Unexpected behavior in Python with "is" operator

Question

The code is pretty self-explanatory. I was messing around with Python and this (a is b==b==a is c) unexpectedly returned True. How i saw it, the order of operations is == first and then 'is' (according to google). So the code should be equivalent to (a is True is c) which returns False.

Anyway i tried a lot of possible order of operation and all of them seem to return False. Am i missing something really obvious?

I'm using 3.4.2

a,b,c=['wtf' for i in range(3)]

print(a is b==b==a is c) #prints True
print(b==b==a) #prints True
print(a is True is c) #prints False
print((a is (b==b==a)) is c) #prints False
print(a is (b==b==a) is c) #prints False
print((a is b==b==a) is c) #prints False
print((a is b==b)==a is c) #prints False
print((a is b)==b==a is c) #prints False
print(a is ((b==b)==a) is c) #prints False
print(a is ((b==b)==a is c)) #prints False
print((a is b)==(b==(a is c))) #prints False
print((a is b)==b==(a is c)) #prints False

how would `a is True` ever evaluate to True unless a was actually True? — Padraic Cunningham, May 17 '15 at 14:09
I know b==b==a is True. So what i was expecting to happen is for (a is b==b==a is c) to be equivalent to (a is True is c), and both should be false, since i used 'is', and a is not True. — Costea Vlad Alexandru, May 17 '15 at 14:19
@user2015213, but a is not True a is a string, `a is b -> True , b==b==a -> True, a is c ->True` — Padraic Cunningham, May 17 '15 at 14:20
You might also want to look at this http://stackoverflow.com/questions/28329498/why-does-a-space-effect-the-identity-comparison-of-equal-strings/28329522#28329522 — Padraic Cunningham, May 17 '15 at 14:22
and this https://docs.python.org/2/reference/expressions.html#comparisons — Padraic Cunningham, May 17 '15 at 14:24

score 2 · Accepted Answer · answered May 17 '15 at 14:20

2

In Python comparison operators chain, so a is b==b==a is c is equivalent to

(a is b) and (b == b) and (b == a) and (a is c)

and as a,b and c are all the same object, all comparisons evaluate to True.

answered May 17 '15 at 14:20

Daniel

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wait, why does the 'is' enter the chain too? shouldn't it just be (a is ((a==b) and (b==b)) is c)? – Costea Vlad Alexandru May 17 '15 at 14:28
nevermind, i got it. *all* comparation operators chain. – Costea Vlad Alexandru May 17 '15 at 14:29

Unexpected behavior in Python with "is" operator

1 Answers1