返回，返回无，根本没有返回？

本文翻译自：return, return None, and no return at all?

Consider three functions: 考虑三个功能：

def my_func1():print "Hello World"return Nonedef my_func2():print "Hello World"returndef my_func3():print "Hello World"

They all appear to return None. 它们似乎都返回None。 Are there any differences between how the returned value of these functions behave? 这些函数的返回值的行为方式之间有什么区别吗？ Are there any reasons to prefer one versus the other? 是否有任何理由偏爱一个？

#1楼

参考：https://stackoom.com/question/12CN4/返回-返回无-根本没有返回

#2楼

They each return the same singleton None -- There is no functional difference. 它们每个都返回相同的单例None在功能上没有区别。

I think that it is reasonably idiomatic to leave off the return statement unless you need it to break out of the function early (in which case a bare return is more common), or return something other than None . 我认为离开return语句是合理的习惯做法，除非您需要它早点退出该函数（在这种情况下，裸return比较常见），或者返回None以外的东西。 It also makes sense and seems to be idiomatic to write return None when it is in a function that has another path that returns something other than None . 这也很有意义，并且在函数中具有返回除None其他值的函数时，写return None似乎是惯用的。 Writing return None out explicitly is a visual cue to the reader that there's another branch which returns something more interesting (and that calling code will probably need to handle both types of return values). 明确地将return None明确写出是读者的视觉提示，因为还有另一个分支返回了更有趣的内容（并且调用代码可能需要处理两种类型的返回值）。

Often in Python, functions which return None are used like void functions in C -- Their purpose is generally to operate on the input arguments in place (unless you're using global data ( shudders )). 在Python中，返回None的函数通常与C中的void函数一起使用-它们的目的通常是对输入参数进行适当的操作 （除非您使用全局数据（ shudders ））。 Returning None usually makes it more explicit that the arguments were mutated. 返回None通常可以使参数更明确地变得更明确。 This makes it a little more clear why it makes sense to leave off the return statement from a "language conventions" standpoint. 这使我们更加清楚为什么从“语言约定”的角度出发，放弃return语句是有意义的。

That said, if you're working in a code base that already has pre-set conventions around these things, I'd definitely follow suit to help the code base stay uniform... 就是说，如果您正在使用已经针对这些事情设置了预设约定的代码库，那么我肯定会效仿以帮助使代码库保持一致...

#3楼

On the actual behavior, there is no difference. 在实际行为上，没有区别。 They all return None and that's it. 他们都返回None ，仅此而已。 However, there is a time and place for all of these. 但是，所有这些都有时间和地点。 The following instructions are basically how the different methods should be used (or at least how I was taught they should be used), but they are not absolute rules so you can mix them up if you feel necessary to. 以下说明基本上是应如何使用不同方法的方法（或至少应告诉我如何使用它们的方法），但是它们不是绝对规则，因此您可以根据需要将它们混合使用。

Using `return None` 使用`return None`

This tells that the function is indeed meant to return a value for later use, and in this case it returns None . 这说明该函数确实是要返回一个值供以后使用，在这种情况下，它返回None 。 This value None can then be used elsewhere. 此值None可以在其他地方使用。 return None is never used if there are no other possible return values from the function. 如果该函数没有其他可能的返回值，则从不使用return None 。

In the following example, we return person 's mother if the person given is a human. 在以下示例中，如果给定的person是人类，我们将返回person的mother 。 If it's not a human, we return None since the person doesn't have a mother (let's suppose it's not an animal or something). 如果不是人，则由于该person没有mother （因此假设它不是动物或其他东西），我们将返回None 。

def get_mother(person):if is_human(person):return person.motherelse:return None

Using `return` 使用`return`

This is used for the same reason as break in loops. 出于与break循环相同的原因使用它。 The return value doesn't matter and you only want to exit the whole function. 返回值无关紧要，您只想退出整个函数。 It's extremely useful in some places, even though you don't need it that often. 即使您不经常使用它，它在某些地方也非常有用。

We've got 15 prisoners and we know one of them has a knife. 我们有15 prisoners ，我们知道其中一个有一把刀。 We loop through each prisoner one by one to check if they have a knife. 我们逐个循环检查每个prisoner ，以检查他们是否有刀。 If we hit the person with a knife, we can just exit the function because we know there's only one knife and no reason the check rest of the prisoners . 如果我们用刀子打人，我们就可以退出该功能，因为我们知道只有一把刀子，没有理由检查其余prisoners 。 If we don't find the prisoner with a knife, we raise an alert. 如果我们没有用刀找到prisoner ，我们会发出警报。 This could be done in many different ways and using return is probably not even the best way, but it's just an example to show how to use return for exiting a function. 这可以通过许多不同的方式完成，使用return可能甚至不是最好的方法，但这只是一个示例，说明如何使用return退出函数。

def find_prisoner_with_knife(prisoners):for prisoner in prisoners:if "knife" in prisoner.items:prisoner.move_to_inquisition()return # no need to check rest of the prisoners nor raise an alertraise_alert()

Note: You should never do var = find_prisoner_with_knife() , since the return value is not meant to be caught. 注意：绝对不要执行var = find_prisoner_with_knife() ，因为不打算捕获返回值。

Using no `return` at all 根本没有`return`

This will also return None , but that value is not meant to be used or caught. 这也将返回None ，但是该值并不意味着要使用或捕获。 It simply means that the function ended successfully. 这仅表示该功能已成功结束。 It's basically the same as return in void functions in languages such as C++ or Java. 它基本上与以C ++或Java这样的语言在void函数中return相同。

In the following example, we set person's mother's name and then the function exits after completing successfully. 在下面的示例中，我们设置了人的母亲的名字，然后该函数在成功完成后退出。

def set_mother(person, mother):if is_human(person):person.mother = mother

Note: You should never do var = set_mother(my_person, my_mother) , since the return value is not meant to be caught. 注意：绝对不要执行var = set_mother(my_person, my_mother) ，因为这并不意味着要捕获返回值。

#4楼

Yes, they are all the same. 是的，它们都是一样的。

We can review the interpreted machine code to confirm that that they're all doing the exact same thing. 我们可以查看解释后的机器代码，以确认它们都在做完全相同的事情。

import disdef f1():print "Hello World"return Nonedef f2():print "Hello World"returndef f3():print "Hello World"dis.dis(f1)4   0 LOAD_CONST    1 ('Hello World')3 PRINT_ITEM4 PRINT_NEWLINE5   5 LOAD_CONST    0 (None)8 RETURN_VALUEdis.dis(f2)9   0 LOAD_CONST    1 ('Hello World')3 PRINT_ITEM4 PRINT_NEWLINE10  5 LOAD_CONST    0 (None)8 RETURN_VALUEdis.dis(f3)14  0 LOAD_CONST    1 ('Hello World')3 PRINT_ITEM4 PRINT_NEWLINE            5 LOAD_CONST    0 (None)8 RETURN_VALUE

#5楼

就功能而言，它们都是相同的，它们之间的区别在于代码的可读性和样式（要考虑的重要因素）