This upgrade involves major version jumps for both Flask (1.1.1 → 3.1.3) and its core dependency Werkzeug (1.0.0 → 3.1.6), introducing significant breaking changes that require developer action.

Key Breaking Changes:

• Python Version Requirement: Support for Python versions 2.7, 3.5, 3.6, and 3.7 has been dropped. Your application must be running on Python 3.8 or newer. [5, 7, 10]
• Werkzeug Security Hashing: In Werkzeug 3.0, generate_password_hash now uses scrypt by default instead of pbkdf2:sha256. If your application uses Werkzeug for password management, existing password hashes will fail validation. [5, 7]
• Removal of Deprecated APIs: Functions and parameters deprecated in previous versions have been removed. This includes:
  • Flask's Config.from_json() is removed. Use Config.from_file() instead. [2, 8, 10]
  • Flask's safe_join is removed. Use werkzeug.utils.safe_join instead. [8, 10]
  • Renamed send_file parameters (e.g., attachment_filename is now download_name). [2]
• Dependency Requirements: Flask 3.0 requires Werkzeug 3.0, meaning these upgrades must be handled together. [3, 10]

Recommendation:

1. Verify Python Version: Ensure your production environment and deployment pipeline use Python 3.8 or higher.
2. Address Password Hashing: If using werkzeug.security for passwords, you must implement a strategy to handle the new hashing default. This may involve re-hashing user passwords or explicitly setting the hashing method to the legacy algorithm during a transition.
3. Update Deprecated Code: Review your codebase for use of removed functions like Config.from_json and update them to their modern equivalents.
4. Thorough Testing: Due to the extensive nature of these changes, a full testing cycle is essential to catch issues related to API removals and behavioral changes.

Source: Flask Changes, Werkzeug Changes

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