There's no bug here.

"&" is the bitwise Boolean logical-and operator on integers. For example,

>>> 1 & 2
0
>>> 1 & 3
1

It binds more tightly than the "==" equality-testing operator. To get the result you want, you would need to add more parentheses to override the natural operator precedence:

>>> (a == 10) & (not(a!=b)) & (b == 10)
True

But, more to the point, what you probably really want is the "and" logical operator, not the bitwise integer "&" operator:

>>> a == 10 and (not(a!=b)) and b == 10
True