What changed in Flask from 2 to 3

This is the Flask 3.1.3 security fix release, which fixes a security issue but does not otherwise change behavior and should not result in breaking changes compared to the latest feature release.

PyPI: https://pypi.org/project/Flask/3.1.3/ Changes: https://flask.palletsprojects.com/page/changes/#version-3-1-3

• The session is marked as accessed for operations that only access the keys but not the values, such as in and len. GHSA-68rp-wp8r-4726

This is the Flask 3.1.2 fix release, which fixes bugs but does not otherwise change behavior and should not result in breaking changes compared to the latest feature release.

PyPI: https://pypi.org/project/Flask/3.1.2/ Changes: https://flask.palletsprojects.com/page/changes/#version-3-1-2 Milestone: https://github.com/pallets/flask/milestone/38?closed=1

• stream_with_context does not fail inside async views. #5774
• When using follow_redirects in the test client, the final state of session is correct. #5786
• Relax type hint for passing bytes IO to send_file. #5776

This is the Flask 3.1.1 fix release, which fixes bugs but does not otherwise change behavior and should not result in breaking changes compared to the latest feature release.

PyPI: https://pypi.org/project/Flask/3.1.1/ Changes: https://flask.palletsprojects.com/en/stable/changes/#version-3-1-1 Milestone https://github.com/pallets/flask/milestone/36?closed=1

• Fix signing key selection order when key rotation is enabled via SECRET_KEY_FALLBACKS. GHSA-4grg-w6v8-c28g
• Fix type hint for cli_runner.invoke. #5645
• flask --help loads the app and plugins first to make sure all commands are shown. #5673
• Mark sans-io base class as being able to handle views that return AsyncIterable. This is not accurate for Flask, but makes typing easier for Quart. #5659

This is the Flask 3.1.0 feature release. A feature release may include new features, remove previously deprecated code, add new deprecations, or introduce potentially breaking changes. We encourage everyone to upgrade, and to use a tool such as pip-tools to pin all dependencies and control upgrades. Test with warnings treated as errors to be able to adapt to deprecation warnings early.

PyPI: https://pypi.org/project/Flask/3.1.0/ Changes: https://flask.palletsprojects.com/en/stable/changes/#version-3-1-0 Milestone: https://github.com/pallets/flask/milestone/33?closed=1

• Drop support for Python 3.8. #5623
• Update minimum dependency versions to latest feature releases. Werkzeug >= 3.1, ItsDangerous >= 2.2, Blinker >= 1.9. #5624, #5633
• Provide a configuration option to control automatic option responses. #5496
• Flask.open_resource/open_instance_resource and Blueprint.open_resource take an encoding parameter to use when opening in text mode. It defaults to utf-8. #5504
• Request.max_content_length can be customized per-request instead of only through the MAX_CONTENT_LENGTH config. Added MAX_FORM_MEMORY_SIZE and MAX_FORM_PARTS config. Added documentation about resource limits to the security page. #5625
• Add support for the Partitioned cookie attribute (CHIPS), with the SESSION_COOKIE_PARTITIONED config. #5472
• -e path takes precedence over default .env and .flaskenv files. load_dotenv loads default files in addition to a path unless load_defaults=False is passed. #5628
• Support key rotation with the SECRET_KEY_FALLBACKS config, a list of old secret keys that can still be used for unsigning. Extensions will need to add support. #5621
• Fix how setting host_matching=True or subdomain_matching=False interacts with SERVER_NAME. Setting SERVER_NAME no longer restricts requests to only that domain. #5553
• Request.trusted_hosts is checked during routing, and can be set through the TRUSTED_HOSTS config. #5636

This is a fix release for the 3.0.x feature branch.

PyPI: https://pypi.org/project/Flask/3.0.3/ Changes: https://flask.palletsprojects.com/en/3.0.x/changes/#version-3-0-3 Milestone: https://github.com/pallets/flask/milestone/35?closed=1

• The default hashlib.sha1 may not be available in FIPS builds. Don't access it at import time so the developer has time to change the default. #5448
• Don't initialize the cli attribute in the sansio scaffold, but rather in the Flask concrete class. #5270

This is a fix release for the 3.0.x feature release branch. It fixes bugs but does not otherwise change behavior and should not result in breaking changes.

• Changes: https://flask.palletsprojects.com/en/3.0.x/changes/#version-3.0.2
• Milestone: https://github.com/pallets/flask/milestone/34?closed=1
• PyPI: https://pypi.org/project/Flask/3.0.2/

This is a fix release for the 3.0.x feature release branch.

Fixes an issue where using other JSON providers, such as flask-orjson, previously caused loaded session data to have an incorrect format in some cases.

• Changes: https://flask.palletsprojects.com/en/3.0.x/changes/#version-3-0-1
• Milestone: https://github.com/pallets/flask/milestone/32?closed=1
• PyPI: https://pypi.org/project/Flask/3.0.1/

This is a feature release, which includes new features, removes previously deprecated code, and adds new deprecations. The 3.0.x branch is now the supported fix branch, the 2.3.x branch will become a tag marking the end of support for that branch. We encourage everyone to upgrade, and to use a tool such as pip-tools to pin all dependencies and control upgrades. Test with warnings treated as errors to be able to adapt to deprecation warnings early.

• Changes: https://flask.palletsprojects.com/en/3.0.x/changes/#version-3-0-0
• Milestone: https://github.com/pallets/flask/milestone/20?closed=1