Why does (1 == 2 != 3) evaluate to False in Python?

This is due to the operators’ chaining phenomenon. The Python documentation explains it as:

Comparisons can be chained arbitrarily, e.g., x < y <= z is equivalent
to x < y and y <= z, except that y is evaluated only once (but in both
cases z is not evaluated at all when x < y is found to be false).

And if you look at the precedence of the == and != operators, you will notice that they have the same precedence and hence applicable to the chaining phenomenon.

So basically what happens:

>>>  1==2
=> False
>>> 2!=3
=> True

>>> (1==2) and (2!=3)
  # False and True
=> False