python条件检查中的str(None)

Question

Say I have a variable in python a

if a is None , then is str(None) going to return None or something else?

I ran this in an intepreter an did something like:

a = str(None)
print(a is None)

And it printed False , why is that?

Answer 1

In Python, str will always return a string. In this case, str(None) is the string "None" . "None" is not equal to None since one is a string and the other is the actual value.

Answer 2

With regard to your first question (you should have only one per post, by the way), you can figure this out by running it yourself.

With regard to your second question, you are printing the value of the expression a is None . a is either None or it isn't. Either way, you'll get a Boolean value. In this case, a isn't None but rather a string.

Answer 3

Converting it to a string is changing it to the word "None". Try the below.

a = str(None)
print(type(a))
print(type(None))
print('None' == None)

This will output the below.

<class 'str'>
<class 'NoneType'>
False

A string and a NoneType are not equal.

Answer 4

Maybe you need to print out the result of a, and then you know why the result is False .

a = str(None)
print(type(a))
print(a is None)

Answer 5

The operator is is an identity operator, meaning that it returns True if both variables are the same object. Objects referencing None will always have the same identity, however, str(None) and None will not have the same identity, therefore your print statement returns False .

The is operator has nothing to do with whether something is equal in value, you have to use == operator for that. See examples below.

a = str(None)

>>>print(a == None)
False
>>> print(a is None)
False
>>> print(a is str(None))
False
>>> print(a == str(None))
True