更改命名元组列表的值

Question

我有一个名为 Books 的命名元组列表，我试图将 price 字段增加 20%，这确实会改变 Books 的值。我试着做：

from collections import namedtuple
Book = namedtuple('Book', 'author title genre year price instock')
BSI = [
       Book('Suzane Collins','The Hunger Games', 'Fiction', 2008, 6.96, 20),
       Book('J.K. Rowling', "Harry Potter and the Sorcerer's Stone", 'Fantasy', 1997, 4.78, 12)]
for item in BSI:
    item = item.price*1.10
print(item.price)

但我不断得到：

 Traceback (most recent call last):
 print(item.price)
 AttributeError: 'float' object has no attribute 'price'

我了解我无法在命名元组中设置字段。如何更新price？

我试着把它变成一个函数：

def restaurant_change_price(rest, newprice):
    rest.price = rest._replace(price = rest.price + newprice)
    return rest.price

print(restaurant_change_price(Restaurant("Taillevent", "French", "343-3434", "Escargots", 24.50), 25))

但我得到一个错误替换说：

 rest.price = rest._replace(price = rest.price + newprice)
 AttributeError: can't set attribute

谁能告诉我为什么会这样？

Answer 1

命名元组是不可变的，因此您无法操纵它们。

正确的做法：

如果你想要一些可变的，你可以使用recordtype。

from recordtype import recordtype

Book = recordtype('Book', 'author title genre year price instock')
books = [
   Book('Suzane Collins','The Hunger Games', 'Fiction', 2008, 6.96, 20),
   Book('J.K. Rowling', "Harry Potter and the Sorcerer's Stone", 'Fantasy', 1997, 4.78, 12)]

for book in books:
    book.price *= 1.1
    print(book.price)

PS：如果你没有安装它，你可能需要pip install recordtype。

不好的做法：

您也可以继续使用namedtuple 和_replace() 方法。

from collections import namedtuple

Book = namedtuple('Book', 'author title genre year price instock')
books = [
   Book('Suzane Collins','The Hunger Games', 'Fiction', 2008, 6.96, 20),
   Book('J.K. Rowling', "Harry Potter and the Sorcerer's Stone", 'Fantasy', 1997, 4.78, 12)]

for i in range(len(books)):
    books[i] = books[i]._replace(price = books[i].price*1.1)
    print(books[i].price)

Answer 2

在 Python >= 3.7 中，您可以使用 dataclass 装饰器和新的变量注释功能来生成可变记录类型：

from dataclasses import dataclass


@dataclass
class Book:
    author: str
    title: str
    genre: str
    year: int
    price: float
    instock: int


BSI = [
    Book("Suzane Collins", "The Hunger Games", "Fiction", 2008, 6.96, 20),
    Book(
        "J.K. Rowling",
        "Harry Potter and the Sorcerer's Stone",
        "Fantasy",
        1997,
        4.78,
        12,
    ),
]

for item in BSI:
    item.price *= 1.10
    print(f"New price for '{item.title}' book is {item.price:,.2f}")

输出：

New price for 'The Hunger Games' book is 7.66
New price for 'Harry Potter and the Sorcerer's Stone' book is 5.26

Answer 3

这看起来像是 Python 数据分析库 pandas 的任务。做这种事情真的非常容易：

In [6]: import pandas as pd
In [7]: df = pd.DataFrame(BSI, columns=Book._fields)
In [8]: df
Out[8]: 
           author                                  title    genre  year  \
0  Suzane Collins                       The Hunger Games  Fiction  2008   
1    J.K. Rowling  Harry Potter and the Sorcerers Stone  Fantasy  1997   

   price  instock  
0   6.96       20  
1   4.78       12  

In [9]: df['price'] *= 100
In [10]: df
Out[10]: 
           author                                  title    genre  year  \
0  Suzane Collins                       The Hunger Games  Fiction  2008   
1    J.K. Rowling  Harry Potter and the Sorcerer's Stone  Fantasy  1997   

   price  instock  
0    696       20  
1    478       12  

现在不是比与namedtuples 一起工作要好得多吗？