p>
Productions are expressions constructed from terms and the following
operators, in increasing precedence:
</p>
<pre class="grammar">
|   alternation
()  grouping
[]  option (0 or 1 times)
{}  repetition (0 to n times)
</pre>

<p>

Group       = "(" Expression ")" .
Option      = "[" Expression "]" .
Repetition  = "{" Expression "}" .
</pre>

<p>
Productions are expressions constructed from terms and the following
operators, in increasing precedence:
</p>
<pre class="grammar">
|   alternation
()  grouping
[]  option (0 or 1 times)
{}  repetition (0 to n times)
</pre>

<p>

 * to explicitly add another equivalence group.
 *
 * @author Gregory Kick
 * @since 10.0
 */
@GwtCompatible
@ElementTypesAreNonnullByDefault
public final class EquivalenceTester<T> {
  private static final int REPETITIONS = 3;

  private final Equivalence<? super T> equivalence;
  private final RelationshipTester<T> delegate;
  private final List<T> items = Lists.newArrayList();

 * to explicitly add another equivalence group.
 *
 * @author Gregory Kick
 * @since 10.0
 */
@GwtCompatible
@ElementTypesAreNonnullByDefault
public final class EquivalenceTester<T> {
  private static final int REPETITIONS = 3;

  private final Equivalence<? super T> equivalence;
  private final RelationshipTester<T> delegate;
  private final List<T> items = Lists.newArrayList();

 * </ul>
 *
 * @author Jim McMaster
 * @author Jige Yu
 * @since 10.0
 */
@GwtCompatible
@ElementTypesAreNonnullByDefault
public final class EqualsTester {
  private static final int REPETITIONS = 3;

  private final List<List<Object>> equalityGroups = Lists.newArrayList();
  private final RelationshipTester.ItemReporter itemReporter;

  /** Constructs an empty EqualsTester instance */
  public EqualsTester() {

 * </ul>
 *
 * @author Jim McMaster
 * @author Jige Yu
 * @since 10.0
 */
@GwtCompatible
@ElementTypesAreNonnullByDefault
public final class EqualsTester {
  private static final int REPETITIONS = 3;

  private final List<List<Object>> equalityGroups = Lists.newArrayList();
  private final RelationshipTester.ItemReporter itemReporter;

  /** Constructs an empty EqualsTester instance */
  public EqualsTester() {