# KeyError: 'session'

- **ID:** `python/flask-keyerror-session`
- **Domain:** python
- **Category:** data_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

Trying to access a session key that does not exist, often due to missing secret_key or disabled sessions.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| 3.x | active | — | — |

## Workarounds

1. **Set secret_key at app initialization** (95% success)
   ```
   app = Flask(__name__)
app.secret_key = 'your-secret-key'
# Then use session
session['user'] = 'Alice'
   ```
2. **Check session key existence before access** (90% success)
   ```
   if 'user' in session:
    user = session['user']
else:
    user = 'Guest'
   ```

## Dead Ends

- **Setting secret_key after first request** — Secret key must be set before any session access. (75% fail)
- **Using session without importing** — Flask session requires from flask import session. (60% fail)