What is the `dataclasses.field()` metadata parameter?

Question

Accepted Answer

**field metadata** — attach custom data to dataclass fields.

```python
from dataclasses import dataclass, field, fields

@dataclass
class User:
    name: str = field(metadata={"max_length": 100, "required": True})
    age: int = field(metadata={"min_value": 0, "max_value": 150})
    email: str = field(metadata={"pattern": r"^[^@]+@[^@]+$"})

# Access metadata
for f in fields(User):
    print(f.name, f.metadata)
    # name {'max_length': 100, 'required': True}
    # age {'min_value': 0, 'max_value': 150}

# Use metadata for validation
def validate(obj):
    for f in fields(obj):
        value = getattr(obj, f.name)
        meta = f.metadata
        if "max_length" in meta and len(str(value)) > meta["max_length"]:
            raise ValueError(f"{f.name} too long")
        if "min_value" in meta and value < meta["min_value"]:
            raise ValueError(f"{f.name} too small")
```

**Use cases:**
- Form/validation frameworks.
- ORM field mapping.
- Documentation generation.

Python: What is the `dataclasses.field()` metadata parameter?