# TypeError: BaseModel.__init__() takes 1 positional argument but 2 were given

- **ID:** `python/fastapi-pydantic-v1-v2-migration`
- **Domain:** python
- **Category:** type_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

Pydantic v1 code calling a v2 model with positional args, or using v1-only APIs like .dict() vs .model_dump().

## Version Compatibility

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

## Workarounds

1. **** (94% success)
   ```
   item = Item(name='x')
data = item.model_dump()
   ```
2. **** (85% success)
   ```
   from pydantic.v1 import BaseModel
   ```

## Dead Ends

- **** — FastAPI 0.100+ requires pydantic v2; other deps break. (85% fail)
- **** — Does not address .dict/.json API changes elsewhere. (80% fail)