[英]How to correctly handle cancelled tasks in Python's `asyncio.gather`
[英]How to build list of tasks for asyncio.gather in Python 3.8
下面我附上了一個測試程序來演示我遇到的asyncio.gather拋出TypeError的問題。
我的目標:進行多個並發異步調用,以從連接到我的計算機的 USB 攝像頭陣列中將攝像頭圖像捕獲到文件中。 當所有相機都完成異步捕獲后,我想恢復處理。
此處顯示的異步協程take_image()對“ffmpeg”應用程序進行系統調用,該應用程序將圖像從指定相機捕獲到指定文件。
import asyncio
import os
import subprocess
import time
async def take_image(camera_id, camera_name, image_file_path, image_counter):
image_capture_tic = time.perf_counter()
try:
run_cmd = subprocess.run( ["ffmpeg", '-y', '-hide_banner', '-f', 'avfoundation', '-i', camera_id,
'-frames:v', '1', '-f', 'image2', image_file_path], universal_newlines=True,
stdout=subprocess.PIPE, stderr=subprocess.PIPE) # Note, ffmpeg writes to stderr, not stdout!
except Exception as e:
print("Error: Unable to capture image for", image_file_path)
return "NO IMAGE!"
image_capture_toc = time.perf_counter()
print(f"{image_counter}: Captured {camera_name} image in: {image_capture_toc - image_capture_tic:0.0f} seconds")
return camera_name
下面顯示的main()例程獲取多個攝像頭的列表,並遍歷列表中的每個攝像頭, main() 使用asyncio.create_task()為每個攝像頭創建一個異步任務。 每個任務都添加到任務列表中。
一旦所有圖像捕獲任務都已啟動,我使用await asyncio.gather(tasks)等待它們完成。
async def main():
tic = time.perf_counter()
camera_list = [('0', 'FHD Camera #1'), ('1', 'FHD Camera #2'), ('2', 'FHD Camera #3'), ]
image_counter = 1
tasks = []
for camera_pair in camera_list:
camera_id, camera_name = camera_pair
image_file_name = 'img' + str(image_counter) + "-cam" + str(camera_id) + "-" + camera_name + '.jpg'
image_file_path = os.path.join("/tmp/test1/img", image_file_name)
# schedule all image captures calls *concurrently*:
tasks.append(asyncio.create_task(take_image(camera_id, camera_name, image_file_path, image_counter),
name=image_file_name))
image_counter = image_counter + 1
await asyncio.gather(tasks) # <-- This line throws a TypeError!
toc = time.perf_counter()
print(f"Captured list of {image_counter - 1} cameras in: {toc - tic:0.0f} seconds")
asyncio.run(main())
不幸的是,當我嘗試運行這個程序時,我收到了這個錯誤:
類型錯誤:不可散列類型:“列表”
和以下追溯:
Traceback (most recent call last):
File "scratch_10.py", line 41, in <module>
asyncio.run(main())
File "/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/asyncio/runners.py", line 43, in run
return loop.run_until_complete(main)
File "/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/asyncio/base_events.py", line 608, in run_until_complete
return future.result()
File "scratch_10.py", line 36, in main
await asyncio.gather(tasks)
File "/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/asyncio/tasks.py", line 805, in gather
if arg not in arg_to_fut:
TypeError: unhashable type: 'list'
我一直試圖通過 asyncio 的 3.8 文檔來解決問題,但我不明白出了什么問題。
如何讓每個 take_image 請求異步運行,然后在每個任務完成后在我的調用例程中恢復處理?
gather
采用位置 arguments,而不是單個可迭代的參數。 你需要解開你的清單。
await asyncio.gather(*tasks)
你應該嘗試: (Task1,Task2,Task3)這就是我為任務所做的並且它有效
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