In Python, why does ws.send_text("1") have to wait for load_dataset("beans") to be executed before it is completed?

Analysis of the execution order of await keyword in Python asynchronous programming

This article discusses the execution order of await keyword in Python asynchronous programming, especially the scenario of combining FastAPI and WebSocket. A common misconception is that the code after await is executed immediately, and this is not always the case.

The following code example demonstrates this problem:

 from fastapi import FastAPI, WebSocket
from datasets import load_dataset

app = FastAPI()

@app.websocket("/")
async def h(ws: WebSocket):
    await ws.accept()
    await ws.send_text("1")
    dataset = load_dataset("beans") # blocking operation await ws.send_text("2")

Intuitively, we expect "1" to be sent first, then load the dataset, and finally "2". But in reality, the sending of "1" will block until load_dataset("beans") completes. This is because load_dataset("beans") is a synchronous (blocking) function that prevents subsequent execution of the asynchronous function.

In order to observe the execution order more clearly, we modify the code and add the timestamp:

 from datetime import datetime
from datasets import load_dataset
from fastapi import FastAPI, WebSocket
from fastapi.responses import HTMLResponse

app = FastAPI()

# ... (HTML code remains the same) ...

@app.websocket("/ws")
async def h(ws: WebSocket):
    await ws.accept()
    await ws.send_text(f"1: {datetime.now()}")
    dataset = load_dataset("beans")
    print(f"time: {datetime.now()} => dataset: {dataset}")
    await ws.send_text(f"2: {datetime.now()}")
    While True:
        data = await ws.receive_text()
        await ws.send_text(f"Message text was: {data}, datetime: {datetime.now()}")

Run the modified code and you will find that "1" is indeed sent to the client first, and the browser receives the websocket information. This proves that ws.send_text("1") is executed first. However, the blocking feature of load_dataset("beans") results in a sending delay of "2".

in conclusion

await keyword only waits for the asynchronous operation to complete. If await is followed by a synchronous function, the execution of this synchronous function will block the entire asynchronous function until the synchronous function is executed. Therefore, in asynchronous functions, blocking operations should be avoided as much as possible, or asynchronous alternatives should be used to improve efficiency and response speed. In the above example, if load_dataset has an asynchronous version, it should be used first.

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