[Python-Dev] PEP 557: Data Classes (original) (raw)

Eric V. Smith eric at trueblade.com
Fri Sep 8 10:57:09 EDT 2017

I've written a PEP for what might be thought of as "mutable namedtuples with defaults, but not inheriting tuple's behavior" (a mouthful, but it sounded simpler when I first thought of it). It's heavily influenced by the attrs project. It uses PEP 526 type annotations to define fields. From the overview section:

@dataclass class InventoryItem: name: str unit_price: float quantity_on_hand: int = 0

 def total_cost(self) -> float:
     return self.unit_price * self.quantity_on_hand

Will automatically add these methods:

def init(self, name: str, unit_price: float, quantity_on_hand: int = 0) -> None: self.name = name self.unit_price = unit_price self.quantity_on_hand = quantity_on_hand def repr(self): return f'InventoryItem(name={self.name!r},unit_price={self.unit_price!r},quantity_on_hand={self.quantity_on_hand!r})' def eq(self, other): if other.class is self.class: return (self.name, self.unit_price, self.quantity_on_hand) == (other.name, other.unit_price, other.quantity_on_hand) return NotImplemented def ne(self, other): if other.class is self.class: return (self.name, self.unit_price, self.quantity_on_hand) != (other.name, other.unit_price, other.quantity_on_hand) return NotImplemented def lt(self, other): if other.class is self.class: return (self.name, self.unit_price, self.quantity_on_hand) < (other.name, other.unit_price, other.quantity_on_hand) return NotImplemented def __le__(self, other): if other.__class__ is self.__class__: return (self.name, self.unit_price, self.quantity_on_hand) <= (other.name, other.unit_price, other.quantity_on_hand) return NotImplemented def __gt__(self, other): if other.__class__ is self.__class__: return (self.name, self.unit_price, self.quantity_on_hand) > (other.name, other.unit_price, other.quantity_on_hand) return NotImplemented def ge(self, other): if other.class is self.class: return (self.name, self.unit_price, self.quantity_on_hand) >= (other.name, other.unit_price, other.quantity_on_hand) return NotImplemented

Data Classes saves you from writing and maintaining these functions.

The PEP is largely complete, but could use some filling out in places. Comments welcome!

Eric.

P.S. I wrote this PEP when I was in my happy place.

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