在类中编写代码和在Python中的def __init __（self）有什么区别？ [英] What is the difference writing code in a class and in def __init__(self) in Python?

问题描述

可能重复：

我理解当一个类被调用时，它将在任何事情之前运行 __ init __ 中的代码。我仍然没有看到它们之间的区别，并直接在类下面写代码。

例如：

  class main（）：
x = 1 
 
 def disp（self）：
 print self.x 
  

  class main（）：
 def __init __（self）：
 self.x = 1 
 
 def disp（self）：
 print self.x 
  pre> 
 
 
对我来说，两者都有相同的功能。 
解决方案
这里有一个是一个几个关键的区别，在 __ init __ 之间，写在类下面，以及你写的。
 
 
 
使用 x = 1  
 
 
 
首先，你是对的 - 这两个代码有效（特别是因为我们在这里使用 int 对象，这对于可变对象是不同的）：
 
 
 
 请注意，他们不会实际上执行相同的操作 - 请参阅对此答案的评论以了解详情。 
  class main（object）：
x = 1 
  
 
 
  class main（object）：
 def __init __（self）：
 self.x = 1 
  
这就是为什么许多非标准Python库，例如 mongoengine 和 django 模型，有一个标准，你在其中创建类，而不使用 __ init __ 语句，以便不覆盖内置的，但仍允许您创建类属性，例如， Django示例：
  class mymodel（models.model）：
 name = models。 CharField（max_length = 20）
 url = models.UrlField（）
  
其他海报指出，两者之间有一个区别，当 x = 1 在 __ init __ 函数之外，它是类本身的一部分，即使没有初始化 - 参见 Zagorulkin Dmitry 的更多细节的答案。 
 
 
 
其他注意事项
 
 
 
更多用于 __ init __ ，而不仅仅是设置变量。最重要的一个是在初始化期间接受参数的能力。据我所知，没有一个方法，没有 __ init __ 函数。 
 
 
 
让我们假设我们创建一个 Person 类，当我们创建一个 Person 时，我们提供他们的年龄，然后自动计算出生年份。
  import datetime 
 class Person（object）：
 def __init __（self，age）：
 self .age = age 
 self.birth_year =（datetime.date.today（） -  datetime.timedelta（days = age * 365））。year 
  
使用中：
 > ;> joe = Person（23）
>>> joe.age 
 23 
>>> joe.birth_year 
 1990 
  
这不可能没有 __ init __ ，因为我们' t否则传递初始化 age 参数。
 

  
Possible Duplicate:

  Variables inside and outside of a class __init__() function  
I understand that when a class is called it will run the code in __init__ before anything. I still don't see the difference between that, and writing the code directly under the class.

For example:
class main():
  x = 1

  def disp(self):
    print self.x




class main():
  def __init__(self):
    self.x = 1

  def disp(self):
    print self.x
To me, both have the same functionality. (Maybe I am missing out something.) I would like to know which is more (ahem) pythonic and why.
解决方案
There are a couple key differences here, both between __init__ and writing it just under the class, as well as what you wrote.

Working with x = 1

First, you are right--these two items of code do effectively the same thing for your purposes (specifically because we're working with int objects here, this would be different for mutable objects):

Note that they don't actually do the same thing--please see the comments on this answer for clarification.
class main(object):
    x = 1

class main(object):
    def __init__(self):
        self.x = 1
This is why many non-standard Python libraries, like mongoengine and django models, have a standard where you create classes without using an __init__ statement so as not to override the built-in one, but still allowing you to create class attributes, e.g., a Django example:
class mymodel(models.model):
    name = models.CharField(max_length=20)
    url = models.UrlField()
However, as the other poster points out, there is a difference between the two in that when x=1 is outside of the __init__ function, it is part of the class itself even when not intialized--see Zagorulkin Dmitry's answer for more detail on that.  In most cases, though, that distinction won't be relevant for you.

Other considerations

There are more uses for __init__ beyond just setting variables.  The most important one offhand is the ability to accept arguments during initialization.  To my knowledge, there is not a way to do this without an __init__ function.  I'll show you what I mean by this in this example.

Let's say we're creating a Person class, and when we create a Person, we supply their age, and then their birth year is automatically calculated from that for us.
import datetime
class Person(object):
    def __init__(self, age):
        self.age = age
        self.birth_year = (datetime.date.today() - datetime.timedelta(days=age*365)).year
In use:
>>>joe = Person(23)
>>>joe.age
23
>>>joe.birth_year
1990
This wouldn't be possible without __init__, since we couldn't pass the initialization the age argument otherwise.

                        
这篇关于在类中编写代码和在Python中的def __init __（self）有什么区别？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！