如何在Python中从__init__返回一个值？

I have a class with an __init__ function.

我有一个带__init__函数的类。

How can I return an integer value from this function when an object is created?

如何在创建对象时从此函数返回整数值？

I wrote a program, where __init__ does command line parsing and I need to have some value set. Is it OK set it in global variable and use it in other member functions? If so how to do that? So far, I declared a variable outside class. and setting it one function doesn't reflect in other function ??

我写了一个程序，其中__init__执行命令行解析，我需要设置一些值。可以在全局变量中设置它并在其他成员函数中使用它吗？如果是这样怎么办？到目前为止，我在课外宣布了一个变量。并设置一个功能不反映在其他功能??

7 个解决方案

#1

__init__ is required to return None. You cannot (or at least shouldn't) return something else.

__init__必须返回None。你不能（或至少不应该）返回别的东西。

Try making whatever you want to return an instance variable (or function).

尝试制作任何想要返回实例变量（或函数）的内容。

>>> class Foo:
...     def __init__(self):
...         return 42
... 
>>> foo = Foo()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() should return None

#2

Why would you want to do that?

你为什么想这么做？

If you want to return some other object when a class is called, then use the __new__() method:

如果要在调用类时返回其他对象，请使用__new __（）方法：

class MyClass(object):
    def __init__(self):
        print "never called in this case"
    def __new__(cls):
        return 42

obj = MyClass()
print obj

#3

From the documentation of __init__:

从__init__的文档：

As a special constraint on constructors, no value may be returned; doing so will cause a TypeError to be raised at runtime.

作为构造函数的特殊约束，不能返回任何值;这样做会导致在运行时引发TypeError。

As a proof, this code:

作为证明，这段代码：

class Foo(object):
    def __init__(self):
        return 2

f = Foo()

Gives this error:

给出了这个错误：

Traceback (most recent call last):
  File "test_init.py", line 5, in <module>
    f = Foo()
TypeError: __init__() should return None, not 'int'

#4

Sample Usage of the matter in question can be like:

#5

The __init__ method, like other methods and functions returns None by default in the absence of a return statement, so you can write it like either of these:

在缺少return语句的情况下，__ init__方法与其他方法和函数一样，默认情况下返回None，因此您可以像以下任一方式一样编写它：

class Foo:
    def __init__(self):
        self.value=42

class Bar:
    def __init__(self):
        self.value=42
        return None

But, of course, adding the return None doesn't buy you anything.

但是，当然，添加回报“无”不会给你带来什么。

I'm not sure what you are after, but you might be interested in one of these:

我不确定你在追求什么，但你可能对以下其中一个感兴趣：

class Foo:
    def __init__(self):
        self.value=42
    def __str__(self):
        return str(self.value)

f=Foo()
print f.value
print f

prints:

打印：

42
42

#6

__init__ doesn't return anything and should always return None.

__init__不返回任何内容，应始终返回None。

#7

Just wanted to add, you can return classes in __init__

只想添加，你可以在__init__中返回类

@property
def failureException(self):
    class MyCustomException(AssertionError):
        def __init__(self_, *args, **kwargs):
            *** Your code here ***
            return super().__init__(*args, **kwargs)

    MyCustomException.__name__ = AssertionError.__name__
    return MyCustomException

The above method helps you implement a specific action upon an Exception in your test

上述方法可帮助您在测试中对异常执行特定操作

#1