The inconsistency is that for immutable objects (or more generally, objects that have __add__ but not __iadd__), '+=' does create a name binding. After 'a = 30

The inconsistency is that for immutable objects (or more generally, objects that have __add__ but not __iadd__), '+=' does create a name binding. After 'a = 300; b = 50000', a += b is exactly the same as a = a + b In this case, a new object is created with the value of a + b which then gets bound to the name a.