{"id":349,"library":"flask","title":"Flask","description":"Flask is a lightweight WSGI web application framework in Python, designed with simplicity and flexibility in mind. The current version is 3.1.3, released on March 28, 2026, following a regular release cadence with feature and fix updates as needed.","status":"active","version":"3.1.3","language":"python","source_language":"en","source_url":"https://github.com/pallets/flask/","tags":["web framework","Python","Flask","web development"],"install":[{"cmd":"pip install flask","lang":"bash","label":"Install Flask"}],"dependencies":[{"reason":"Provides WSGI utilities and functions for Flask","package":"werkzeug","optional":false},{"reason":"Template engine for rendering HTML in Flask","package":"jinja2","optional":false},{"reason":"Provides cryptographic signing for Flask","package":"itsdangerous","optional":false},{"reason":"Command-line interface toolkit for Flask","package":"click","optional":false}],"imports":[{"note":"Ensure 'flask' is installed in your environment; otherwise, you'll encounter an ImportError.","symbol":"Flask","correct":"from flask import Flask"},{"note":"Used for rendering HTML templates; requires Jinja2.","symbol":"render_template","correct":"from flask import render_template"},{"note":"Provides access to request data; ensure 'werkzeug' is installed.","symbol":"request","correct":"from flask import request"}],"quickstart":{"code":"from flask import Flask, render_template, request\n\napp = Flask(__name__)\n\n@app.route('/')\ndef index():\n    return render_template('index.html')\n\nif __name__ == '__main__':\n    app.run(debug=True)","lang":"python","description":"A minimal Flask application that renders an 'index.html' template when accessing the root URL."},"warnings":[{"fix":"Update your Flask application to handle session changes introduced in version 3.1.3.","message":"Flask 3.1.3 includes a security fix that may affect session behavior. Review the release notes for details.","severity":"breaking","affected_versions":"3.1.3"},{"fix":"Use absolute imports or adjust your project structure to support relative imports properly.","message":"Relative imports can cause issues if not used correctly. Ensure your project structure supports them to avoid ImportError.","severity":"gotcha","affected_versions":"All"},{"fix":"Review application logs for clues about activity before the timeout. Use a debugger to identify infinite loops or deadlocks. Optimize long-running operations or increase the test runner's timeout setting if appropriate. Check for resource leaks or excessive memory/CPU usage.","message":"The application experienced a timeout during testing. This can be caused by infinite loops, deadlocks, resource exhaustion, or long-running operations that exceed the test runner's timeout threshold.","severity":"breaking","affected_versions":"All"}],"env_vars":null,"last_verified":"2026-05-12T13:13:15.978Z","next_check":"2026-06-26T00:00:00.000Z","problems":[{"fix":"Install Flask using pip: `pip install Flask`. If using a virtual environment, ensure it is activated before installation.","cause":"The Flask library is not installed in the Python environment where the code is being executed, or the wrong Python interpreter/virtual environment is active.","error":"ModuleNotFoundError: No module named 'flask'"},{"fix":"Manually push an application context using `with app.app_context():` around the code block that requires it.","cause":"This error occurs when Flask functionality that relies on an active application context (like `current_app`, `g`, or database operations with extensions like Flask-SQLAlchemy) is accessed outside of a request or CLI command.","error":"RuntimeError: Working outside of application context"},{"fix":"Ensure that every execution path within a view function explicitly returns a valid Flask response, such as a string, a `render_template()` call, a `jsonify()` response, or a tuple like `('Error message', 400)`.","cause":"A Flask view function must return a valid response object, string, tuple, or an instance of `werkzeug.exceptions.HTTPException`. This error occurs when the function implicitly returns `None` or ends without an explicit return statement.","error":"TypeError: The view function did not return a valid response"},{"fix":"Explicitly specify the allowed HTTP methods in the route decorator using the `methods` argument: `@app.route('/my_route', methods=['GET', 'POST'])`.","cause":"The HTTP method used in the client's request (e.g., POST, PUT, DELETE) is not permitted for the requested URL by the Flask route definition. By default, Flask routes only accept GET requests.","error":"Method Not Allowed (405 Error)"}],"ecosystem":"pypi","meta_description":null,"install_score":100,"install_tag":"verified","quickstart_score":70,"quickstart_tag":"verified","pypi_latest":null,"install_checks":{"last_tested":"2026-05-12","tag":"verified","tag_description":"installs cleanly on critical runtimes, fast import, recently tested","results":[{"runtime":"python:3.10-alpine","python_version":"3.10","os_libc":"alpine (musl)","variant":"default","exit_code":0,"wheel_type":null,"failure_reason":null,"install_time_s":null,"import_time_s":0.49,"mem_mb":11.7,"disk_size":"22.3M"},{"runtime":"python:3.10-slim","python_version":"3.10","os_libc":"slim (glibc)","variant":"default","exit_code":0,"wheel_type":null,"failure_reason":null,"install_time_s":null,"import_time_s":0.35,"mem_mb":11.7,"disk_size":"23M"},{"runtime":"python:3.11-alpine","python_version":"3.11","os_libc":"alpine (musl)","variant":"default","exit_code":0,"wheel_type":null,"failure_reason":null,"install_time_s":null,"import_time_s":0.59,"mem_mb":13.2,"disk_size":"25.0M"},{"runtime":"python:3.11-slim","python_version":"3.11","os_libc":"slim (glibc)","variant":"default","exit_code":0,"wheel_type":null,"failure_reason":null,"install_time_s":null,"import_time_s":0.49,"mem_mb":13.2,"disk_size":"26M"},{"runtime":"python:3.12-alpine","python_version":"3.12","os_libc":"alpine (musl)","variant":"default","exit_code":0,"wheel_type":null,"failure_reason":null,"install_time_s":null,"import_time_s":0.53,"mem_mb":12.9,"disk_size":"16.7M"},{"runtime":"python:3.12-slim","python_version":"3.12","os_libc":"slim (glibc)","variant":"default","exit_code":0,"wheel_type":null,"failure_reason":null,"install_time_s":null,"import_time_s":0.52,"mem_mb":12.9,"disk_size":"17M"},{"runtime":"python:3.13-alpine","python_version":"3.13","os_libc":"alpine (musl)","variant":"default","exit_code":0,"wheel_type":null,"failure_reason":null,"install_time_s":null,"import_time_s":0.48,"mem_mb":13.9,"disk_size":"16.3M"},{"runtime":"python:3.13-slim","python_version":"3.13","os_libc":"slim (glibc)","variant":"default","exit_code":0,"wheel_type":null,"failure_reason":null,"install_time_s":null,"import_time_s":0.49,"mem_mb":13.8,"disk_size":"17M"},{"runtime":"python:3.9-alpine","python_version":"3.9","os_libc":"alpine (musl)","variant":"default","exit_code":0,"wheel_type":null,"failure_reason":null,"install_time_s":null,"import_time_s":0.4,"mem_mb":12.1,"disk_size":"22.1M"},{"runtime":"python:3.9-slim","python_version":"3.9","os_libc":"slim (glibc)","variant":"default","exit_code":0,"wheel_type":null,"failure_reason":null,"install_time_s":null,"import_time_s":0.34,"mem_mb":12.1,"disk_size":"23M"}]},"quickstart_checks":{"last_tested":"2026-04-23","tag":"verified","tag_description":"quickstart runs on critical runtimes, recently tested","results":[{"runtime":"python:3.10-alpine","exit_code":0},{"runtime":"python:3.10-slim","exit_code":0},{"runtime":"python:3.11-alpine","exit_code":0},{"runtime":"python:3.11-slim","exit_code":0},{"runtime":"python:3.12-alpine","exit_code":0},{"runtime":"python:3.12-slim","exit_code":0},{"runtime":"python:3.13-alpine","exit_code":0},{"runtime":"python:3.13-slim","exit_code":0},{"runtime":"python:3.9-alpine","exit_code":2},{"runtime":"python:3.9-slim","exit_code":2}]}}