Python:dataclasses

lastmod 2023-07-07created 2021-08-055 edits

This module provides a decorator and functions that automatically add generated special methods such as init() and repr() to user-defined classes. It was originally described in PEP 557.

How to use

from dataclasses import dataclass

@dataclass
class InventoryItem:
    """Class for keeping track of an item in inventory."""
    name: str
    unit_price: float
    quantity_on_hand: int = 0

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

An init() function like the one below is added automatically.

def __init__(self, name: str, unit_price: float, quantity_on_hand: int = 0):
    self.name = name
    self.unit_price = unit_price
    self.quantity_on_hand = quantity_on_hand

dataclass

@dataclass
class Base:
    x: Any = 15.0
    y: int = 0

@dataclass
class C(Base):
    z: int = 10
    x: int = 15

For C, which inherits from Base, the following constructor is generated.

def __init__(self, x: int = 15, y: int = 0, z: int = 10):

Customizing fields

To control characteristics such as initialization arguments and whether a field is included in __repr__, use the field function.

# -*- coding: utf-8 -*-

from typing import Optional, Any, List
from datetime import datetime
from dataclasses import dataclass, field


@dataclass
class Project:
    uid: Optional[int] = None
    group_uid: Optional[int] = None
    slug: Optional[str] = None
    name: Optional[str] = None
    description: Optional[str] = None
    features: Optional[List[str]] = None
    extra: Optional[Any] = None
    created_at: Optional[datetime] = None
    updated_at: Optional[datetime] = None

    _group_slug: Optional[str] = field(default=None, init=False, repr=False)

    def remove_sensitive(self) -> None:
        self.uid = None
        self.group_uid = None

    @property
    def group_slug(self) -> Optional[str]:
        return self._group_slug

    @group_slug.setter
    def group_slug(self, value: Optional[str]):
        self._group_slug = value

default_factory

If provided, it must be a zero-argument callable that is called when a default value is needed for this field.

Among other uses, this can be used to specify fields with mutable default values, as discussed below.

It is an error to specify both default and default_factory.

@dataclass
class C:
    mylist: list[int] = field(default_factory=list)

c = C()
c.mylist += [1, 2, 3]

Favorite site

dataclasses — Data Classes — Python 3.9.6 documentation

author Zer0 sitemap