Python / Uvicorn Fundamentals Interview Questions
What is uvloop and how does it improve Uvicorn's performance?
uvloop is a high-performance, drop-in replacement for Python's built-in asyncio event loop. It is implemented in Cython on top of libuv - the same C library used by Node.js - making it significantly faster than the standard asyncio loop for network I/O operations.
| Aspect | asyncio (built-in) | uvloop |
|---|---|---|
| Implementation | Pure Python + C | Cython / libuv (C) |
| Performance | Baseline | ~2-4x faster on I/O-heavy workloads |
| Installation | Built into Python | pip install uvloop (included in uvicorn[standard]) |
| Drop-in replacement | N/A | Yes - same API as asyncio |
| PyPy compatibility | Yes | No - requires CPython |
# Uvicorn automatically uses uvloop when installed # Install via standard extras: pip install "uvicorn[standard]" # Or force uvloop explicitly: uvicorn main:app --loop uvloop # Force asyncio (e.g. for debugging or PyPy): uvicorn main:app --loop asyncio # Check which event loop is active (in your app): import asyncio print(type(asyncio.get_event_loop())) # will show uvloop.Loop when active # Programmatic config: uvicorn.run("main:app", loop="uvloop")
When NOT to use uvloop: uvloop only works with CPython (not PyPy or alternative runtimes). For PyPy deployments, use uvicorn.workers.UvicornH11Worker instead of the standard worker. Also, some libraries that depend on specific asyncio internals may not be compatible with uvloop.
What underlying C library does uvloop use for its event loop implementation?
In which scenario should you NOT use uvloop with Uvicorn?
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