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ntoledo319

Posted on • Originally published at eolkits.com

Python 3.13 on AWS Lambda — cgi, telnetlib, crypt, lib2to3 removed (PEP 594)

#aws #lambda #python #serverless

AWS Lambda launched the python3.13 runtime in November 2024 — Amazon Linux 2023, Python 3.13.x, LTS support through October 2029. If you're still on python3.9 or python3.10, the create-block deadline hits 2027-02-01; python3.11 create-blocks at 2027-07-31. Many teams are jumping straight to python3.13 to bank the longest runway.

What bites them isn't the runtime label change. It's PEP 594 — the "dead batteries" cleanup that deprecated 19 stdlib modules in Python 3.11 (warnings only) and deleted all of them in Python 3.13. Cold start, hard crash, ModuleNotFoundError. Here are the four that actually appear in Lambda functions and their transitive dependencies.

ModuleNotFoundError: No module named 'cgi'

The cgi and cgitb modules are gone in Python 3.13.

# Breaks on python3.13
from cgi import parse_qs, FieldStorage, escape
import cgitb; cgitb.enable()
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Fixes:

  • cgi.parse_qs / cgi.parse_qsl → urllib.parse.parse_qs / urllib.parse.parse_qsl
  • cgi.escape → html.escape
  • Multipart form parsing (FieldStorage) → multipart library, or your framework's built-in (werkzeug.formparser, starlette.requests.Request.form(), etc.)
  • Drop cgitb.enable() — use structured logging (logging.exception) instead

The most common place this surfaces in Lambda is a dependency that still imports cgi for query-string parsing — pip show -f <package> to locate it, then upgrade.

ModuleNotFoundError: No module named 'telnetlib' 

import telnetlib  # removed in Python 3.13
tn = telnetlib.Telnet("host", 23)
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Fixes:

  • Network/device automation: replace with telnetlib3 (asyncio-native, maintained) or Exscript.
  • Better: switch to SSH where the device supports it — paramiko or netmiko.
  • Pinning to python3.12 still includes telnetlib, but python3.12 itself will eventually deprecate — treat that as a stopgap, not a fix.

ModuleNotFoundError: No module named 'crypt'

The Unix password-hashing module was removed because its underlying algorithms (DES-crypt, MD5-crypt) are cryptographically broken and belong nowhere near new code.

import crypt
hashed = crypt.crypt("password", crypt.mksalt(crypt.METHOD_SHA512))
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Fixes:

  • New password hashing: passlib (bcrypt, argon2-cffi) or hashlib.scrypt / hashlib.pbkdf2_hmac.
  • Verify existing crypt(3) hashes: passlib.hash.sha512_crypt.verify(password, stored_hash) — it implements the same crypt(3) schemes so you can verify old hashes during a migration window.
  • There is no stdlib drop-in replacement. Don't reach for hashlib.md5 — that's not a password hash.

This one shows up most often in provisioning scripts and user-management Lambda functions inherited from old EC2 tooling.

ModuleNotFoundError: No module named 'lib2to3'

lib2to3 (and the 2to3 command-line tool) were removed because the internal grammar couldn't track new Python syntax — it was itself becoming obsolete.

from lib2to3 import pygram, pytree  # gone in Python 3.13
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Fixes:

  • Code parsing/transformation: LibCST or parso.
  • If this comes from a dependency (build tools, older linters, black < 24), upgrade it — modern versions have already removed the lib2to3 dependency.
  • For a one-off Python 2→3 migration: run 2to3 from a Python ≤ 3.12 environment before upgrading the runtime.

Grep your dependencies before deploying

PEP 594 removed 19 modules in total. The four above are the ones that appear in Lambda workloads, but also removed were: aifc, audioop, chunk, cgitb, imghdr, mailcap, msilib, nis, nntplib, ossaudiodev, pipes, sndhdr, spwd, sunau, uu, xdrlib.

A quick scan before you flip the runtime:

# Run from your virtualenv or package directory
python3.13 -c "
import importlib, sys
dead = ['cgi','cgitb','telnetlib','crypt','lib2to3','aifc','audioop',
        'chunk','imghdr','mailcap','nis','nntplib','ossaudiodev',
        'pipes','sndhdr','sunau','uu','xdrlib']
for m in dead:
    try:
        importlib.import_module(m)
        print(f'WARN: {m} somehow importable')
    except ModuleNotFoundError:
        pass
print('Scan complete')
"
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Better yet, test your handler locally on a Python 3.13 environment before deploying to Lambda. The errors above are cold-start killers — they won't appear until the function is invoked.

Timeline recap

Runtime Deprecated Create blocked Update blocked
python3.9 2025-12-15 2027-02-01 2027-03-03
python3.10 2026-03-31 2027-02-01 2027-03-03
python3.11 2027-06-30 2027-07-31 2027-08-31
python3.12 not yet deprecated — —
python3.13 not yet deprecated — —

Sources: AWS Lambda runtime deprecation table, Python 3.13 What's New, PEP 594.

The free EOLkits scanner detects deprecated Lambda runtimes and flags known packages that still import the removed modules — in your browser, nothing uploaded. Full per-error fixes and the migration deadline timeline: eolkits.com/migrate/lambda-python-3.11-eol.

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