boolean

[snippsat]

It has to do with chaining comparison operators.
So what's really happens is:

>>> 'b' in 'abc' == 'a' in 'abc'
False

>>> # The chaining is 
>>> ('b' in 'abc') and ('abc' == 'a') and ('a' in 'abc')
False

>>> # After ('abc' == 'a') the result is False and last ('a' in 'abc') is not evaluated   
>>> ('abc' == 'a') 
False

Should avoid to use == like this as it can give unexpected result.
If most use it so dos () break the chaining.

>>> ('b' in 'abc') == ('a' in 'abc')
True