如何在python中“测试”NoneType？

How to "test" NoneType in python?

提问by CrveniZg

I have a method that sometimes returns a NoneType value. So how can I question a variable that is a NoneType? I need to use ifmethod, for example

我有一个有时会返回 NoneType 值的方法。那么我如何质疑一个 NoneType 的变量呢？例如，我需要使用if方法

if not new:
    new = '#'

I know that is the wrong way and I hope you understand what I meant.

我知道那是错误的方式，我希望你明白我的意思。

采纳答案by thefourtheye

So how can I question a variable that is a NoneType?

那么我如何质疑一个 NoneType 的变量呢？

Use isoperator, like this

使用is运算符，像这样

if variable is None:

Why this works?

为什么这有效？

Since Noneis the sole singleton object of NoneTypein Python, we can use isoperator to check if a variable has Nonein it or not.

由于None是NoneTypePython 中唯一的单例对象，我们可以使用is运算符来检查变量是否None存在。

Quoting from isdocs,

引用is文档，

The operators isand is nottest for object identity: x is yis true if and only if xand yare the same object. x is not yyields the inverse truth value.

对象标识的运算符is和is not测试：x is y当且仅当x和y是同一个对象时为真。x is not y产生逆真值。

Since there can be only one instance of None, iswould be the preferred way to check None.

由于只能有一个 实例None，因此is将是首选的检查方式None。

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Hear it from the horse's mouth

从马口中听到

Quoting Python's Coding Style Guidelines - PEP-008(jointly defined by Guido himself),

引用Python 的编码风格指南 - PEP-008（由 Guido 本人共同定义），

Comparisons to singletons like Noneshould always be done with isor is not, never the equality operators.

与单例的比较None应该总是用isor来完成is not，而不是等号运算符。

回答by elhoucine

if variable is None:
   ...

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if variable is not None:
   ...

回答by cardamom

It can also be done with isinstanceas per Alex Hall's answer:

也可以isinstance按照Alex Hall 的回答来完成：

>>> NoneType = type(None)
>>> x = None
>>> type(x) == NoneType
True
>>> isinstance(x, NoneType)
True

isinstanceis also intuitive but there is the complication that it requires the line

isinstance也很直观，但复杂的是它需要线

NoneType = type(None)

NoneType = type(None)

which isn't needed for types like intand float.

像intand这样的类型不需要float。

回答by Martin Müsli

As pointed out by Aaron Hall's comment:

正如 Aaron Hall 的评论所指出的：

Since you can't subclass NoneTypeand since Noneis a singleton, isinstanceshould not be used to detect None- instead you should do as the accepted answer says, and use is Noneor is not None.

由于您不能子类化NoneType并且None是单例，isinstance因此不应用于检测None- 相反，您应该按照公认的答案进行操作，并使用is Noneor is not None。

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Original Answer:

原答案：

The simplest way however, without the extra line in addition to cardamom's answeris probably:
isinstance(x, type(None))

然而，最简单的方法，除了豆蔻的答案之外没有额外的行可能是：
isinstance(x, type(None))

So how can I question a variable that is a NoneType? I need to use if method

那么我如何质疑一个 NoneType 的变量呢？我需要使用 if 方法

Using isinstance()does not require an iswithin the if-statement:

Usingisinstance()不需要is在if-statement 中：

if isinstance(x, type(None)): 
    #do stuff

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Additional information
You can also check for multiple types in one isinstance()statement as mentioned in the documentation. Just write the types as a tuple.

附加信息
您还可以检查文档中isinstance()提到的一个语句中的多种类型。只需将类型写为元组。

isinstance(x, (type(None), bytes))

回答by Pesko

Python 2.7 :

蟒蛇2.7：

x = None
isinstance(x, type(None))

or

或者

isinstance(None, type(None))

==> True

==> 真

回答by Joshua Pachner

Not sure if this answers the question. But I know this took me a while to figure out. I was looping through a website and all of sudden the name of the authors weren't there anymore. So needed a check statement.

不确定这是否回答了问题。但我知道这花了我一段时间才弄明白。我正在浏览一个网站，突然之间作者的名字不再存在了。所以需要一个检查语句。

if type(author) == type(None):
     my if body
else:
    my else body

Author can be any variable in this case, and Nonecan be any type that you are checking for.

在这种情况下，作者可以是任何变量，也None可以是您要检查的任何类型。

回答by alexmosk25

I hope this example will be helpful for you)

我希望这个例子对你有帮助）

print(type(None) # NoneType

So, you can check type of the variable name

因此，您可以检查变量名称的类型

#Example
name = 12 # name = None
if type(name) != type(None):
    print(name)
else:
    print("Can't find name")