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ntoledo319
ntoledo319

Posted on • Originally published at eolkits.com

"ModuleNotFoundError: No module named 'smtpd'" / "'asyncore'" — Python 3.12's PEP 594 stdlib cleanup

#python #aws #lambda #migration

You moved a Lambda function (or any service) to the python3.12 runtime and something that worked fine on 3.11 now dies at import time with one of:

ModuleNotFoundError: No module named 'smtpd'
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ModuleNotFoundError: No module named 'asyncore'
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Nothing in your code changed. These modules are just gone.

Why this happens

smtpd, asyncore, and asynchat were all deprecated in Python 3.6 with explicit DeprecationWarnings, and all three were removed outright in Python 3.12 as part of PEP 594 — the standard library's long-running "dead batteries" cleanup (the same PEP that also dropped distutils, imp, and several others).

They tend to surface in a few specific places:

  • smtpd — a local DebuggingServer or test SMTP server used in integration tests, or a script that fakes outbound mail for local development.
  • asyncore / asynchat — older network code written before asyncio existed, or a dependency (often something mail- or FTP-related) that never migrated off the callback-based event loop these modules implemented.

Either way, the pattern is the same one behind most Python-3.12 Lambda breakage: code that was already deprecated-but-working for years suddenly hard-fails the moment the runtime bump turns the warning into a missing import.

Fix — smtpd

Replace it with aiosmtpd, the maintained asyncio-based successor:

pip install aiosmtpd
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# Before (removed in 3.12)
from smtpd import DebuggingServer
import asyncore
server = DebuggingServer(('localhost', 1025), None)
asyncore.loop()

# After
import asyncio
from aiosmtpd.controller import Controller

class PrintHandler:
    async def handle_DATA(self, server, session, envelope):
        print(envelope.content.decode('utf8', errors='replace'))
        return '250 Message accepted for delivery'

controller = Controller(PrintHandler(), hostname='localhost', port=1025)
controller.start()
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If you only need the removed module back verbatim rather than rewriting call sites, the standard-smtpd package on PyPI restores it as a drop-in shim — a faster stopgap, not the long-term fix.

Fix — asyncore / asynchat

Port the code to asyncio, which is the supported replacement for the transport/protocol model asyncore implemented:

# Before (removed in 3.12): asyncore.dispatcher subclass with handle_read/handle_write
# After: an asyncio.Protocol
import asyncio

class MyProtocol(asyncio.Protocol):
    def data_received(self, data):
        # equivalent to handle_read
        ...

    def connection_lost(self, exc):
        # equivalent to handle_close
        ...
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If the import comes from a third-party dependency rather than your own code, upgrade it first — most actively maintained libraries that used asyncore shipped an asyncio-based release well before 3.12 removed it. For a genuinely unmaintained dependency, vendoring the pure-Python asyncore.py/asynchat.py from a Python 3.11 source tree is a working (if inelegant) stopgap while you plan the real port.

Catch every PEP 594 removal before the deploy, not after

Both of these tend to arrive together, and rarely alone — a codebase old enough to import smtpd or asyncore is usually old enough to still import imp or rely on distutils too. Smoke-test the whole surface on 3.12 before flipping the runtime label:

python3.12 -c "import your_module"
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Running that against every entry point (handler, layer, and any script the deploy pipeline invokes) surfaces the full removal list at once, instead of one ModuleNotFoundError per cold start.

Free EOLkits scanner flags every deprecated Python/Node Lambda runtime in an account in about 30 seconds, nothing uploaded — I maintain it. Full references: eolkits.com/fix/python-no-module-named-smtpd and eolkits.com/fix/python-no-module-named-asyncore.

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