# Spot the bug: Data Engineering

> One snippet. Find the bug, or prove there isn't one.

Daily challenge: 2026-08-19 (UTC) · Senior

```python
from typing import Callable

class PipelineEventBus:
    def __init__(self):
        self._subscribers: list[Callable] = []

    def subscribe(self, callback: Callable):
        self._subscribers.append(callback)

    def publish(self, event: dict):
        for sub in self._subscribers:
            sub(event)

bus = PipelineEventBus()
bus.subscribe(lambda e: metrics.record(e))
bus.subscribe(lambda e: alerting.notify(e))
bus.subscribe(lambda e: audit_log.write(e))

def on_batch_complete(batch_id: str):
    bus.publish({"type": "batch_complete", "batch_id": batch_id})
```

## Your verdict

- Security flaw
- Logic bug
- Code style
- No issue

One attempt per day. The answer shows up right after.

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