x = 3 if (y == 1) else 2
It does exactly what it sounds like: “assign 3 to x if y is 1, otherwise assign 2 to x”. Note that the parens are not necessary, but I like them for readability. You can also chain it if you have something more complicated:
x = 3 if (y == 1) else 2 if (y == -1) else 1
Though at a certain point, it goes a little too far.
Note that you can use if ... else in any expression. For example:
(func1 if y == 1 else func2)(arg1, arg2)
Here func1 will be called if y is 1 and func2, otherwise. In both cases the corresponding function will be called with arguments arg1 and arg2.
Analogously, the following is also valid:
x = (class1 if y == 1 else class2)(arg1, arg2)
where class1 and class2 are two classes.