Bug Description

The Python task runner replaces sys.stderr with io.StringIO() to capture error output. However, StringIO does not implement a working fileno() method, which causes failures when executing code that uses external libraries (like Prisma, psycopg2, or other database drivers) that attempt to call sys.stderr.fileno().

Environment

• n8n Version: 1.117.3
• Deployment: Self-hosted (Docker with Native Python Runners)
• Python Runner: External task runner mode (n8nio/runners:1.117.3)

To Reproduce

1. Enable Native Python Runners with external task runner
2. Install Prisma Python client in the runner container:

   pip install prisma

3. Run this code in an n8n Python Code node:

   from prisma import Prisma
   
   db = Prisma()
   db.connect()
   settings = db.setting.find_many()
   db.disconnect()
   
   return {'count': len(settings)}

Expected behavior

The code should execute successfully and return results from the database query.

Debug Info

Debug info

core

• n8nVersion: 1.117.3
• platform: docker (self-hosted)
• nodeJsVersion: 22.18.0
• nodeEnv: production
• database: postgres
• executionMode: regular
• concurrency: -1
• license: enterprise (production)
• consumerId: 7527b28f-d26b-4db7-8d5b-2dc34312a99a

storage

• success: all
• error: all
• progress: false
• manual: true
• binaryMode: memory

pruning

• enabled: true
• maxAge: 336 hours
• maxCount: 10000 executions

client

• userAgent: mozilla/5.0 (windows nt 10.0; win64; x64) applewebkit/537.36 (khtml, like gecko) chrome/141.0.0.0 safari/537.36
• isTouchDevice: false

Generated at: 2025-10-29T06:02:07.808Z

Operating System

Docker

n8n Version

1.117.3

Node.js Version

22.18.0

Database

PostgreSQL

Execution mode

main (default)

Hosting

self hosted