如何在Python中从__init__返回值?-Python 实用宝典

如何在Python中从__init__返回值?

我有一个带__init__功能的课。 创建对象时如何从该函数返回整数值? 我写了一个程序,在其中进行__init__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,该怎么做?到目前为止,我在课外声明了一个变量。并将其设置为一个功能不会反映在其他功能中?

问题:如何在Python中从__init__返回值?

我有一个带__init__功能的课。

创建对象时如何从该函数返回整数值?

我写了一个程序,在其中进行__init__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,该怎么做?到目前为止,我在课外声明了一个变量。并将其设置为一个功能不会反映在其他功能中?

I have a class with an __init__ function.

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

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 ??


回答 0

__init__必须返回无。您不能(或至少不应该)返回其他东西。

尝试做任何您想返回的实例变量(或函数)。

>>> 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

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

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

>>> 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

回答 1

你为什么想这么做?

如果要在调用类时返回其他对象,请使用以下__new__()方法:

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

obj = MyClass()
print obj

Why would you want to do that?

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

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

obj = MyClass()
print obj

回答 2

从以下文档中__init__

作为对构造函数的特殊限制,不能返回任何值。这样做将导致在运行时引发TypeError。

作为证明,此代码:

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

f = Foo()

给出此错误:

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

From the documentation of __init__:

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

As a proof, this code:

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

f = Foo()

Gives this error:

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

回答 3

有问题的示例用法如下:

class SampleObject(object):

    def __new__(cls, item):
        if cls.IsValid(item):
            return super(SampleObject, cls).__new__(cls)
        else:
            return None

    def __init__(self, item):
        self.InitData(item) #large amount of data and very complex calculations

...

ValidObjects = []
for i in data:
    item = SampleObject(i)
    if item:             # in case the i data is valid for the sample object
        ValidObjects.append(item)

我没有足够的声誉,所以我无法发表评论,这太疯狂了!我希望我可以将其发布为对weronika的评论

Sample Usage of the matter in question can be like:

class SampleObject(object):

    def __new__(cls, item):
        if cls.IsValid(item):
            return super(SampleObject, cls).__new__(cls)
        else:
            return None

    def __init__(self, item):
        self.InitData(item) #large amount of data and very complex calculations

...

ValidObjects = []
for i in data:
    item = SampleObject(i)
    if item:             # in case the i data is valid for the sample object
        ValidObjects.append(item)

I do not have enough reputation so I can not write a comment, it is crazy! I wish I could post it as a comment to weronika


回答 4

__init__与其他方法和函数一样,该方法默认情况下在没有return语句的情况下返回None,因此您可以像以下任何一种一样编写它:

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

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

但是,当然,添加return None并不会给您带来任何好处。

我不确定您要追求的是什么,但是您可能会对其中之一感兴趣:

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

f=Foo()
print f.value
print f

印刷品:

42
42

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:

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.

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

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

f=Foo()
print f.value
print f

prints:

42
42

回答 5

__init__不返回任何东西,应该总是返回None

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


回答 6

您可以将其设置为类变量,然后从主程序中读取它:

class Foo:
    def __init__(self):
        #Do your stuff here
        self.returncode = 42
bar = Foo()
baz = bar.returncode

You can just set it to a class variable and read it from the main program:

class Foo:
    def __init__(self):
        #Do your stuff here
        self.returncode = 42
bar = Foo()
baz = bar.returncode

回答 7

只需添加即可,您可以在 __init__

@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

上面的方法可以帮助您对测试中的异常执行特定的操作

Just wanted to add, you can return classes in __init__

@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


回答 8

init()返回无值,完美解决

class Solve:
def __init__(self,w,d):
    self.value=w
    self.unit=d
def __str__(self):
    return str("my speed is "+str(self.value)+" "+str(self.unit))
ob=Solve(21,'kmh')
print (ob)

输出:我的速度是21 kmh

init() return none value solved perfectly

class Solve:
def __init__(self,w,d):
    self.value=w
    self.unit=d
def __str__(self):
    return str("my speed is "+str(self.value)+" "+str(self.unit))
ob=Solve(21,'kmh')
print (ob)

output: my speed is 21 kmh


回答 9

好吧,如果您不再关心对象实例,则可以替换它!

class MuaHaHa():
def __init__(self, ret):
    self=ret

print MuaHaHa('foo')=='foo'

Well, if you don't care about the object instance anymore ... you can just replace it!

class MuaHaHa():
def __init__(self, ret):
    self=ret

print MuaHaHa('foo')=='foo'

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