These are special methods surrounded by double underscores, such as __init__, __repr__, __str___ __setitem__, __getitem__, __enter__, __exit__, __add__, __mult__, __next__ and so on... The interpreter invokes them in certain situations, such as when creating an object, converting it to string via built-in str() function. They are used for operator overloading, i.e. to make expressions such as a + b or a * b, or a[ i ] work for custom defined classes and not just for numbers, or arrays.