I was wondering, what is "is not"? Is it similar to "!=" ?

Question

How's going everybody? ｖ(⌒ｏ⌒)ｖ
(If you're wondering where I ran into this problem, this is from the Learn Python Track, in the Python Collections course, from the video "Shopping List Take 3", at 6:53)
Let's say I have this code
```
position = input("In what position (index) do you want to insert your item into the list?")
if position is not None:
my_list.append(position)
else:
break
```
Do you notice the "is not"?
How does that compare to "!="?
if I put instead

if position != None:

is that doing something different?
Thank you very much ^_^

Steven Parker · Accepted Answer

One is a negative identity test ("is not") and the other is a negative equality test ("!=").  Identity means that both items are references to the same thing, where equality means that both items have the same value.
Now there's no difference when using "None", because it is always a reference to a specific internal object.  But it can be quite different with other things.
To illustrate, let's simplify it a bit by using the positive versions of these comparisons ("is" and "=="):
a = 999  
b = a  
a is b :point_left: True (because a and b both refer to the same thing internally)   But...
a = 999  
b = 999  
a is b :pointleft: _False (because a and b refer to different internal storage locations)  
a == b :pointleft: True (because they do have the same _value)
:warning: if you try this example with small values (256 and below) internal optimization will confuse the results.

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Liam Hayes

Liam Hayes

I was wondering, what is "is not"? Is it similar to "!=" ?

1 Answer

Steven Parker

Steven Parker

Liam Hayes

Liam Hayes