flask

# Flask Skill

Production-tested patterns for Flask with the application factory pattern, Blueprints, and Flask-SQLAlchemy.

**Latest Versions** (verified January 2026):
- Flask: 3.1.2
- Flask-SQLAlchemy: 3.1.1
- Flask-Login: 0.6.3
- Flask-WTF: 1.2.2
- Werkzeug: 3.1.5
- **Python**: 3.9+ required (3.8 dropped in Flask 3.1.0)

---

## Quick Start

### Project Setup with uv

```bash
# Create project
uv init my-flask-app
cd my-flask-app

# Add dependencies
uv add flask flask-sqlalchemy flask-login flask-wtf python-dotenv

# Run development server
uv run flask --app app run --debug
```

### Minimal Working Example

```python
# app.py
from flask import Flask

app = Flask(__name__)

@app.route("/")
def hello():
    return {"message": "Hello, World!"}

if __name__ == "__main__":
    app.run(debug=True)
```

Run: `uv run flask --app app run --debug`

---

## Known Issues Prevention

This skill prevents **9** documented issues:

### Issue #1: stream_with_context Teardown Regression (Flask 3.1.2)
**Error**: `KeyError` in teardown functions when using `stream_with_context`
**Source**: [GitHub Issue #5804](https://github.com/pallets/flask/issues/5804)
**Why It Happens**: Flask 3.1.2 introduced a regression where `stream_with_context` triggers `teardown_request()` calls multiple times before response generation completes. If teardown callbacks use `g.pop(key)` without a default, they fail on the second call.

**Prevention**:
```python
# WRONG - fails on second teardown call
@app.teardown_request
def _teardown_request(_):
    g.pop("hello")  # KeyError on second call

# RIGHT - idempotent teardown
@app.teardown_request
def _teardown_request(_):
    g.pop("hello", None)  # Provide default value
```

**Status**: Will be fixed in Flask 3.2.0 as side effect of PR #5812. Until then, ensure all teardown callbacks are idempotent.

---

### Issue #2: Async Views with Gevent Incompatibility
**Error**: `RuntimeError` when handling concurrent async requests with gevent
**Source**: [GitHub Is