使用 contextvar 跟蹤 Python 中的異步循環

Question

我正在嘗試在 Python 中使用一個簡單的異步示例，主要遵循這里的出色答案。

我的目標是設置一個上下文變量，並通過不斷附加來跟蹤一系列調用。 我知道可以使用.get()方法訪問上下文變量，並使用.set()方法更改它們的值。 然而，在以下情況下，盡管從控制台可以明顯看出一系列對 function sum()的調用，但變量並未被修改。

編輯：根據下面 Michael Butscher 的評論，我將原始上下文變量（它是一個字符串）替換為一個列表： output_list並使用.append()迭代地修改了該列表。 現在，這確實使我能夠查看最終的 output，但不能查看各個sum()方法中的中間值。

完整代碼：

import asyncio
import contextvars
import time

output_list = contextvars.ContextVar('output_list', default=list())

async def sleep():
    print(f'Time: {time.time() - start:.2f}')
    await asyncio.sleep(1)

async def sum(name, numbers):
    total = 0
    for number in numbers:
        print(f'Task {name}: Computing {total}+{number}')
        await sleep()
        total += number
    output_list.set(output_list.get().append(f"{name}"))
    print(f'Task {name}: Sum = {total}\n')
    print(f'Partial output from task {name}:', output_list.get())

start = time.time()

loop = asyncio.get_event_loop()
tasks = [
    loop.create_task(sum("A", [1, 2])),
    loop.create_task(sum("B", [1, 2, 3])),
]
loop.run_until_complete(asyncio.wait(tasks))
loop.close()

end = time.time()
print(f'Time: {end-start:.2f} sec')
print("Final output_str =", output_list.get())

如何迭代地跟隨上下文變量列表output_list的擴展？

我想要的控制台 output 是：

Task A: Computing 0+1
Time: 0.00
Task B: Computing 0+1
Time: 0.00
Task A: Computing 1+2
Time: 1.02
Task B: Computing 1+2
Time: 1.02
Task A: Sum = 3

Partial output from task A: ['A']
Task B: Computing 3+3
Time: 2.02
Task B: Sum = 6

Partial output from task B: ['A', 'B']
Time: 3.03 sec
Final output_str = ['A', 'B']

相反，我得到：

Task A: Computing 0+1
Time: 0.00
Task B: Computing 0+1
Time: 0.00
Task A: Computing 1+2
Time: 1.02
Task B: Computing 1+2
Time: 1.02
Task A: Sum = 3

Partial output from task A: None
Task B: Computing 3+3
Time: 2.02
Task B: Sum = 6

Partial output from task B: None
Time: 3.03 sec
Final output_str = ['A', 'B']

Answer 1

根據asyncio文檔：

任務支持 contextvars 模塊。 創建任務時，它會復制當前上下文，然后在復制的上下文中運行其協程。

因此，如果您在程序頂部聲明cvar = contextvars.ContextVar('cvar', default='x') ，當您創建任務時，這將復制當前上下文，如果您修改cvar ，它只會影響副本但不影響原始上下文。 這就是您在最終 output 中獲得'' （空字符串）的主要原因。

要實現您想要的“跟蹤”，您必須使用全局變量才能在任何地方修改它。 但是，如果您想使用asyncio和contextvars來了解它是如何工作的，請參見下面的示例：

import asyncio
import contextvars
import time

output = contextvars.ContextVar('output', default='No changes at all') 

async def sleep():
    print(f'Time: {time.time() - start:.2f}')
    await asyncio.sleep(1)

async def sum(name, numbers):
    total = 0
    for number in numbers:
        print(f'Task {name}: Computing {total}+{number}')
        await sleep()
        total += number
        output.set(output.get()+name) #Here we modify the respective context
    print(f'Task {name}: Sum = {total}\n')
    print(f'Partial output from task {name}:', output.get())
    return output.get() #Here we return the variable modified
start = time.time()

# main() will have its own copy of the context
async def main():
    output.set('Changed - ') # Change output var in this function context
    # task1 and task2 will copy this context (In this contect output=='Changed - ')
    task1 = asyncio.create_task(sum("A", [1, 2])) #This task has its own copy of the context of main()
    task2 = asyncio.create_task(sum("B", [1, 2, 3])) #This task also has its own copy of the context of main()
    done, pending = await asyncio.wait({task1,task2})
    resultTask1 = task1.result() # get the value of return of task1
    resultTask2 = task2.result() # get the value of return of task1
    print('Result1: ', resultTask1)
    print('Result2: ', resultTask2)
    print('Variable output in main(): ',output.get()) # However, output in main() is sitill 'Changed - '
    output.set(output.get()+'/'+resultTask1+'/'+resultTask2) #Modify the var in this context
    print('Variable modified in main(): ', output.get())
    return output.get() #Return modified value

x = asyncio.run(main()) # Assign the return value to x

end = time.time()
print(f'Time: {end-start:.2f} sec')
print("Final output (without changes) =", output.get())
output.set(x)
print("Final output (changed) =", output.get())

##### OUTPUT #####
# Time: 0.00
# Task B: Computing 0+1
# Time: 0.00
# Task A: Computing 1+2
# Time: 1.01
# Task B: Computing 1+2
# Time: 1.01
# Task A: Sum = 3

# Partial output from task A: Changed - AA
# Task B: Computing 3+3
# Time: 2.02
# Task B: Sum = 6

# Partial output from task B: Changed - BBB
# Result1:  Changed - AA
# Result2:  Changed - BBB
# Variable output in main():  Changed -
# Variable modified in main():  Changed - /Changed - AA/Changed - BBB
# Time: 3.03 sec
# Final output (without changes) = No changes at all
# Final output (changed) = Changed - /Changed - AA/Changed - BBB

如您所見，不可能同時修改同一個變量。 當task1正在修改它的副本時， task2也在修改它的副本。