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# Nested callbacks example.
This example shows a `Span` for a top-level operation, and how it can be passed down on a list of nested callbacks (always one at a time), have it as the active one for each of them, and finished **only** when the last one executes. For Python, we have decided to do it in a **fire-and-forget** fashion.
Implementation details:
- For `threading`, `gevent` and `asyncio` the `Span` is manually passed down the call chain, activating it in each corotuine/task.
- For `tornado` and `contextvars`, the active `Span` is not passed down nor activated because the context is implicitly propagated.
`threading` implementation:
```python
def submit(self):
span = self.tracer.scope_manager.active.span
def task1():
with self.tracer.scope_manager.activate(span, False):
span.set_tag('key1', '1')
def task2():
with self.tracer.scope_manager.activate(span, False):
span.set_tag('key2', '2')
...
```
`tornado` implementation:
```python
@gen.coroutine
def submit(self):
span = self.tracer.scope_manager.active.span
@gen.coroutine
def task1():
self.assertEqual(span, self.tracer.scope_manager.active.span)
span.set_tag('key1', '1')
@gen.coroutine
def task2():
self.assertEqual(span,
self.tracer.scope_manager.active.span)
span.set_tag('key2', '2')
```
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