7.7. Don’t Chain Inequality != Operators¶
Chaining comparison operators like 18 < age < 35 or chaining assignment operators like six = halfDozen = 6 are handy shortcuts for (18 < age) and (age < 35) and six = 6; halfDozen = 6 , respectively. But don’t chain the != comparison operator. You might think the follow- ing code checks whether all three variables have different values from each other, because the following expression evaluates to True :
>>> a= 'cat'
>>> b= 'dog'
>>> c= 'moose'
>>> a!= b != c
True
But this chain is actually equivalent to (a != b) and (b != c) . This means that a could still be the same as c and the a != b != c expression would still be True :
>>> a= 'cat'
>>> b= 'dog'
>>> c= 'cat'
>>> a!= b != c
True
This bug is subtle and the code is misleading, so it’s best to avoid using chained != operators altogether.