Special methods (sometimes also called magic methods) are how user defined classes interact with the Python virtual machine. For example, for a class to support addition it must implement the __add__ and __radd__ special methods. When the expression a + b is evaluated the Python virtual machine will call type(a).__add__(a, b) and if that is not implemented it will call type(b).__radd__(b, a).

Since these special methods are always called by the virtual machine with a fixed number of parameters, if the method is implemented with a different number of parameters it will fail at runtime with a TypeError.