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How a Werkzeug version mismatch can silently break Flask on Ubuntu 24.04

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A developer running Flask 3.0.3 on an Ubuntu 24.04 staging server traced a severe memory-swap incident to a version conflict between Werkzeug 2.x and Flask 3.x, which require Werkzeug 3.0 for API compatibility. Ubuntu 24.04's apt package manager pins an older Werkzeug build, meaning installing Flask via apt can create an environment where imports succeed but routing fails under production load. The recommended fix is to use a Python 3.12 virtual environment and pin dependencies explicitly with a constraints file, bypassing the system-managed package directory entirely. A verification script is also advised to confirm correct versions and flag environments consuming over 150MB of RSS memory at import time. The article emphasises that Flask itself is not a production WSGI server and must be paired with a properly configured server such as uWSGI or Gunicorn for deployment.

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