Flask

Frontend & Mobile

The Python micro framework for building web applications.

Latest: 3.1.33 releases2 common errorsUpdated Feb 19, 2026View on GitHub

Release History

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3.1.31 fix

Feb 19, 2026

Flask 3.1.3 is a security fix release addressing an issue where session access was not correctly tracked for key-only operations.

3.1.23 fixes

Aug 19, 2025

Flask 3.1.2 fixes async view handling for `stream_with_context`, corrects session state with `follow_redirects` in the test client, and relaxes the `send_file` type hint.

3.1.14 fixes

May 13, 2025

Flask 3.1.1 is a bug-fix release that corrects signing-key selection, improves CLI type hints, ensures the help command loads the app, and updates sans-io typing for async iterables.

Common Errors

OSError2 reports

TemplateAssertionError1 report

TemplateAssertionError in Flask usually arises when Jinja2, the templating engine, encounters an undefined variable or attribute within your HTML template. To fix this, ensure all variables used in your template are properly passed from the Flask view function, and double-check for typos in variable names within both your Python code and your HTML templates.