无 Python 错误/错误? [英] None Python error/bug?
问题描述
在 Python 中,你有 None
单例,它在某些情况下表现得很奇怪:
所以首先,
显示 None
不是类型,但 NoneType
是.然而,当您运行 isinstance(a,NoneType)
时,它会响应一个错误:NameError: name 'NoneType' is not defined
现在,鉴于此,如果您有一个输入默认设置为 None
的函数,并且需要检查,您可以执行以下操作:
如果变量是无:#做一点事别的:#做一点事
我不能执行以下操作的原因是什么:
if isinstance(variable,None): #or NoneType#做一点事别的:#做一点事
我只是想找一个详细的解释,这样我才能更好地理解这一点
很好的应用
假设我想使用 isinstance
以便在 variable
是多种类型(包括 None
)时我可以做一些事情:>
if isinstance(variable,(None,str,float)):#做一点事
你可以试试:
<预><代码>>>>变量 = 无>>>isinstance(变量,类型(无))真的>>>变量 = 真>>>isinstance(变量,类型(无))错误的isinstance 接受 2 个参数 isinstance(object, classinfo)
在这里,通过传递 None
您将 classinfo
设置为 None,因此出现错误.您需要传入类型.
In Python you have the None
singleton, which acts pretty oddly in certain circumstances:
>>> a = None
>>> type(a)
<type 'NoneType'>
>>> isinstance(a,None)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: isinstance() arg 2 must be a class, type, or tuple of classes and types
So first off, <type 'NoneType'>
displays that None
is not a type, but that NoneType
is. Yet when you run isinstance(a,NoneType)
, it responds with an error: NameError: name 'NoneType' is not defined
Now, given this, if you have a function with an input default set to None
, and need to check, you would do the following:
if variable is None:
#do something
else:
#do something
what is the reason that I cannot do the following instead:
if isinstance(variable,None): #or NoneType
#do something
else:
#do something
I am just looking for a detailed explanation so I can better understand this
Edit: good application
Lets say I wanted to use isinstance
so that I can do something if variable
is a variety of types, including None
:
if isinstance(variable,(None,str,float)):
#do something
You can try:
>>> variable = None
>>> isinstance(variable,type(None))
True
>>> variable = True
>>> isinstance(variable,type(None))
False
isinstance takes 2 arguments isinstance(object, classinfo)
Here, by passing None
you are setting classinfo
to None, hence the error. You need pass in the type.
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