What is asyncio.run_in_executor?

Question

Accepted Answer

**run_in_executor** runs **blocking code** in a thread/process pool.

```python
import asyncio
from concurrent.futures import ThreadPoolExecutor, ProcessPoolExecutor

# Blocking function
def blocking_read(file_path):
    with open(file_path) as f:
        return f.read()

def cpu_heavy(n):
    return sum(i * i for i in range(n))

async def main():
    loop = asyncio.get_event_loop()

# Thread pool (default)
    content = await loop.run_in_executor(
        None,  # Default thread pool
        blocking_read,
        "file.txt"
    )

# Custom thread pool
    with ThreadPoolExecutor(max_workers=4) as executor:
        content = await loop.run_in_executor(
            executor,
            blocking_read,
            "file.txt"
        )

# Process pool for CPU-bound
    with ProcessPoolExecutor(max_workers=4) as executor:
        result = await loop.run_in_executor(
            executor,
            cpu_heavy,
            10**7
        )

asyncio.run(main())
```

**vs to_thread:**
- `run_in_executor`: explicit executor control.
- `to_thread`: simpler, uses default executor.

**Use cases:**
- File I/O.
- CPU-bound tasks.
- Blocking library calls.

Python: What is asyncio.run_in_executor?