如何在多回调触发后使用twisted.internet.defer.DeferredList？

Let's break down what this tool does and how to use it effectively in Twisted applications:

Core Functionality #

• Combine multiple Deferreds into a single callback chain: Instead of handling each Deferred's success or failure individually, wrap a group of them in a DeferredList. This combined Deferred will only trigger its callbacks after all wrapped Deferreds have completed (whether successfully or with an error).
• Structured result output: When the DeferredList finishes, it returns a list of tuples. Each tuple follows the format (success, result):
  • success: A boolean (True if the corresponding Deferred succeeded, False if it failed)
  • result: The value passed to callback() if successful, or the Failure object from errback() if failed

Key Note: Reusing Deferreds After Adding to DeferredList #

A crucial point to remember—you can still interact with the original Deferred objects after adding them to a DeferredList. This is particularly useful for suppressing those pesky "Unhandled error in Deferred" warnings. For example:

If a Deferred might fail, even though the DeferredList will catch its error, Twisted may still throw an unhandled error warning unless you add an errback to the original Deferred later. Here's a quick example of how to handle this:

from twisted.internet.defer import Deferred, DeferredList
from twisted.internet import reactor

# Create a Deferred and add it to a DeferredList
d = Deferred()
dl = DeferredList([d])

# Later, add an errback to the original Deferred to suppress warnings
d.addErrback(lambda failure: print(f"Quietly handled error: {failure.getErrorMessage()}"))

# Trigger a failure for the Deferred
d.errback(Exception("Something went wrong with this task"))

# Define a callback for the DeferredList to process all results
def handle_all_results(results):
    for success, res in results:
        if not success:
            print(f"DeferredList caught failure: {res.getErrorMessage()}")

dl.addCallback(handle_all_results)
reactor.run()

In this case, the DeferredList still tracks the Deferred's failure state, but we've added an errback to the original d to prevent the unhandled error warning from appearing.

内容的提问来源于stack exchange，提问作者Hatshepsut