In diesem Fall ist es der `str`:

```Python
"{$callback_url}/invoices/{$request.body.id}"
```

Wenn Ihr API-Benutzer (der externe Entwickler) also einen Request an *Ihre API* sendet, via:

```
https://yourapi.com/invoices/?callback_url=https://www.external.org/events
```

mit einem JSON-Körper:

```JSON
{
    "id": "2expen51ve",

import java.lang.reflect.Method;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests creation (typically through a constructor or static factory
 * method) of a collection. Can't be invoked directly; please see {@link
 * com.google.common.collect.testing.CollectionTestSuiteBuilder}.
 *
 * @author Chris Povirk
 */
@GwtCompatible

import com.google.common.collect.testing.features.CollectionSize;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests {@code retainAll} operations on a list. Can't be invoked
 * directly; please see {@link com.google.common.collect.testing.ListTestSuiteBuilder}.
 *
 * @author Chris Povirk
 */
@GwtCompatible

import com.google.common.collect.testing.features.MapFeature;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests {@code get} operations on a map. Can't be invoked directly;
 * please see {@link com.google.common.collect.testing.MapTestSuiteBuilder}.
 *
 * @author Kevin Bourrillion
 * @author Chris Povirk
 */
@GwtCompatible

import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.Map.Entry;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests {@code clear()} operations on a map. Can't be invoked directly;
 * please see {@link com.google.common.collect.testing.MapTestSuiteBuilder}.
 *
 * @author George van den Driessche
 * @author Chris Povirk
 */
@GwtCompatible

import java.lang.reflect.Method;
import java.util.List;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests {@code add(Object)} operations on a list. Can't be invoked
 * directly; please see {@link com.google.common.collect.testing.ListTestSuiteBuilder}.
 *
 * @author Chris Povirk
 */
@GwtCompatible

import com.google.common.collect.testing.features.CollectionSize;
import java.util.NoSuchElementException;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests {@code element()} operations on a queue. Can't be invoked
 * directly; please see {@link com.google.common.collect.testing.CollectionTestSuiteBuilder}.
 *
 * @author Jared Levy
 */
@GwtCompatible

import com.google.common.collect.testing.features.CollectionSize;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests {@code peek()} operations on a queue. Can't be invoked directly;
 * please see {@link com.google.common.collect.testing.CollectionTestSuiteBuilder}.
 *
 * @author Jared Levy
 */
@GwtCompatible

import com.google.common.collect.testing.features.CollectionSize;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests addAll operations on a set. Can't be invoked directly; please
 * see {@link com.google.common.collect.testing.SetTestSuiteBuilder}.
 *
 * @author Kevin Bourrillion
 */
@GwtCompatible

import com.google.common.collect.testing.features.CollectionSize;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests {@code addAll(Collection)} operations on a list. Can't be
 * invoked directly; please see {@link com.google.common.collect.testing.ListTestSuiteBuilder}.
 *
 * @author Chris Povirk
 */
@GwtCompatible
@Ignore("test runners must not instantiate and run this directly, only via suites we build")