使用数据类读取文本文件，将属性设置为日期时间对象

Question

我正在努力强迫自己养成使用数据classes 的习惯。我正在读取一个包含数据行的文本文件，其中两列包含一个日期。当我将文件读入 class 时，为什么日期没有被转换为日期时间对象？

import datetime
from dataclasses import dataclass

@dataclass
class AC07:
    NAME: str
    MADE_FROM_DATE: datetime.datetime
    MADE_UPTO_DATE: datetime.datetime
    
counter = 0
deck = []
with open(filename, encoding="latin-1") as file:
    for line in file:
        args = line.strip().split("|")[1:]  # not interested in first column
        data = AC07(*args)
        deck.append(data)
        counter += 1
        if counter == 10:
            break
        
print (type(deck[0].MADE_FROM_DATE))
<class 'str'>
# Expected behavior <class datetime.datetime>

Answer 1

如有错误请指正，但我相信数据类不会强制类型。 datetime.datetime 在你的例子中（几乎）只是一个注释，除了两个实现细节。检查 documentation。具体来说，

The dataclass() decorator examines the class to find fields. A field is defined as class variable that has a type annotation. With two exceptions described below, nothing in dataclass() examines the type specified in the variable annotation.

这两个异常与强制数据类型无关：

Class variables One of two places where dataclass() actually inspects the type of a field is to determine if a field is a class variable as defined in PEP 526. It does this by checking if the type of the field is typing.ClassVar. If a field is a ClassVar, it is excluded from consideration as a field and is ignored by the dataclass mechanisms. Such ClassVar pseudo-fields are not returned by the module-level fields() function.

Init-only variables The other place where dataclass() inspects a type annotation is to determine if a field is an init-only variable. It does this by seeing if the type of a field is of type dataclasses.InitVar. If a field is an InitVar, it is considered a pseudo-field called an init-only field. As it is not a true field, it is not returned by the module-level fields() function. Init-only fields are added as parameters to the generated __init__() method, and are passed to the optional __post_init__() method. They are not otherwise used by dataclasses.

Answer 2

现在我使用@vasco-ludovico 发布的答案更好地理解了它，为了解决我的问题，我使用了以下代码：

with open(filename, encoding="latin-1") as file:
    for line in file:
        args = line.strip().split("|")[1:]  # not interested in first column
        data = AC07(*args)
        data.MADE_FROM_DATE = datetime.datetime.strptime(data.MADE_FROM_DATE, "%d/%m/%Y")
        deck.append(data)
        counter += 1
        if counter == 10:
            break

使用数据类读取文本文件，将属性设置为日期时间对象

Using a dataclass to read a text file setting an attribute to be a datetime object

python

datetime

python-dataclasses