What is the dataclass field function?

Question

Accepted Answer

**field()** customizes **dataclass fields** with defaults, metadata, and factories.

```python
from dataclasses import dataclass, field
from typing import ClassVar

@dataclass
class User:
    name: str
    email: str
    age: int = 0
    tags: list[str] = field(default_factory=list)
    _id: int = field(init=False, repr=False)

def __post_init__(self):
        self._id = hash(self.name + self.email)

# Usage
user = User(name="Alice", email="alice@example.com")
print(user)  # User(name='Alice', email='alice@example.com', age=0, tags=[])

user.tags.append("admin")
print(user.tags)  # ["admin"]

# With metadata
@dataclass
class Product:
    name: str = field(metadata={"max_length": 100})
    price: float = field(metadata={"min_value": 0})

# Frozen dataclass
@dataclass(frozen=True)
class Point:
    x: float
    y: float

p = Point(1.0, 2.0)
# p.x = 3.0  # FrozenInstanceError!

# Slot dataclass (Python 3.10+)
@dataclass(slots=True)
class Optimized:
    x: int
    y: int
```

**field() options:**
- `default`: default value.
- `default_factory`: callable for mutable defaults.
- `init`: include in __init__.
- `repr`: include in __repr__.
- `compare`: include in __eq__.
- `metadata`: dict for custom data.

Python: What is the dataclass field function?