namedtuple

最新推荐文章于 2025-09-09 20:40:49 发布

weixin_30438813

最新推荐文章于 2025-09-09 20:40:49 发布

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CC 4.0 BY-SA版权

文章标签：数据库

原文链接：http://www.cnblogs.com/zzwcng/p/10983446.html

本文探讨了在Python中使用namedtuple、__slots__及dataclass优化小类如Point的内存使用，通过实例展示了如何创建具默认值的Account类，以及在csv、sqlite等场景下使用namedtuple的优势。同时，介绍了namedtuple的多种实用方法，如_make、_asdict、_replace等，以及如何通过继承增强其功能。

在写一些比较小的类的时候，例如 Point(x, y) ，不用去定义一个类Point

class Point:     
    def __init__(self, x=1.0, y=1.0):
        self.x = x
        self.y = y

可使用namedtuple替换:

Point = namedtuple('Point', ['x', 'y'], defaults=[1.0, 1.0])

假设有一个列表points，它是一系列点的集合

points = [Point() for _ in range(100000)]

此时使用namedtuple可减少内存开销（Named tuple instances do not have per-instance dictionaries, so they are lightweight and require no more memory than regular tuples.）

也可使用__slots__来限制属性和方法，减少内存开销

class Point:     
    __slots__ = x, y
    def __init__(self, x=1.0, y=1.0):
        self.x = x
        self.y = y

也可使用dataclass

from dataclasses import dataclass

@dataclass
class Point:
    x: float = 1.0
    y: float = 1.0

示例：

设置默认值

from collections import namedtuple

>>> Account = namedtuple('Account', ['type', 'balance'], defaults=[0]) >>> Account._field_defaults {'balance': 0}

>>> Account = namedtuple('Account', ['type', 'balance'], defaults=[0，0]) >>> Account._field_defaults
{'type': 0, 'balance': 0}

csv、sqlite

EmployeeRecord = namedtuple('EmployeeRecord', 'name, age, title, department, paygrade')

import csv
for emp in map(EmployeeRecord._make, csv.reader(open("employees.csv", "rb"))):
    print(emp.name, emp.title)

import sqlite3
conn = sqlite3.connect('/companydata')
cursor = conn.cursor()
cursor.execute('SELECT name, age, title, department, paygrade FROM employees')
for emp in map(EmployeeRecord._make, cursor.fetchall()):
    print(emp.name, emp.title)

方法

>>> t = [11, 22]
>>> Point._make(t)
Point(x=11, y=22)

>>> p = Point(x=11, y=22)
>>> p._asdict()
OrderedDict([('x', 11), ('y', 22)])

>>> p = Point(x=11, y=22)
>>> p._replace(x=33)
Point(x=33, y=22)
>>> for partnum, record in inventory.items():
...     inventory[partnum] = record._replace(price=newprices[partnum], timestamp=time.now())

>>> p._fields            # view the field names
('x', 'y')
>>> Color = namedtuple('Color', 'red green blue')
>>> Pixel = namedtuple('Pixel', Point._fields + Color._fields)
>>> Pixel(11, 22, 128, 255, 0)
Pixel(x=11, y=22, red=128, green=255, blue=0)

>>> getattr(p, 'x')
11

>>> class Point(namedtuple('Point', ['x', 'y'])):
...     __slots__ = ()
...     @property
...     def hypot(self):
...         return (self.x ** 2 + self.y ** 2) ** 0.5
...     def __str__(self):
...         return 'Point: x=%6.3f  y=%6.3f  hypot=%6.3f' % (self.x, self.y, self.hypot)
>>> for p in Point(3, 4), Point(14, 5/7):
...     print(p)
Point: x= 3.000  y= 4.000  hypot= 5.000
Point: x=14.000  y= 0.714  hypot=14.018

>>> Point3D = namedtuple('Point3D', Point._fields + ('z',))

>>> Book = namedtuple('Book', ['id', 'title', 'authors'])
>>> Book.__doc__ += ': Hardcover book in active collection'
>>> Book.id.__doc__ = '13-digit ISBN'
>>> Book.title.__doc__ = 'Title of first printing'
>>> Book.authors.__doc__ = 'List of authors sorted by last name'

>>> Account = namedtuple('Account', 'owner balance transaction_count')
>>> default_account = Account('<owner name>', 0.0, 0)
>>> johns_account = default_account._replace(owner='John')
>>> janes_account = default_account._replace(owner='Jane')