flask-session

Set a strong, unique, and secret key in your Flask application configuration. It's best practice to load this from an environment variable for production.
```python
app = Flask(__name__)
app.config['SECRET_KEY'] = 'your_super_secret_key_here' # In production, load from env var
# Or, for Flask 0.10 and later:
# app.secret_key = 'your_super_secret_key_here'
```

Configure the `SESSION_TYPE` in your Flask application to one of the supported backends (e.g., 'filesystem', 'redis', 'memcached', 'mongodb', 'sqlalchemy', 'cachelib').
```python
from flask import Flask
from flask_session import Session

app = Flask(__name__)
app.config['SECRET_KEY'] = 'your_secret_key'
app.config['SESSION_TYPE'] = 'filesystem' # Or 'redis', 'memcached', etc.
sess = Session()
sess.init_app(app)
```

Ensure the Redis server is running and accessible from your Flask application. Verify the `SESSION_REDIS` configuration points to the correct Redis instance.
```python
from flask import Flask
from flask_session import Session
from redis import Redis

app = Flask(__name__)
app.config['SECRET_KEY'] = 'your_secret_key'
app.config['SESSION_TYPE'] = 'redis'
app.config['SESSION_REDIS'] = Redis(host='localhost', port=6379, db=0)
sess = Session()
sess.init_app(app)
```
Also, check your Redis server status (e.g., `redis-cli ping` or `sudo systemctl status redis`) and firewall rules.

Install `flask-session` using pip in your active Python environment. If using a virtual environment, ensure it's activated.
```bash
pip install Flask-Session
# If using Python 3 and have multiple Python versions:
pip3 install Flask-Session
```

Ensure `SECRET_KEY` is set and loaded correctly (especially outside `if __name__ == '__main__':` blocks for production). Confirm `SESSION_TYPE` is properly configured for a persistent backend (e.g., 'filesystem', 'redis'). If deploying with a proxy or HTTPS, set `SESSION_COOKIE_SECURE=True` and `SESSION_COOKIE_SAMESITE='Lax'` or `'None'` (if cross-site) along with a `SECRET_KEY`.
```python
app = Flask(__name__)
app.config['SECRET_KEY'] = 'your_strong_secret_key'
app.config['SESSION_TYPE'] = 'filesystem'
app.config['SESSION_PERMANENT'] = False # If you want non-permanent sessions
app.config['SESSION_COOKIE_SECURE'] = True # Use True in production with HTTPS
app.config['SESSION_COOKIE_SAMESITE'] = 'Lax' # Or 'None' with SECURE=True for cross-site
sess = Session()
sess.init_app(app)
```