Iterator<String> iterator = asList("one", "two", "three", "four", "five").iterator();
    IllegalArgumentException expected =
        assertThrows(IllegalArgumentException.class, () -> getOnlyElement(iterator));
    assertThat(expected)
        .hasMessageThat()
        .isEqualTo("expected one element but was: <one, two, three, four, five>");
  }

  public void testGetOnlyElement_noDefault_moreThanFiveElements() {

# Background Tasks - `BackgroundTasks`

You can declare a parameter in a *path operation function* or dependency function with the type `BackgroundTasks`, and then you can use it to schedule the execution of background tasks after the response is sent.

You can import it directly from `fastapi`:

```python
from fastapi import BackgroundTasks
```

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